THE PHYSIOLOGICAL STRUCTURE AND FUNCTION OF THE KIDNEY
The kidneys are bean-shaped, reddish-brown organs that are located retroperitoneally on either side of the vertebral column, extending from the 12th thoracic vertebra to the 3rd lumbar vertebra. Each kidney is approximately 11.0 cm in length, 5.0-7.0 cm in diameter, and 2.5 cm in thickness. Externally at the portion of the kidney there is a notch called the hilum, where renal artery and vein, lymphatics, nerves and renal pelvis are located. Internally the kidneys are composed of the cortex and the medulla. Uriniferous tubule is the functional unit of the kidney. It consists of the nephron and the collecting duct. It is estimated that a pair of kidneys contain 2.5 million nephrons. The nephron is a unique and complex structure. Nephrons are classified into cortical and juxtamedullary nephrons. Each nephron consists of the Bowman’s capsule, proximal convoluted tubule, loop of Henle, and distal convoluted tubule. Many distal tubules empty into one collecting duct or tubule. Bowman’s capsule is a cup-like structure that surrounds the capillary network called glomerulus, two of them together being called renal corpuscle. The corpuscle has a vascular pole and urinary pole. Renal glomerulus is a vascular component of the nephron which consists of glomerular capillaries and intraglomerular mesangium. The glomerular basement membrane is a three layered structure consisting of lamina rara
externa, lamina densa and lamina rara interna. The filtration of blood takes place in the renal capsule across the glomerular filtration membrane or blood-urine barrier, which is made up of the following layers: fenestrated endothelium, glomerular basement membrane and slit membrane.
The glomerular mesangium comprises mesangial cells and mesangial matrix. From the capsular space the glomerular filtrate drains into the proximal convoluted tubule, loop of Henle and distal convoluted tubule.
The urine passes from the distal convoluted tubule into the collecting tubule and collecting duct.
The main function of the kidneys is to excrete the waste products of the metabolism, to regulate the body concentration of water and salt in order to maintain the appropriate acid-base balance of plasma, and serves as an endocrine organ secreting erythropoietin, renin and prostaglandins (1).
DIABETIC NEPHROPATHY
The development of diabetic nephropathy is characterized by a progressive increase in the excretion of protein, particularly albumin, an early and continuing rise in systemic blood pressure, and a late decline in glomerular filtration rate, leading eventually to end-stage renal failure. In addition to these central abnormalities, type 1 and type 2 diabetes patients with
E-mail: salahbenhamed@hotmail.com
2Vuk Vrhovac Institute, University Clinic for Diabetes,
Endocrinology and Metabolic Diseases, Dugi dol 4a, HR-10000 Zagreb, Croatia
MICROALBUMINURIA AND DIABETES MELLITUS
Salah R. Saleh Ben Hamed1, Pajica Pavkoviæ2, @eljko Metelko2nephropathy can be distinguished from their normoalbuminuric peers by the presence of other metabolic and clinical abnormalities (2).
EPIDEMIOLOGY: INCIDENCE AND PREVALENCE
In diabetic patients with proteinuria the relative mortality is about 40 times higher than in diabetics without proteinuria. Renal damage is a serious complication of diabetes mellitus (DM). It is estimated that death due to renal disease is 17 times more common in diabetics than in nondiabetics. Diabetic nephropathy is the most important cause of death in type 1 diabetic patients, of whom 30%-40% eventually develop end-stage renal failure. Death due to diabetic nephropathy with renal failure is less common in type 2 DM. The relative risk of renal mortality in diabetic patients diagnosed after the age of 45 years is estimated to be twofold that in nondiabetic patients. However, proteinuria as an indicator of renal involvement is not uncommon in patients with type 2 DM. It is now established that in both type 1 and 2 DM, urinary excretion of small amounts of albumin (microalbuminuria) is predictive of morbidity and mortality due to renal complications and cardiovascular disease (3).
When diabetic nephropathy is diagnosed by the classical methods such as detection of proteinuria on urine analysis or decrease in creatinine clearance, little can be done to prevent the progressive downhill course to renal failure. However, the progression of diabetic nephropathy in type 1 diabetic individuals with microalbuminuria has been reported to be retarded by good glycemic control. In addition, tight blood pressure control has also been noted to reduce microalbuminuria, particularly with the use of ACE inhibitors.
Five years from the onset of diabetes, the risk of nephropathy rises rapidly, peaks during the second decade and then declines, although nephropathy is often considered together with retinopathy as a microangiopathic complication.
Reports on nephropathy developing in some patients with apparently well controlled diabetes and not developing in some patients even after years of severe hyperglycemia implicate some background predisposition (4).
ETIOLOGY OF DIABETIC NEPHROPATHY
Diabetes mellitus is one of the systemic diseases affecting the kidneys. Diabetic nephropathy would have developed in about one third of patients who have had type 1 DM for more than 20 years. The incidence of nephropathy in type 2 DM is uncertain. Mortality and morbidity in these patients are due to cardiovascular disease accelerated by hypertension and hyperlipidemia (3).
Biochemical, hormonal, immunologic and rheologic factors are important in the pathogenesis of diabetic nephropathy.
A) Biochemical factors include hyperglycemia and glycosylated proteins in blood and basement membrane of the kidneys (5).
B) Hormonal factors: higher levels of growth hormone are believed to promote basement membrane thickening in diabetes with coincidental hypopituitarism and increased basement membrane thickening in experimental animals given growth hormone injections (6).
C) Immunologic factors: both exogenous and endogenous insulin anti-insulin antibody complex mediated immunologic factors also contribute to the basement membrane thickening (insulin autoantibodies, IAA) (7).
D) Rheologic factors: reduced red blood cell (RBC) deformability due to glycosylation and fibrin deposition resulting from altered permeability and hypercoagulability. With the deposition of fibrin, electron microscopic lesions are converted into light microscopic ones, although basement membrane is thickened in diabetic nephropathy due to various factors mentioned above. So, kidney functions as a ’poor filter’ (8).
PATHOGENESIS OF DIABETIC NEPHROPATHY
Both type 1 and type 2 DM are characterized by hyperglycemia, a relative or absolute lack of insulin, and the development of diabetes induced vascular changes. A large number of human studies provide support for the concept that the microvascular complications of diabetes mellitus are dependent on
hyperglycemia. There are four potential biochemical pathways linking hyperglycemia to the changes within the kidney, which can plausibly be linked to the functional, structural changes characterizing diabetic nephropathy. These are: polyol pathway, nonenzymatic glycation, glucose auto-oxidation and de novosynthesis of diglycerol leading to protein kinase C and phospholipase A2 activation.
FUNCTIONAL CHANGES IN DIABETIC NEPHROPATHY
Cardinal functional changes characterize the natural history of diabetic nephropathy: changes in the glomerular filtration rate (GFR); proteinuria and albuminuria; and arterial blood changes. There are 5 stages which apply mostly to type 1 DM but some may also be seen in type 2 DM (9).
Changes in glomerular filtration rate (GFR)
Several studies have shown that in both type 2 and type 1 diabetic patients, GFR is elevated in the newly diagnosed patients and is significantly related to the increase in the kidney size. So, GFR is higher than normal in stage 1 of glomerular hypertrophy and hyperfiltration. It may exceed 150 ml/min. With the increase in protein excretion there is a tendency of GFR to fall to lower level but not below the normal range during the 2nd and 3rd stages of diabetic nephropathy. With the onset of persistent proteinuria, GFR progressively falls and culminates in the end stage renal disease (ESRD) in months to a year if left untreated.
GFR correlates positively with HbA1c. Raised urinary albumin excretion is also associated with HbA1c in patients with incipient nephropathy. The rate of progression of nephropathy is correlated with metabolic control. So far there is no clinical indication for renal biopsy in patients with microalbuminuria including morphometry (10).
Proteinuria and albuminuria
Proteinuria, more specifically albuminuria, is the earliest and most sensitive predictor of diabetic nephropathy. Physiologically, the upper normal range of urinary protein excretion is 150 mg/24 h. Direct
determination of urinary albumin greatly improves sensitivity. The upper limit of albumin excretion of 30 mg/24 h is equivalent to approximately 20 ìg/min (11). Microalbuminuria (below 200 ìg/min or 300 mg/day) cannot be detected by albustix method but with more sensitive methods such as ELISA and microalbustix-distix (12).
Overt nephropathy, the onset of clinical phase of diabetic nephropathy, is signaled by the presence of persistent proteinuria, i.e. albumin excretion rate (AER) >200 ìg/min or 300 mg/day, usually accompanied by retinopathy, hypertension, declining GFR and plasma lipid abnormalities. With the progression of nephropathy AER increases to an arbitrary level of >300 mg/24 h, and in later stages proteinuria may be within the nephritic range.
Arterial pressure changes
Patients are usually normotensive until albuminuria supervenes. Once microalbuminuria has appeared, blood pressure starts to rise. Concomitantly an abnormal blood pressure profile is noted with a diminished nocturnal fall and increase during exercise. Hypertension is the consequence of renal parenchymal changes in type 1 DM.
Type 2 diabetic patients may be hypertensive for years prior to the onset of overt diabetes. At the time of diagnosis of type 2 DM, hypertension is found in approximately 70%-80% of patients. Still blood pressure raises further in those patients who subsequently develop diabetic nephropathy (13).
Mogensen et al. have characterized the progression of diabetic renal disease according to stages (14), as shown in Table 1.
MICROALBUMINURIA Definition of microalbuminuria
The term microalbuminuria has been used to describe an amount of albumin in the urine which is less than can be detected by ordinary clinical tests such as albustix, but is otherwise still associated with future disease (15,16). Microalbuminuria has been defined using different units of measurements. According to the Gento-Montecatini Convention, microalbuminuria is present when the urinary albumin excretion rate
(UAER) in 24-hour urine or short time collected urine during daytime is in the range of 30 to 300 mg/24-h (20 to 200 µg/min), which is equivalent to 0.46 to 4.6
µmol/24-h (17). Urine samples should be collected when the patient is at rest and his diabetes is under average clinical control. No measurement should be made in patients with ketosis or poor control until proper control is established. If excretion is lower than 20 µg/min, the patient is considered to have normoalbuminuria, and if excretion is higher than 200
µg/min, he is considered to have macroalbuminuria or clinical proteinuria. Microalbuminuria should be present in at least two of three urine samples collected over a period of several months (18,19).
In some conditions, false-positive results can be observed, e.g.:
* hyperglycemia * acute intercurrent
disease
* hypertension * heavy exercise
* urinary tract infection * cardiovascular
(UTI) decompensation
*stress febrile conditions * contamination with seminal or menstrual fluid
Renal structural changes in type 1 diabetes with microalbuminuria
In the '60s, several reports indicated that morphological changes were present at onset of type 1 diabetes. Later studies clearly showed the glomeruli to be normal at that time. Structural changes in the glomeruli, termed diabetic glomerulopathy, are an aspect that has attracted most attention. The increased thickness of the glomerular basement membrane and mesangial expansion with accumulation of the matrix are the fundamental changes. Another glomerular structural change of completely different nature is the glomerular hypertrophy present in the earliest stage of diabetes. In advanced stages it is further expressed as a compensatory enlargement accompanying the developing glomerulopathy. Further, extraglomerular changes may play an important role. One of the characteristics is arteriolar hyalinosis affecting afferent and efferent arterioles. Semiquantitative studies of the hyalinosis of arterioles have dealt with a very broad severity of glomerulopathy and interstitial expansion as well as with renal function. Also, expansion of the
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cortical interstitium is part of the whole picture. Further, measurable enlargement has been described in the juxtaglomerular apparatus and the vascular pole region (20).
Renal structural changes in type 2 diabetes with microalbuminuria
Although 80% or more of diabetic patients receiving renal replacement therapy have type 2 diabetes, the renal pathology and natural history of diabetic nephropathy in type 2 diabetes has been studied much less intensely than in type 1 diabetes, thus many important questions remain unclear (21).
Measuring microalbuminuria
Techniques for measuring microalbuminuria are qualitative (or semiquantitative) and quantitative. Several qualitative tests for microalbuminuria based on the binding methods are described. Semiquantitative or qualitative screening tests for microalbuminuria should be positive in >95% of patients with microalbuminuria to be useful for screening. Positive results must be confirmed by quantitative analysis in a laboratory.
Screening tests must have high detection rates for abnormal samples. Further studies are needed before the dipstick tests for microalbuminuria can be recommended as replacements for the quantitative tests. The use of qualitative tests at the point of care is only reasonable when it can be shown to avoid quantitative testing in a major proportion of patients and to ensure detection of those patients with early renal disease.
The Micral microalbumin urine test strip (Roche Diagnostic)
This test is an immunochemical strip specific for albumin. Albumin in the sample is bound by soluble conjugate of antibodies and the β-galactosidase enzyme marker. Conjugate-albumin complexes are separated and the β-galactosidase enzyme reacts with a substrate to produce a red dye. The intensity of the color produced is proportional to the albumin concentration in the urine.
Clinitec Microalbumin (Bayer Diagnostic)
This test strip is based on dye binding by albumin method. It uses the high affinity dye bis (3’,3’’-diiodo- 4’,4’’-dihydroxy-5’,5’’-dinitrophenyl)-3,4,5,6-tetrabromosulfonephthalein. At a constant pH, the strip turns blue in the presence of albumin, and color is directly related to albumin concentraiton in the urine sample.
Quantitative assays for urine albumin use immuno-chemistry with antibodies to human albumin.
There are four methodologies available: radio-immunoassay (RIA), enzyme-linked immunosorbent assay (ELISA), radioimmunodiffusion (RID), and immunoturbidimetry. All these four methods have similar sensitivity, precision and range.
Radioimmunoassay (RIA)
Standard RIA is performed in liquid phase with the presence of excess antigen. Albumin in the sample competes with a fixed amount of I125-labeled albumin for the binding sites of specific antibodies. Free and bound albumin are separated by the addition of a second antibody immunoadsorbent, followed by centrifugation and decanting. The radioactivity in the
pellet is measured with ã-counter. Albumin
concentration in the sample is inversely proportional to the radioactivity. The sample values are determined by comparison with a calibration curve. The advantages are sensitivity, precision and inexpensiveness, whereas the disadvantage is short shelf life and radioactivity of the reagents.
Radioimmunodiffusion (RID)
This method requires long incubation and cannot be automated so it is not widely accepted. Monospecific antiserum to human albumin is incorporated into an agar gel. Samples and calibrators are added to the wells and allowed to diffuse into the agar at equilibrium, the antigen-antibody complexes precipitate and after staining the distance of migration is being measured.
Enzyme-linked immunosorbent assay (ELISA)
Competitive and ’sandwich’ ELISAs are available. The competitive ELISA is faster but it is reported to be less sensitive. ELISA can be semiautomated with the use of a microplate reader. In all ’sandwich’ techniques the primary antibody (antialbumin
anti-serum) is fixed on the plastic plate. Then the samples, calibrators and controls are added. The antibody-antigen complexes are detected and quantified by second antibody conjugated to an enzyme label.
Immunoturbidimetry
In the urine sample albumin forms an insoluble complex with antibodies to human albumin. Polyethylene glycol (PEG) accelerates the complex formation. The turbidity caused by these complexes is measured on a spectrophotometer at 340 nm and it is the measure of albumin concentration. This is a simple and less expensive method, offering the possibility of rapid analysis of large numbers of samples.
Samples
There are several proposals for suitable urine samples for screening diabetic patients for early renal involvement:
1) a random urine sample is not very useful because there may be great variation in urine volume output due to fluid intake, and therefore the excretion rate may not correlate very closely to albumin concentration;
2) random early morning urine sample: the first urine sample voided in the morning is usually rather concentrated and on using this sample there is usually a pretty good correlation between the excretion rate and concentration of albumin. Therefore, such a sample is suitable for the screening purpose. If the concentration is above 20-25 g/l, the risk of an abnormal excretion rate is rather high. Measuring creatinine in such a sample and expressing albumin concentration over creatinine concentration may make such test more exact, since this corrects it for urine volume; 3) an overnight sample is used by many centers and is
probably the method of choice. If a diabetic clinic is planning to perform screening for early renal disease, this method is recommended;
4) 24-h urine collection is preferred by some centers; and
5) a short-term urine collection may also be used, e.g., while the patient is waiting at the clinic.
Measurements can also be done in connection with clearance studies. In this way a complete measurement of renal function can be done along with clearance studies. In this way a complete measurement of renal function is performed but care should be taken to avoid the first urine samples after water-drinking and high urine output due to early wash-out effect after starting water drinking.
Nonanalytical sources of variation
Transient increases of urinary albumin excretion have been reported with short-term hyperglycemia, exercise, urinary tract infections, marked hypertension, heart failure, and acute febrile illness. Albumin excretion is as variable in nondiabetic adults as in diabetics with normal or elevated albumin excretion, the average intraindividual comparable variability being approximately 40%. In order to improve precision in screening, the urinary albumin to creatinine ratio has been suggested. Values in first morning urine exceeding 2 mg/mmol indicate microalbuminuria (22-26).
SIGNIFICANCE OF
MICROALBUMINURIA AS AN EARLY PREDICTOR AND MARKER FOR DIABETIC NEPHROPATHY
Diabetic nephropathy rapidly becomes an important problem. Early detection of a risk leading to the possibility of intervention before advanced renal damage has occurred is an obviously important goal. This goal is made difficult by the fact that much of the important diabetic renal structural injuries can occur in absolute clinical silence.
The proportion of patients with ESRD caused by diabetes has progressively increased during the last few decades and diabetic nephropathy is now the single most common cause of ESRD in the western world.
Based on studies in type 1 diabetes, it was generally considered that once overt diabetic nephropathy manifesting as persistent proteinuria had set in, it was only possible to slow but not halt the progression towards ESRD. This led investigators during the early ’80s to search for early predictors of diabetic nephropathy through the measurement of low concentrations of albumin in the urine. Initial
retrospective studies in type 1 diabetic patients
observed an ∼80% risk of progression from
microalbuminuria to proteinuria over the subsequent 6-14 years. These early studies, each of which used different AER criteria for microalbuminuia, led to a consensus conference at which a general agreement was reached on the definition of microalbuminuria (AER, 20-200 µg/min). Since then, there has been broad acceptance of microalbuminuria as a marker of an increased risk of diabetic nephropathy. However, concern has been raised that there is a wide range of underlying diabetic glomerular lesions among longstanding type 1 diabetic patients. In some patients with persistent microalbuminuria, renal lesions are quite advanced and treatment in these patients could be less efficient than at earlier stages of the disease. Thus, in these patients microalbuminuria may be a marker rather than a predictor of advanced renal structural changes. Therefore, it is not surprising that patients with microalbuminuria may progress to proteinuria despite strict glycemic control. Thus, it would make sense to try to identify normoalbuminuric patients at an increased risk of diabetic nephropathy in order to select those who are at early stages still amenable to aggressive intervention strategies such as strict glycemic control (27).
Longterm follow-up studies have been undertaken by three diabetes centers which evaluate the power of microalbuminuria to predict overt diabetic nephropathy defined as clinical proteinuria. The technique and urine sample used, as well as the follow-up time differed among the centers, therefore it is not surprising that there are some differences in the predictive levels reported (12,28). However, generally speaking, the results are quite consistent, documenting that an elevated urinary albumin excretion rate in fact predicts overt diabetic nephropathy with a rather high degree of accuracy.
For a predictor of diabetic nephropathy to be optimally useful, it should identify individuals at an increased risk of the development of serious diabetic renal disease early enough in the natural history of the disorder that the evolution of the process can be influenced by intervention strategies.
Microalbuminuria is less common in the first decade of type 1 diabetes, especially during the first 5 years, and by 20-25 years much of the natural history of the disorder has already declared itself within the patient population. On the other hand, because the duration of type 2 diabetes is usually not accurately known, microalbuminuria may even appear before the diagnosis of diabetes. Further, in population-based studies microalbuminuria is not uncommon, especially in elderly people, where it is also strongly related to cardiovascular disease and mortality as in type 1 and type 2 DM.
In diabetic pregnancy, an increase of microalbu-minuria predicts complications (14).
Microalbuminuria in terms of predictive power is still the strongest broadly available marker or predictor of diabetic nephropathy (Table 2). Other markers of diabetic nephropathy are listed in Table 3.
However, we need improved markers and predictors aiming at an early diagnosis of renal changes that may occur in normoalbuminuric patients. These will be addressed in two general categories:
Existing methods
A combination of measures of AER with multiple clinical and renal structural parameters may lead to the development of more precise risk estimates for diabetic nephropathy. These additional variables include age, diabetes duration, blood pressure (including 24-h blood pressure monitoring), GFR, HbA1c, retinopathy, and renal biopsy measurements. Other variables could include plasma prorenin, erythrocyte sodium/lithium
n o i t a r t l i f r e p y H Microalbuminuria Clinicalproteinuria Structurallesions r e w o p e v it c i d e r P Yes Strong Strong Sitllunknown y g o l o i s y h p o h t a p o t p i h s n o it a l e R Likely Yes Yes Notdeifned e g a m a d l a r u t c u r t s o t p i h s n o it a l e R Notclearly Clearlyintype2DM Yes r a l u c s a v r e h t o h t i w n o it a i c o s s A s n o i s e l ? Yes Yes Likely
Table 3. Markers of diabetic nephropathy other than microalbuminuria r e k r a M f o e p y T M D Mainfindings n i r r e f s n a r t r a l u r e m o l G Urine Type1 Increasedexcreitoncomparedtocontrols s t n e it a p e v it a g e n x it s p i d n i n o it e r c x e d e s a e r c n I a i r u n i m u b l a h t i w d e t a l e r r o C a i r u n i m u b l a n a h t a i r u n i r r e f s n a r t r o f e v it i s o p s t n e it a p e r o M a i r u n i m u b l a n a h t y l d e k r a m e r o m , e s i c r e x e r e t f a n o it e r c x e d e s a e r c n I y h t a p o n it e r h t i w s t n e it a p n i d e s a e r c n I 2 e p y T Higherpredicitona/creaitninraitocomparedtocontrolsinabsenceofalbuminuria n i m / g * 5 < R E A U n e h w n e v e s l o r t n o c h t i w d e r a p m o c R E T U n a i d e m r e h g i H e s o h t o t d e r a p m o c n o i s n e t r e p y h d n a s e t e b a i d h t o b h t i w s t n e it a p n i r e h g i h R E A U / R E T U n o it i d n o c e l g n i s r e h t i e h t i w R E A U h t i w d e t a l e r r o C n il u b o l g o r c i m -1 , G A N h t i w d e t a l e r r o C l o r t n o c c i m e c y l g d e v o r p m i h t i w d e s a e r c e d n o it e r c x E s e v i s n e t r e p y h n i t a h t s e m it 3 s e t e b a i d n i n o it e r c x E n it c e n o r b i F Plasma Type1 Higherlevelsindiabeitcswithmicroalbuminuriacomparedwiththosewithoutitand s l o r t n o c h t i w l o r t n o c c i m e c y l g m r e t -t r o h s h t i w e g n a h c o N f o n o it c i d e r p r o , P B l a i r e t r a , I M B , s e t e b a i d f o n o it a r u d , e g a h t i w n o it a i c o s s a o N l o r t n o c c i m e c y l g e n i r U Type2 Higherlevelsindiabeitcscomparedwithcontrols a i r u n i m u b l a o r c i m h t i w s c it e b a i d n i s l e v e l r e h g i H e c n a r a e l c e n i n it a e r c h t i w d e t a l e r r o c y l e v it a g e N 1 P n i n i m a L m u r e s m u r e S Type2 Higherlevelsindiabeitcscomparedwithcontrols,eveninstageof a i r u n i m u b l a o r c i m y h t a p o r h p e n g n i n e s r o w h t i w s t n e it a p n i s l e v e l r e h g i H e g a r o c 1 A b H h t i w d e t a l e r r o c t o N e n i r U Type1 Higherlevelsindiabeitcscomparedwithcontrols n il u b o l g o r c i m -2 d n a -1 , G A N , n i a h c t h g il a p p a k , n i m u b l a , c 1 A b H h t i w d e t a l e r r o C 2 e p y T Higherinpaitentswithdiabetescomparedwithcontrols h t i w d e t a i c o s s a a i r u n i m u b l a f o e e r g e d r a l i m i s h t i w s t n e it a p c it e b a i d n o n n i e r o m n o it e r c x E s t n e it a p c i r u n i m u b l a o m r o n n i n e v e n il u b o l g o r c i m -1 d n a G A N V I e p y T n e g a l l o c m u r e S Type2 Higherinpaitentswithdiabetescomparedwithcontrols,eveninstageofmicroalbuminuria p u -w o l l o f n o y h t a p o r h p e n g n i n e s r o w d n a y h t a p o i g n a o r c i m h t i w s t n e it a p n i r e h g i H e g a r o c 1 A b H h t i w d e t a l e r r o c t o N e n i r U Type2 Elevatedinpaitentswithdiabetescomparedwithcontrols n o it e r c x e n i n i m a l h t i w d e t a l e r r o C s t n e it a p c i r u n i m u b l a o m r o n n i n e v e n il u b o l g o r c i m -1 d n a G A N h t i w d e t a i c o s s A e t a f l u s n a r a p e H n a c y l g o e t o r p e n i r U Type1 Higherinpaitentswithdiabetescomparedwithcontrols t i t u o h t i w e s o h t d n a a i r u n i m u b l a o r c i m h t i w e s o h t n e e w t e b e c n e r e f f i d o N s n i e t o r p r a l u b u T n il u b o l g o r c i m -2 e n i r U Type1 Increasedlevelscomparedtocontrols a i r u n i m u b l a e l i h w p u -w o l l o f r a e y -3 n o s l o r t n o c h t i w d e r a p m o c s l e v e l d e s a e r c n I d e g n a h c n u s n i a m e r a i r u n i m u b l a o r c i m h t i w d e t a l e r r o c t o N y h t a p o r h p e n t r e v o h t i w s t n e it a p n i d e s a e r c e d n o it e r c x e n i n o it a i r a v l a n r u i d n a i d e M P B R h t i w d e t a l e r r o C 1 P n i n i m a l h t i w d e t a l e r r o C l o r t n o c c i m e c y l g f o e e r g e d e h t h t i w t o n t u b s e t e b a i d f o n o it a r u d e h t h t i w d e t a i c o s s A , e r u s s e r p d o o l b , e s o d n il u s n i , s e t e b a i d f o n o it a r u d , c 1 A b H h t i w n o it a l e r r o c o N n o it e r c x e e s o c u l g e n i r u d n a a i r u n i m u b l a f o t n e m p o l e v e d d n a n o it e r c x e n il u b o l g o r c i m -2 h t i w n o it a l e r o N r e t a l s r a e y 2 1 2 e p y T Increasedexcreitoncomparedtocontrolsafter5-yearfollow-up s l o r t n o c d n a s c it e b a i d n e e w t e b e c n e r e f f i d o N s t n e it a p c i r u n i m u b l a o r c i m d n a c i r u n i m u b l a o m r o n n e e w t e b n o it e r c x e n i e c n e r e f f i d o N s e n o e v i s n e t o m r o n o t d e r a p m o c s t n e it a p e v i s n e t r e p y h n i r e h g i H l o r t n o c c i m e c y l g h t i w d e s a e r c e d s l e v e L t o h i n a M s u h c s o m e l b a f o l o h o c l a h t i w d e t a e r t s t n e it a p n i r e w o l s l e v e l y r a n i r U , a m s a l p ( d o o l B e l o h w , m u r e s ) d o o l b 2 e p y T Higherindiabeitcscomparedtocontrols a i r u n i m u b l a o r c i m h t i w s t n e it a p n i s l e v e l r e h g i H a i r u n i m u b l a o r c i m h t i w d e t a i c o s s A t o h i n a M s u h c s o m e l b a f o l o h o c l a h t i w d e t a e r t s t n e it a p n i r e w o l s l e v e l d o o l B g n i d n i b -l o n it e R n i e t o r p e n i r U Type1 Increasedcomparedwithcontrols a i r u n i m u b l a e l i h w p u -w o l l o f r a e y -3 n o s l o r t n o c o t d e r a p m o c n o it e r c x e d e s a e r c n I d e g n a h c n u d e n i a m e r R E A r e h g i h h t i w s t n e it a p n i n o it c i d e r p r e h g i H a i m e c y l g u e e t u c a r e t f a y l l a i c e p s e , s t n e it a p c i r u n i m u b l a o m r o n n i n o it e r c x e d e s a e r c n I α α α α β β β
y g y p , p a i r u n i m u b l a o r c i m t u o h t i w r o h t i w s t n e it a p n e e w t e b e c n e r e f f i d o N n il u b o l g o r c i m -2 h t i w d e t a l e r r o C n o it e r c x e n e g it n a r e d r o b -h s u r b h t i w d e t a l e r r o C n o it e r c x e e s o c u l g e n i r u r o e r u s s e r p d o o l b , e s o d n il u s n i , c 1 A b H h t i w d e t a l e r r o C 2 e p y T Correlatedwithalbuminexcreiton(protein/creaitnineraito) n o it e r c x e G A N d n a n il u b o l g o r c i m -2 e s a e r c n i h t i w d e t a i c o s A s t n e it a p e v i s n e t o m r o n d n a e v i s n e t r e p y h n e e w t e b e c n e r e f f i d o N l a m r o n e r e w s l e v e l m u r e s n e h w d e s a e r c n i n o it e r c x e y r a n i r U f o e c n e s e r p e h t f o e v it c e p s e r r i n o it c i d e r p h t i w s t n e it a p n i r e h g i h n o it e r c x E a i r u n i m u b l a o r c i m m u r e S Type1 Lowerinchildrenwithtype1DM l o r t n o c c il o b a t e m n i t n e m e v o r p m i m r e t -t r o h s h t i w d e s a e r c n I c 1 A b H h t i w d e t a l e r t o N l l a f s r o H -m m a T n i e t o r p e n i r U Type1 Increasedexcreitoncomparedtocontrols,especiallyinpaitentswithnephropathy s e t e b a i d f o n o it a r u d d e s a e r c n i h t i w n o it e r c x e d e s a e r c n I s r a e y 0 1 > s e t e b a i d f o n o it a r u d d e s a e r c n i h t i w n o it e r c x e d e s a e r c n I l a m r o n f o e t i p s n i s r a e y 5 1 < s e t e b a i d f o n o it a r u d h t i w n o it e r c x e d e s a e r c e D n o it e r c x e n i m u b l a n il u b o l g o r c i m -1 Urine Type1 Higherpredicitoncomparedwithcontrols e g n a r l a m r o n e h t n i h t i w l l it s s i n o it e r c x e n i m u b l a n e h w n o it e r c x e d e s a e r c n I 2 e p y T Levelsdecreasedwithpredicitoninglycemiccontrol n o it e r c x e n i m u b l a h t i w d e t a l e r r o C n o it e r c x e f o n o it c i d e r p h t i w d e t a l e r r o C 1 n i e t o r p e n i r U Urine Type1 UP1morepredicitonoftubulardysfuncitonthan 1-microglobuiln -D -l y t e c a -N e s a d i n i m a e s o c u l g e n i r U Type1 Higherexcreitoncomparedtocontrols e g n a r l a m r o n e h t n i h t i w l l it s s i n o it e r c x e n i m u b l a n e h w n o it e r c x e r e h g i H n o it e r c x e n i m u b l a h t i w d e t a l e r r o C s e t e b a i d f o n o it a r u d h t i w d e t a l e r r o C l o r t n o c c il o b a t e m h t i w d e t a l e r r o C n o it e r c x e n i m u b l a h t i w d e t a l e r r o C n o it e r c x e n o it c i d e r p h t i w d e t a l e r r o C y h t a p o n it e r f o e c n e s e r p e h t n i s l e v e l d e s a e r c n I l i r p o t p a c h t i w t n e m t a e r t y b d e s a e r c e d s l e v e L 2 e p y T Increasedexcreitoncomparedwithcontrols a i r u n i m u b l a o r c i m t u o h t i w s t n e it a p n i n e v e n o it e r c x e d e s a e r c n I d n a s r a e y 0 1 -3 n e e w t e b d e u a e t a l p , s e t e b a i d f o r a e y d r i h t n i d e s a e r c n i y t i v it c A n o it a r u d r a e y -0 1 r e t f a d e s a e r c n i n o it c n u f l a n e r d n a l o r t n o c c i m e c y l g h t i w d e t a l e r r o C l o r t n o c c i m e c y l g h t i w d e s a e r c e d s l e v e L n o it e r c x e n o it c i d e r p h t i w d e t a l e r r o C m u r e S Type1 Increasedlevelsinpaitentswithnewlymanifestedmicroalbuminuria r e h g i h h t i w d e t a i c o s s a p u -w o l l o f r a e y -6 r e t f a y h t a p o n it e r f o e c n e s e r P e n il e s a b t a s l e v e l G A N y h t a p o n it e r t u o h t i w s t n e it a p n i y l n o p u -w o l l o f r a e y -4 r e t f a l e v e l n i e s a e r c n I d o i r e p s i h t t u o h g u o r h t c 1 A b H h t i w d e t a l e r r o C e s a t u m s i d e d i x o r e p u s d n a r o t a v it c a n e g o n i m s a l p e u s s it h t i w d e t a l e r r o C n o i s n e t r e p y h h t i w d e t a i c o s s a t o N 2 e p y T AcitvityofbothisozymesAandBcorrelatedwiththedegreeofcilnical s e g n a h c r a l u c s a v c it e b a i d d n a s m o t p m y s e s a r e t s e n il o h C Urine Type2 Increasedacitvitycomparedtocontrols n o it e r c x e G A N h t i w d e t a l e r r o C f o o it a r e n i n it a e r c / n i m u b l a e n i r u h t i w s t n e it a p n e e w t e b e c n e r e f f i d o N l o m / g m 2 > r o l o m m / g m 2 < l y m a t u l G -e s a d it p e p s n a r t e n i r U Type2 Increasedcomparedtocontrolseveninabsenceofmicroalbuminuria ) R F G ( n o it c n u f l a n e r d n a l o r t n o c c i m e c y l g h t i w d e t a l e r r o C e n i n a l A e s a d it p e p o n i m a e n i r U Type2 Increasedcomparedtocontrols s n e g it n a r a l u b u T n e g it n a r e d r o b -h s u r B e n i r U Type2 Increasedcomparedtocontrols n il u b o l g o r c i m -2 d n a B A R , n o it e r c x e n i m u b l a h t i w d e t a l e r r o C e n i m a s o t c u r f h t i w d e t a l e r r o C ; e s a d i n i m a e s o c u l g -D -l y t e c a -N = G A N ; e t a r n o it e r c x e n i r r e f s n a r t y r a n i r u = R E T U . e t a r n o it e r c x e n i m u b l a y r a n i r u = R E A U ; n e it o r p g n i d n i b -l o n it e r = P B R r e k r a M f o e p y T M D Mainfindings α α β β γ β β
countertransport activity, lipid levels, smoking history, and family history of cardiovascular disease and diabetic nephropathy.
Development of new technologies
1) Identification of genes associated with increased or decreased risk of diabetic nephropathy.
2) Measurement of substances in blood or urine, such as extracellular matrix molecules, products of glycation, or growth factors.
3) Measurement of tubular function.
4) Measurement of cellular functions (e.g., in cultured skin fibroblasts) which may be associated with the risk of diabetic nephropathy, including extracellular matrix molecules and growth factors. 5) Less invasive methods such as fine-needle
aspiration to sample renal tissue for structural or biochemical changes associated with biochemical risk.
6) Development and application of new imaging technologies (e.g., positron emission tomography and magnetic resonance imaging) as tools to detect early renal diabetic biochemical or structural changes.
These tests need initially to be validated, at least in part, by their association with important renal lesions as ascertained in renal biopsies (27). It is worth to mention here that determination of the serum creatinine level is still one of the most widely applied tools for the detection of significant renal disease. Although this test is fast, inexpensive and convenient, several associated shortcomings must be appreciated by clinicians. Patients with low muscle mass and decreased protein intake may have lower than expected levels of serum creatinine. Patients with advanced renal failure have changes in the renal tubular secretion of creatinine and production of the creatinase enzyme in the bacterial flora of the intestinal tract, which tends to alter serum creatinine. The total effect of these factors is that the serum creatinine level is misleadingly low and the creatinine clearance overestimates the true GFR in the presence of advanced renal failure (29).
SIGNIFICANCE OF
MICROALBUMINURIA AS A PREDICTOR OF MORTALITY
In the follow-up study first communicated in 1983, it was shown that microalbuminuria in type 2 DM is strongly predictive of an increased mortality risk. This observation has since been confirmed in numerous studies. Other factors could be associated with poor prognosis such as long-term hyperglycemia and high blood pressure. Microalbuminuria seems though to be the strongest overall predictor of mortality as has also been found in nondiabetic population-based studies (14).
TRANSITION FROM
NORMOALBUMINURIA TO MICROALBUMINURIA
The transition from normoalbuminuria to microalbuminuria is a very important phase in the course of type 1 DM. Longitudinal studies have now revealed that patients who develop microalbuminuria already show albumin excretion on the upper normal range before the microalbuminuric level is reached. It has also been shown that patients developing microalbuminuria have a higher level of glycated hemoglobin. Indeed, the study by Feldt-Rasmussen et al.showed that glycated hemoglobin in these patients was always above 7.5%-8% as evaluated by different measurements (30). If glycated hemoglobin is below 8%, the risk of developing microalbuminuria is very small, on the other hand, there are patients who maintain excretion rate in spite of a rather high level of glycated hemoglobin. Blood pressure, on the other hand, appears to be normal before the development of microalbuminuria but rises a few years after microalbuminuria has been clearly established. Importantly, this group of authors suggest that the early phase of microalbuminuria is more closely related to poor metabolic control rather than blood pressure elevation. However, soon after development of microalbuminuria an elevation in blood pressure, although small, is clearly established. The blood pressure rise early in microalbuminuria is not large, but the increase rate in blood pressure, e.g., per year, is probably as important as the actual pressure level (13,30).
MANAGEMENT OF
MICROALBUMINURIA IN DIABETES
Good education about the late complications of diabetes is very important. At the microalbuminuria stage, tight glycemic control and protein restriction are still effective components of treatment. In patients with microalbuminuria the treatment with ACE inhibitors is essential. Adjusting the ACE inhibitor to the maximum allowable dosage normalizes or significantly decreases microalbuminuria. A practical approach to the management of microalbuminuria in diabetic patients is shown in Fig. 1.
CONCLUSION
Microalbumiuria has emerged as a very powerful clinical predictor of overt renal and cardiovascular disease.
This was recently confirmed in the post DCCT and HOPE studies. Although the correlation to the structural changes may not be perfect, especially in type 2 patients, microalbuminuria certainly predicts
clinical proteinuria in both types of diabetes, and in type 2 also cardiovascular mortality. The power of microalbuminuria is strengthened by continuous follow-up at a diabetes clinic (10,12,20).
New technologies have failed to replace microalbuminuria or to add accurate prediction of the disease. Identification of genes has so far been clinically too unreliable and in some studies no association was found. Other substances to be measured in urine have not added any new development. It has been proposed that extracellular-matrix molecules or products of glycation could be of importance, but this still needs to be confirmed. Measurements of cellular function in skin and lymphocytes are interesting research tools, but not useful enough in clinical evaluation. Also, new imaging technologies such as positron emission tomography and magnetic resonance imaging have not been useful although new studies may be required (26,27).
It has been proposed that increasing serum prorenin precedes the onset of microalbuminuria, a highly interesting area that needs further investigations, however, the overlap between nonprogressors and progressors is too large and it is quite cumbersome to evaluate serum prorenin in a diabetic clinic.
Generally it would be useful to have alternatives to the microalbuminuria, especially in evaluation of longterm fate of patients, but so far this has not been possible.
Early treatment with ACE inhibitors and other antihypertensive agents has proven to improve prognosis. However, the observation is very interesting, also because dual blockade of the renal angiotensin system seems to be useful in clinical practice, but it should be mentioned that ACE inhibitors and other antihypertensive agents may normalize micro-albuminuria, so evaluation of the actual clinical situation should be done after discontinuation of therapy for one or two months (14,17,29).
It can thus be concluded that there are no alternatives in the present situation. Microalbuminuria quite accurately predicts renal disease, especially with careful follow-up and measurement of AER in a clinic. No BP >140/85 mm Hg still microalbuminuria yes BP <140/85 mm Hg still microalbuminuria Yes
Glycemic control adequate? Optimize glycemic control
Start antihypertensive therapy e.g., angiotensin converting
enzyme inhibitor (ACE) Check and manage risk factors
Monitor albumin/creatinine ratio (ACR) every 3-6 months Check and manage risk factors
Continue monitoring Check and manage risk factors BP ≤130/85 mm Hg
5-30% MA reduction per year
Monitor BP Repeat ACR every 3-6 months
Goals:
1. Stable or declining MA 2. Stable GFR 3. BP ≤120-130/85 mm Hg
Titrate antihypertensive therapy or
use combination therapy Check and manage risk factors
Risk factors: A. High lipid levels B. Smoking C. Alcohol excess D. Overweight
E. Family history of hypertension and renal disease F. Lack of exercise
Concomitant diseases: 1- Retinopathy 2- Cardiovascular 3- Neuropathy
Figure 1.A practical approach to the management of
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