Chapter 7 Decision analysis for screening tests for GDM: Case identification . 151
7.3 Decision making in screening tests for GDM
Economic evaluations have been applied in different areas of medical and public health programmes, such as primary prevention (risk factor reduction), secondary prevention, screening and diagnostic test programmes. In diagnostic procedures, clinical decision analysis requires a model to link the diagnostic accuracy data and the long-term effectiveness of available treatment. The expected value of diagnostic tests depends
strongly on prevalence of disease, risk association with the test, diagnostic accuracy, and the relationship between the net benefit of treating truly diseased patients and the net harm of unnecessarily treating non-diseased patients. In an economic analysis, savings from advanced disease prevention must be included in the model along with the diagnostic tests and treatment costs. Specifically, the challenge in diagnostic studies is the definition of the optimal cut-off point, which differentiates between positive and negative test results. There is no study, however, on the economic evaluation of screening and diagnostic tests for GDM in Scotland. This study populated a probabilistic model of decision analysis in order to estimate the incremental cost-effectiveness ratios (ICER) of four alternative strategies versus the strategy “no screening”.
7.3.1 Perspective
In economic modelling it is important to decide at the outset what the end use of the model will be. NICE also recommends that all economic evaluations should be based on UK population preference and this was adopted within this chapter (NICE, 2008c). This study preformed a decision analysis from a health care provider’s perspective to inform policy makers on this important issue by establishing the optimal cost-effective screening and diagnostic tests of GDM. The cost of implementing each screening test and related treatment is derived from the study perspective which includes hospital costs and professional fees for the initial screening tests for risk factors. Health sector perspectives concern the health-related cost and impact on the government and on the private sector. Moreover, the next component that is an important perspective is valuation. An economic evaluation of GDM defines the currency reference that will be used to present the resource expenditure associated with a given cost in “pound sterling”.
7.3.2 Interventions and comparators
The model was developed to compare the expected costs and health outcomes of five possible screening strategies of GDM in all pregnant women without a prior diagnosis of diabetes in Scotland, during 24-28 weeks. This study described GDM as carbohydrate intolerance of varying degrees of severity, with onset or first recognition during pregnancy.
This economic evaluation of screening tests for GDM selected screening strategies based on the screening strategy recommendations in the UK list in Table 4.8, and a combination of the test results from section 3.4. Four screening guidelines were selected included SIGN 2001, NICE 2008, Consensus 2010 and SIGN 2010 for the
reasons outlined below and based on an experts suggestion (Robert Lindsay). Table 7.1 shows a summary of the types and methods of screening tests which were used in each strategy. The two guidelines proposed in SIGN 2001 and SIGN 2010 were selected because they were proposed by The Scottish Intercollegiate Guidelines Network (SIGN, 2001) (SIGN, 2010). Although, those two guidelines were published by the same organisation, they recommend different screening test methods and procedures.
Although SIGN 2010 is the most recent up-date to this report, it remains unclear as to which of the guidelines is the most effective, as there is no cost-effectiveness study in either the SIGN 2001 or the SIGN 2010. The inclusion of both guidelines in this study therefore allows for a comparison of their relative cost effectiveness. Both of these guidelines have also been compared to the other guidelines referred to in this study.
The third screening test used was the NICE 2008. NICE 2008 published Diabetes in Pregnancy Clinical Guideline 63 which is used throughout England & Wales (NICE, 2008d). NICE 2008 was selected because it is used all over England & Wales and has similar population characteristics to Scotland (Office for National Statistics, 2012) (The Scottish Government, 2011). Lastly, the fourth method selected was the New Consensus Criteria for GDM (Moses, 2010), and was chosen for this study as it is the result of an international consensus, based on the HAPO study as outlined in Diabetes Care (Metzger et al., 2010a). According to the combination of test approaches mentioned in section 3.4, all guidelines that recommend screening tests for GDM employ a negative dominant strategy (NDS).
Moreover, the four screening tests in this model (other than ‘no screening’) include either a one step approach where a diagnostic test is performed without prior screening or a two step approach where a diagnostic test is performed to confirm a positive screening test. In addition, the screening tests use either universal or selective screening procedures. For example, both the NICE 2008 and SIGN 2010 guidelines recommend selective screening using exactly the same risk factors for GDM screening, but different test methods. Universal screening is the screening of all pregnant women during 24 -28 weeks with various tests depending on the strategies recommended in SIGN 2001 and the 2010 Consensus Criteria. Additionally, the various screening test procedures and test methods used in the four different strategies can be matched to the various combinations of test results discussed in section 3.4. Discounting was not applied, as the time horizon was shorter than a year.
Table 7.1 Screening and diagnostic strategies used in cost effectiveness analysis
Strategies S1:SIGN 2001 S2: NICE 2008 S3:2010 consensus S4: SIGN 2010 Reference (SIGN, 2001) (NICE, 2008d) (Moses, 2010) (SIGN, 2010) Screening type Universal screening Selective screening Universal screening Selective screening
2 step approach 2 step approach 1 step approach 2 step approach Screening methods. Screening test strategy is denoted by “S”.
S1: SIGN 2001, the screening of all pregnant women with random plasma glucose. The threshold for positive screening is a random glucose value greater than or equal to 5.5mmol/l. Patients with positive random glucose subsequently underwent a fasting plasma glucose test and/or 2 hour 75g OGTT, which was used as the actual diagnosis test for gestational diabetes mellitus. The positive threshold was greater than or equal to 5.5mmol/l or 75g OGTT 2-hour greater than or equal to 9.0mmol/l respectively. This test is recommended by SIGN 2001.
S2. NICE 2008, selected pregnant women with one or more specific high risk factors underwent the fasting plasma glucose test, as shown in
Table 7.2
. A fasting plasma glucose test value greater than or equal to 5.5mmol/l meets the threshold for the diagnosis of gestational diabetes. The patient needs a further diagnosis test. The positive screening test must be confirmed by fasting glucose and 2 hour 75g OGTT for diagnostic test. The threshold for a positive diagnosis is a fasting glucose value greaterthan or equal to 7mmol/l and/or a 2 hour 75g OGTT value greater than or equal to 7.8 mmol/l. This test is recommended by NICE 2008.
S3. 2010 Consensus, all pregnant women underwent fasting plasma glucose or 1 and 2h 75g OGTT screening tests. Gestational diabetes mellitus was diagnosed if the fasting plasma glucose value was greater than or equal to 5.1mmol/l and/or 1 hour greater than or equal to 10mmol/l, 2 hour greater than or equal to 8.5mmol/l respectively. Both screening tests and diagnosis tests used the same threshold. These criteria were proposed by the 2010 consensus.
S4. SIGN 2010, all pregnant women with one or more high risk factors were requested to undertake fasting plasma glucose and 1 and 2h 75g OGTT screening tests, as shown in table 3. The threshold for positive diagnosis is fasting plasma glucose greater than or equal to 5.1 mmol/l and/or 1-hour greater than or equal to 10mmol/l, 2-hour greater than or equal to 8.5mmol/l respectively. This strategy was conducted by SIGN 2010.
S5. No screening test, All pregnant women “do nothing” or in other words no pregnant women receive screening or diagnostic tests for GDM, as a comparator intervention. The conditions of “do nothing” represent the standard of care.
Universal screening is the screening of all pregnant women during 24 – 28 weeks with various tests depending on the strategy (S1: SIGN 2001 and S3: 2010 consensus). On the other hand, selective screening is performed during the first interview or visit to a clinic by screening all pregnant women for risk factors. Women who have positive risk factors undergo further screening tests depending on the strategy (S2: NICE 2008 and S4: SIGN 2010). It is assumed that all pregnant women that perform risk factor screening at first visit and undergo screening tests are either positive or negative for risk factors, at 24-28 weeks. The various risk factors are introduced in each of the guidelines, as mentioned in section 4.6.2.1. The most common risk factors referred to in this model are summarised and listed in Table 7.2.
Table 7.2 Clinical risk factors for gestational diabetes mellitus Clinical Risk Factors for Gestational diabetes mellitus
• Body mass index (BMI) above 30 kg/m2
• Previous macrosomia baby weighing 4.5 kg or above
• Age over 30 years
• Previous gestational diabetes
• Family history of diabetes (first-degree relative with diabetes)
• Family origin with a high prevalence of diabetes:
– South Asian (specifically women whose country of origin is India, Pakistan or Bangladesh)
– Middle Eastern (specifically women whose country or family origin is Saudi Arabia, United Arab Emirates, Iraq, Jordan, Syria, Oman, Qatar, Kuwait, Lebanon or Egypt).
– Other, South-east Asian, Aborigine, Hispanic, African, Black Caribbean, Pacific Islander and Indigenous Australian ancestry