The major objectives for the development of comparability ratios for National Hospital Discharge Survey (NHDS) and National Ambulatory Medical Care Survey (NAMCS) data are to provide measures of the expected discontinuity between trend data tabulated by diagnosis and coded according to the ICD-9-CM and ICDA-8. These measures, the comparability ratios, can also be used to estimate what numbers of cases would have been tabulated using one coding revision when data coded using only the other revision are available. The effects
on estimates are illustrated in figure 1. The graph on the left shows a slowly rising trend in the number of hospitalizations due to infective and parasitic diseases until 1979, when the NIZDS coding system was changed from ICDA–8 to ICD–9–CM.
After a sharp decline of about 35 percent between 1978 and 1979, the number of discharges resumed its upward trend until 1983. However, the graphs on the right in figure 1 iktrate what the trends would have been if the discharges after 1978 were con-verted to their equivalent ICDA–8 estimates by multiplying
120
110
100
90
80
70
60
50
40
30
20
10
0
(001 -1 39)
,/-”
/ /
~4.-d 0
,/-/
ICD-9-CM (001 -1 39)
I I I I I I I I I I I
1971 1973 1975 1977 1979 1981 1983 1971 1973 1975 1977 1979 1981 1983
Year Year
Figura 1. Estimated number of hospital discharges for which the first-listed or principal diagnosis was classified to chapter 1, Infectious and pereeitic diseases, ICDA-8 or ICD-9-CM
them by the comparability ratios (upper curve), or if the dis-charges from 1978 and before were converted to their ICD–9–
CM estimates by dividing them by the comparability ratios (lower curve). What appeared to have been a dramatic reduc-tion in hospitalizareduc-tions for infective and parasitic diseases is shown to be an artifact of the differences in what conditions were classitled as infective and parasitic diseases between the coding system in use in 1978 and that used for 1979 data. In fact, the adjusted data show an increase in hospitalizations be-tween 1978 and 1979, rather than a decrease. In the case of infective and parasitic diseases, the comparability ratio for NHDS (see table 1) is 1.5475. Therefore, adjusting for the changes in the coding systems produces large changes in the data. Diabetes, however, has a comparability ratio of 1.0053 for NHDS, indicating that the numbers of discharges classified to diabetes using either coding system will be equivalent.
Fi~re 2 displays trends in NHDS estimates using either ICDA–8 or ICD–9–CM for the following groups of diagnoses:
● Cancer of the uterus and other female genital organs (ICDA-8 codes 180-184, comparability ratio of 1.3580).
. Hypertensive disease (ICDA–8 codes 400–404, described as essential hypertension and hypertensive heart disease in ICD–9–CM, codes 401–402 and 404, with a comparabil-ity ratio of 0.6973).
● Chronic ischemic heart disease (ICDA–8 code 412, com-parability ratio of 1.4186).
● Acute myocardial infarction (ICDA–8 code 410, compa-rability ratio of 1.0257).
Estimates for cancer of the uterus hospitalizations are about 35 percent greater when ICDA–8 is used in comparison with esti-mates based on ICD–9–CM data. As explained earlier, this is due primarily to the classification of carcinoma-in-situ of the cervix to code 180.0 (malignant neoplasm of the cervix) ac-cording to NHDS coding instructions used with ICDA–8.
When using ICD–9–CM, these cases of carcinoma-in-situ of the cervix were assigned to codes 230–234. The trends for hypertension disease show large differences, depending on the coding revision used. In this case, there are large increases in the estimates for essential hypertension and hypertensive heart disease when coded to ICD–9–CM that are attributable to major reskucturing of the coding deffitions. In particular, what was coded to chronic ischemic heart disease in ICDA–8 (code 412) included cases with any mention of ischernic heart disease and hypertension. When these same cases are coded to ICD–
9–CM, the rules lead to selecting hypertensive heart disease (code 402). Conversely, chronic ischemic heart disease esti-mates are dramatically lower when ICD–9–CM data are used than when coded in ICDA–8, largely because of the shift of cases from 412 in ICDA–8 to 402 in ICD–9–CM. Acute myocardial infarction estimates, however, are within a few per-cent of one another when either coding revision is used.
Because coding rules, narrative descriptions used by phy-sicians, and morbidity rates change over time and may vary with the demographic characteristics of the patient population, there is a risk in applying the comparability rates developed for one data base to another. This is readily evident when compar-ing the comparability ratios for NHDS (table 1) with those for NAMCS (table 2) for the same chapters of the two revisions to the classification coding system. The entire chapter 1, Infective and parasitic diseases, for NHDS has a comparability ratio of 1.5475, while the corresponding ratio for NAMCS is 1.1780.
Thus, it is clear that comparability ratios based on hospital discharge diagnoses are not useful for estimating the effects of coding system revisions on diagnoses tabulated from physician office visits.
Trends for selected disease categories as the diagnosis reported for physician office visits tabulated in NAMCS appear in figure 3. The conditions selected for this figure are those with comparability ratios that differ substantially from unity and that represent diagnoses accounting for 5 million or more office visits to physicians (see table 2). The conditions are as follows:
● Chronic ischemic heart disease (ICDA–8 code 412, com-parability ratio of 1.3436).
. Bronchitis, unqualified (ICDA–8 code 490, comparability ratio of 1.2275).
. Asthma (ICDA–8 code 493, comparability ratio of 0.7248).
● Arthritis and rheumatism (ICDA–8 codes 7 10–7 18, com-parability ratio of 1.2479).
The visit rates for each of these disease groups shift by 22-38 percent, depending upon the coding system revision used. Al-though the trend lines for any given disease are parallel when consistently expressed in either ICDA–8 or ICD–9–CM, a failure to recognize the effects of the coding system changes in NAMCS-published data between 1978 and 1979 may lead to erroneous conclusions about morbidity trends and utilization of services in physicians’ offices for certain groups of diagnoses,
16
Cancer of uterus and other female genital organs Hypertensive disease
([CDA-8,180-184) (ICDA-8,400-404)
(ICD-9-CM,179-184, 198.6) (ICD-9-CM,401–402, 404)
50 p 50
r-% _@H-\ “e\_
c f
40 m
w 40 / ICD-9-CM
3
:. ~> 0
30 Ii
m 30 c
ICDA-8 ICDA-8
20
----H- @
c
10 ICD-9-CM v
.!?10 Q
o I I I I I I I I I I [ I o 1 I I I I I I I I I I
1971
140
130
120
110 [
1973 1975 1977 1979
Year
Chronic ischemic heart disease (ICDA-8, 412) (ICD–9-CM,412,414)
1981 1983 1971 1973 1975 1977 1979 1981 1983
Year
100
90
80
70
r
;~
1971 1973 1975 1977 1979 1981 1983
Yeer
wc al .-c
90
80
70
60
50
40
30
Acute myocardial infarction (ICDA-8,41O) (ICD-9-CM,41O)
ICDA-8
ICD-9-CM
20
10
I
o
I
I I I I I I I I 1 I I I1971 1973 1975 1977 1979 1981 1983
Year
I
I Figure2. Trends inestimetes forselected conditions using either lCDA-8- orlCD-9-CM-coded date with adjustments forcomparabili~National Hospitel Discharge Survay
1.4
1.3
1.2
: 1.1
m 2 210+-.
% (nc .a 0.9 .-c
ln .=
.~
> 0.8
0.7
0.6
/ / 0
Chronic ischemic heart disease (ICDA-8,412)
(ICD-9-CM, 412,414)
.-c UI
%.s
>
,.
\
\
‘\
\
I I I I I !
1975 1976 1977 1978 1979 1980 1981
Year
1.0
0.9
0.8
0.7
0.6
0.5
0.4
(ICDA-8, 493) (ICD-9-CM, 493)
A / \\
‘/ /
\\ ICD-9-CM
----‘\
I
0.3
o’~
t1975 1976 7977 1978 1979 1980 1981
Year
.-c
Bronchitis (ICDA-8, 490) (ICD-9-CM, 490) 1.2
1.1
1.0
[ 0.9
t
0.8
0.7
0.6
0.5
t
ICD-9-CM
0.4
0 I I I I I I
1975 1976 1977 1978 1979 1980 1981
Year
Arthritis and rheumatism (ICDA-8, 710-718) (ICD-9-CM,71O-719)
ICDA–8
---v
\ i
~/
\, ,/
v
ICD-9-CM
I I I I I
I
1975 1976 1977 1978 1979 1980 1981
Year
Figure 3. Netional estimatas of numbara of physician office visits for selected conditions using ICDA-8- or lCD-9-CM-coded data adjusted forcomparabiiity
18