Administrative data such as hospital discharge and health insurance claims databases have infrequently been used for health services research on dengue despite their considerable strengths which included nationally representative sample to allow generalizability, larger sample size and low cost. The main disadvantage of administrative data is uncertain validity of the ICD codes used to identify patients with dengue. This study was undertaken to address this weakness and is the first to investigate the validity of ICD-10-CM codes for dengue. Overall we demonstrated adequate diagnostic evidence to support the use of ICD-10-CM codes in identifying hospitalized patients with positive NS1 or IgM test for dengue.
The sensitivity of ICD-10-CM codes for dengue is high (94%), but its specificity is more modest at 83% (17% false positive ratio). The lower specificity could have two explanations. Firstly, despite advances in dengue diagnostic tests , the diagnosis of dengue in current practice is still clinically based rather than entirely dictated by positive NS1 or IgM lab results . This is further supported by our findings of lower specificities when patients manifested with bleeding or low platelet count, both commonly associated with dengue. In other words, patients are more likely to be clinically diagnosed of having dengue even when their NS1 or IgM tests were negative. Secondly, our records selection process also contributed to this modest specificity result. We have deliberately restricted selection of records without ICD codes for dengue to B34.9, A83, R50, A92 to A99 codes because they are most likely a priori to be misclassified as false-positive, thus lowering the specificity estimates. If we would have randomly selected all records without ICD codes for dengue as controls, we would then find nearly all to be true negative given the low prevalence (< 3%) of dengue among all hospitalized subjects, thus inflating the estimate of specificity.
Our results have several implications. For the use of administrative data in descriptive studies to estimate the incidence or prevalence of patients hospitalized for dengue illness, the high sensitivity of A90–91 codes ensures that most of the dengue patients will be identified. However, its moderate specificity (83%) will inflate the estimate of the true frequency of dengue in the population, with high number of false positives. Even moderate loss in specificity can lead to substantial over-estimation especially when the incidence or prevalence of the condition under investigation is low. Our finding that the validity of the ICD-10-CM codes has been stable over time is reassuring for studies on the secular trend in dengue incidence or prevalence.
For analytic studies, the ICD-10-CM codes can be used to sample cases of dengue and non-dengue controls to investigate outcome of subjects’ exposure to dengue. Assuming non-differential misclassification (ie, classification by ICD-10-CM codes is independent of the outcome of interest) and assuming having dengue increases risk of the outcome, the modest specificity (83%) of A90–91 codes will cause cases to have more false positive subjects among them to dilute the risk estimate of the case group, while the high sensitivity (94%) will minimize the number of false negative subjects in the control group, thus lessening its risk inflation. The end result will be a biased estimate of the relative risk towards null.
Limitations of study
As this study involved only seven hospitals, the findings cannot be generalized to all hospitals in Malaysia. In addition, NS1 and/or IgM were used as the diagnostic standards to validate the ICD-10-CM codes for dengue. In this case, both NS1 and IgM are not perfect tests, so some of the dengue cases may be missed. Nevertheless, both NS1 and IgM are the only diagnostic tests that are available in almost all hospitals in Malaysia. Lastly, as this study involved only data from 2010 and 2013, the observed trend of ICD-10-CM validity may not be entirely true for the data of other years.