Diagnostic E-codes for commonly used, narrow therapeutic index medications poorly predict adverse drug events

ObjectiveWe sought to examine the validity of specific hospital discharge codes in identifying drug toxicity precipitating hospitalization, among elderly users of high-risk medications.Study Design and SettingWe conducted a cross-sectional evaluation assessing the diagnostic test characteristics of International Classification of Diseases-9 External-Cause-of-Injury codes (E-codes) compared with a reference standard of medical record review. This study was nested within a prospective cohort of elders using warfarin, digoxin, or phenytoin as identified in the Pharmaceutical Assistance Contract for the Elderly benefit program.ResultsWe identified 4,803 subjects contributing 11,409 person-years of exposure to at least one of three drug groups. Subjects experienced 8,756 hospitalizations, of which 304 were deemed, by expert review, to be a result of an adverse event of warfarin, digoxin, or phenytoin. The sensitivity, specificity, and positive (PPVs) and negative predictive values for drug-specific E-codes were warfarin—25.5%, 98.3%, 46.6%, and 95.7%; digoxin—84.0%, 99.1%, 56.8%, and 99.8%; and phenytoin—86.7%, 98.7%, 59.1%, and 99.7%.ConclusionsE-codes for digoxin and phenytoin have a high sensitivity, but E-codes for all three medications have poor PPVs, a result that might produce misclassification in studies based solely on discharge coding. Investigators should confirm such rare events via medical record review.