Evaluating the impact of expanding the number of diagnosis codes reported in inpatient discharge databases on the counts and rates of birth defects.

Objective: Public health surveillance programs worldwide implement a variety of case-finding strategies, and many rely at least in part on International Classification of Diseases (ICD)-based diagnostic codes in administrative and clinical databases. Over time, state- and national-level hospital discharge databases have been expanding the number of reported diagnosis code fields. This study aimed to evaluate the impact of these expansions on frequencies and rates of major birth defects, and the classification of birth defects as isolated vs multiple. Methods: We used state-level 2006-2013 Florida Birth Defects Registry data and 2009-2012 data from a nationally representative database (Kids' Inpatient Database). We generated data under different scenarios by varying the number of diagnosis code fields available, and comparing counts and rates of major birth defects generated under each scenario. Results: The expansion from 10 to 31 diagnosis code fields improved ascertainment by preventing the loss of 1 in every 40 birth defect cases with defect-related diagnoses appearing only in code positions 11 to 31. Although there was variation by birth defect, the largest impact of the expansion tended to occur for less severe birth defects diagnosed in sicker infants. When restricting to fewer codes, not only were fewer cases diagnosed, but more were classified as being isolated due to the inability to capture co-occurring defects. Conclusion: Our findings encourage additional research for other health outcomes in patients of all ages. Other disease registries rely at least in part on diagnostic codes documented by healthcare providers in their case-finding activities, irrespective of ascertainment protocols, making routine investigation of these databases essential.