Validation of ICD‐9‐CM codes for major genitourinary birth defects in Military Health System administrative data, 2006–2014

Background The Department of Defense Birth and Infant Health Research program is dedicated to birth defects research and surveillance among military families. Here, we assess and refine the validity of International Classification of Diseases, Ninth Revision, Clinical Modification (ICD‐9‐CM) codes for selected genitourinary birth defects in the Military Health System (MHS). We additionally outline methods for the calculation of positive predictive value (PPV) and negative predictive value (NPV), sensitivity, and specificity using a stratified sampling design. Methods Among military infants born from 2006 through 2014, a random sample of ICD‐9‐CM screen‐positive cases (for six genitourinary birth defects) and screen‐negative cases were selected for chart review. PPV, NPV, sensitivity, and specificity were calculated for individual defects and any included defect (i.e., overall); measures were weighted by the inverse probability of being sampled. Results Of 461,557 infants, 686 were sampled for chart review. Bladder exstrophy was accurately reported (PPV: >90%), while the accuracy of renal dysplasia, renal agenesis/hypoplasia, and hypospadias was moderate (PPVs: 66%–68%) and congenital hydronephrosis was low (PPV: 20%). Specificity and NPVs always exceeded 98%. The overall PPV was 50%; however, excluding congenital hydronephrosis screen‐positive cases and requiring at least two inpatient or outpatient diagnostic codes resulted in a PPV of 85%. Conclusions The validity of major genitourinary birth defect codes varied in MHS administrative data. The accuracy of an overall defect measure improved by omitting congenital hydronephrosis and requiring at least two diagnostic codes. Although PPV is generally useful for research, additional calculation of NPV, sensitivity, and specificity better informs the identification of appropriate selection criteria across surveillance and research settings.