Validation of diagnosis codes for low birth weight and small for gestational age in the Medicaid Analytic eXtract database

Abstract The accuracy of low birth weight (LBW) and small for gestational age (SGA) in administrative health care records is crucial for perinatal studies but there are few published validity studies. Using 1999-2010 Medicaid Analytic eXtract (MAX) data linked to birth certificates (BCs), we identified mother-infant dyads (≥30 days enrollment after delivery, with valid gestational age [GA] and birth weight [BW] data). We identified LBW and SGA according to International Classification of Diseases, Ninth Revision, Clinical Modification (ICD-9-CM) codes. Infants with BW < 10% of the US reference were flagged as SGA. For LBW group diagnoses, we imputed BW using median, mean BW from BCs, and ICD code boundaries of infants in the same LBW group. We calculated the sensitivity, specificity, and positive and negative predictive values to assess performance. We identified 1 536 272 live births. All LBW groups had low Ses and high SPs and NPVs, whereas PPVs varied. Among infants with SGA diagnoses based on GA/BW from the BC, SE of the SGA codes was 13.36%, SP was 99.01%, and PPV was 67.37%. Combining imputation with LBW codes increased SE up to 22.09% (lower boundary) but decreased PPV to 41.53% (lower boundary). The ICD-9-CM codes from administrative health care records had low SE but high SP. Imputation based on GA and BW did not add much value to SGA identification.