Accuracy of ICD-9-CM coding to identify small for gestational age newborns

Abstract

Purpose: This study aimed to evaluate the accuracy of the International Classification of Diseases, Ninth Revision, Clinical Modification (ICD-9-CM) diagnosis code for small for gestational age (SGA) recorded in administrative healthcare records using birthweight and gestational age information recorded in electronic medical records.

Methods: We used billing and medical records from women aged 13-55 years who delivered at a tertiary care center in the USA between 2004 and 2011. Information on birthweight, gestational age at birth, and ICD-9-CM code for SGA, 656.5x, was abstracted from the database. Each infant's birthweight percentile for gestational age was calculated on the basis of published US references; infants below the 10th percentile were classified as SGA. The performance characteristics of SGA ICD-9-CM diagnosis code against SGA classification based on birthweight and gestational age were calculated, for all deliveries and by strata of demographic and delivery characteristics.

Results: We identified 51 292 singleton live birth deliveries. The prevalence of SGA infants calculated from birthweight and gestational age at birth was higher (13%) than the prevalence based on ICD-9-CM code (2%). Sensitivity of the SGA ICD-9-CM code was 14.2%, specificity was 99.7%, positive predictive value was 86.8%, and negative predictive value was 88.4%. Stratification by demographic and delivery characteristics yielded similar results.

Conclusions: Identification of SGA infants using ICD-9-CM code, 656.5x, from administrative healthcare records has low sensitivity but high specificity; the accuracy did not differ across demographic and delivery characteristics. Thus, although this source of information would underestimate the prevalence of SGA, it could produce valid relative risk estimates.

Keywords: ICD-9-CM codes; gestational age; healthcare records; pharmacoepidemiology; sensitivity; small for gestational age; specificity.

Figure 1. Study source population

^a Stillbirths were identified from the following ICD-9-CM diagnosis codes: 656.4x, v27.1, v27.3, v27.4, v27.6, and v27.7 ^b All deliveries from 2006 were deleted since > 80% of deliveries from that year had no diagnosis billing codes

Figure 2. Varying thresholds for defining SGA^a for all deliveries (N = 51 292)

^a SGA based on recorded birthweight and gestational age