Using ICD-9 codes to establish prevalence of malformations in newborn infants

BACKGROUND The International Classification of Disease (ICD-9) codes are used to identify, after discharge, diagnoses from a review of the medical record and provide the basis for reimbursement. These codes have been used to establish the prevalence of malformations and to assess potential teratogens. We have analyzed the accuracy of codes 740 to 759.9 to identify newborn infants with malformations.

The diagnoses and ICD-9 codes in the medical records of 1000 consecutive live-born and stillborn infants were compared to the information provided in the medical records of those infants.

One hundred twenty-seven of the 1000 infants had ICD-9 codes between 740 and 759.9. 67 (52.8%) of the codes identified minor features, such as birth marks and minor anomalies. Twenty-three (18.1%) of the codes designated a malformation and were correct. Two types of errors were identified in another 33 infants (26%) whose codes designated a malformation: either the pediatricians' notes described a less severe finding or the fact that there was no such abnormality. In addition, four malformed infants were missed in pregnancies that were either terminated electively or stillborn, as they did not have medical records.

The ICD-9 codes 740 to 759.9 identified accurately some infants (18%) with malformations, but identified incorrectly many others. The accuracy of the coding for identifying malformations would be improved if (1) the findings of the examining pediatricians were considered; (2) normal features of prematurity, such as patent foramen ovale and patent ductus arteriosus, were not considered malformations; (3) minor physical features were not assigned ICD-9 codes within the 740 to 759.9 sequence.