OBJECTIVES: Infant date of birth (DOB) is an important variable when measuring neonatal outcomes, including birth defects, vaccination, and hospitalization. Exact DOB is often unavailable in deidentified databases; DOB proxies can estimate DOB in infants when exact DOB is not available. Though discrepancies between exact DOB and estimated DOB may be small, even small differences can result in misclassification of infant outcomes, particularly in the neonatal period (four weeks following DOB). This study’s objective was to compare exact DOB to estimated DOB using first claim date and enrollment start date.

METHODS: Infants with a DOB between 1/1/2020 and 12/31/2022 were identified in the MarketScan Commercial Database. All infants were required to have health plan enrollment during the calendar year of birth. Two DOB proxies were assigned using 1) date of infant’s first medical or pharmacy claim and 2) health plan enrollment start date. Each proxy was compared to the exact DOB. Days between exact DOB and each proxy were calculated overall and separately for proxies before and after the exact DOB.

RESULTS: 605,742 infants with enrollment during their birth year were included in the study. Most could be assigned a DOB proxy using claim date (95.4%) or enrollment date (96.7%). Almost 30% (28.5%) were assigned an incorrect DOB using claim date; nearly all (97.3%) were assigned an incorrect DOB using enrollment date. For infants with incorrectly assigned DOB, the mean days between the exact DOB and proxy was 48.4 (SD: 73.1) using claim date and 26.8 (SD: 43.6) using enrollment date.

CONCLUSIONS: DOB proxies often incorrectly estimate date of birth in infants. Though claim date proxies performed better than enrollment date proxies, these also failed to correctly assign DOB in many infants. For many infants, DOB proxies also incorrectly identified the neonatal period by 26-48 days on average.