Relationship of Octanoylcarnitine Concentrations to Age at Sampling in Unaffected Newborns Screened for Medium-Chain Acyl-CoA Dehydrogenase Deficiency

Background: Although octanoylcarnitine (C8) concentrations measured from newborn screening dried blood spots are used to identify those at high risk of medium-chain acyl-CoA dehydrogenase deficiency (MCADD), age-related reference values are currently not available for unaffected newborn populations. Because age at sampling may vary within and between screening programs, variations in C8 concentrations by age may affect screening program performance. We determined whether C8 concentrations vary by age at sampling, sex, birth weight, or gestational age in unaffected newborns. Methods: We analyzed C8 concentrations from 227 098 unaffected newborns, including 179 729 from 6 English laboratories participating in a multicenter study and 47 369 from the single laboratory serving the New South Wales (NSW) Newborn Screening Program in Australia. In England, the majority of samples were collected at age 5–8 days and analyzed underivatized by use of tandem mass spectrometry (MS/MS); in NSW, samples were obtained at a median age of 3 days and analyzed derivatized by MS/MS. Information on infants’ sex, birth weight, gestation, hospitalization, and transfusion status was recorded at time of sampling. Results: C8 concentrations did not vary significantly by age at sampling, sex, birth weight, or gestational age and remained relatively constant during the first 2 weeks of life in unaffected babies being screened for MCADD. Conclusions: Newborn MCADD screening programs using this biomarker for screening samples collected after the first day and during the first 14 days of life do not need to adjust cutoff values to account for postnatal age, prematurity, or size at birth.