ABSTRACT
BACKGROUND: Measuring thyrotropin (TSH) eluted from a dried blood spot (DBS) is used to screen an estimated 30 million newborns annually for congenital hypothyroidism (CH). Newborn thyroid screening has eliminated cretinism from the industrialized world and decreased the adverse effects of unrecognized CH on neurocognitive development. Hematocrit, a pre-analytic variable that affects the measurement of TSH from a DBS, contributes to the imprecision of DBS TSH measurement and could account for false-negative and false-positive DBS newborn screening test results. To assess whether variations in hematocrit found in newborns have a clinical effect in DBS-based newborn thyroid screening, the effects of hematocrit variability on the measurement of DBS TSH were studied. METHODS: U.S. Centers for Disease Control and Prevention procedures for manufacturing DBS performance testing standards were used to generate DBSs from blood samples, with hematocrits of 35%, 40%, 45%, 50%, 55%, 60%, and 65% and serum TSH concentrations of 6.3 ± 0.4 and 26.6 ± 8.0 mIU/L. TSH was measured in the eluates of four replicate DBS 3 mm punches at each hematocrit using the Thailand Ministry of Public Health Newborn Screening Operation Center enzyme-linked immunosorbent assay. Data were analyzed using a linear mixed-effects model. RESULTS: Based on the mixed-effects model, hematocrit significantly affected DBS TSH measurement (p < 0.001). A 1% increase in hematocrit resulted in a 0.06 mIU/L decrease in eluate TSH when TSH was 6.3 + 0.4 mIU/L, and a 0.21 mIU/L decrease in eluate TSH when TSH was 26.6 + 8.0 mIU/L. CONCLUSIONS: DBS TSH is significantly affected by the blood sample hematocrit. The pre-analytic variability due to hematocrit is independent of TSH assay sensitivity, specificity, precision, repeatability, and reference intervals. The effect of hematocrit on DBS TSH measurement is clinically relevant, could account for geographic and ethnic variation in the incidence of CH, and may result in both false-positive and false-negative CH screening results. Individual newborn and population-specific hematocrit correction factors may improve the precision of DBS TSH measurement.
