OBJECTIVE: To compare the incidence of low free T4 values reported by a direct equilibrium dialysis method to their incidence reported by 2 non-dialysis methods. STUDY DESIGN: Ninety-five infants, < or = 33 weeks gestational age at birth, admitted to Loma Linda University Children's Hospital before day 3 of life were studied. Infants were grouped by gestational age ranges: < or = 27, 28-30, and 31-33 weeks. Free T4 determinations were measured at 3, 7, and 14 days of life with 3 different free T4 methods. Gestational age-specific newborn reference ranges were available for the direct equilibrium dialysis method only. The only reference ranges available for the non-dialysis free T4 methods were not gestational age specific. Using available reference ranges we classified free T4 values as either low or not low. The incidence of low free T4 values was compared at 3, 7, and 14 days of life. RESULTS: Low direct equilibrium dialysis free T4 values were substantially less frequent than non-dialysis free T4 values. CONCLUSION: Substantial free T4 inconsistencies occur between dialysis and non-dialysis free T4 methods in preterm infants. It is unclear how much of this inconsistency is method dependent and how much is reference range dependent.
OBJECTIVE: To compare the incidence of low free T4 values reported by a direct equilibrium dialysis method to their incidence reported by 2 non-dialysis methods. STUDY DESIGN: Ninety-five infants, < or = 33 weeks gestational age at birth, admitted to Loma Linda University Children's Hospital before day 3 of life were studied. Infants were grouped by gestational age ranges: < or = 27, 28-30, and 31-33 weeks. Free T4 determinations were measured at 3, 7, and 14 days of life with 3 different free T4 methods. Gestational age-specific newborn reference ranges were available for the direct equilibrium dialysis method only. The only reference ranges available for the non-dialysis free T4 methods were not gestational age specific. Using available reference ranges we classified free T4 values as either low or not low. The incidence of low free T4 values was compared at 3, 7, and 14 days of life. RESULTS: Low direct equilibrium dialysis free T4 values were substantially less frequent than non-dialysis free T4 values. CONCLUSION: Substantial free T4 inconsistencies occur between dialysis and non-dialysis free T4 methods in preterm infants. It is unclear how much of this inconsistency is method dependent and how much is reference range dependent.