Evaluation and comparison of radio-, fluorescence, and enzyme-linked immunoassays for serum thyroxine

We have compared three analytical systems for the measurement of serum thyroxine: enzyme-linked immunoassay (EIA), fluorescent immunoassay (FIA) and a radioimmunoassay (RIA). These were evaluated with respect to their precision, accuracy, analytical sensitivity and sample throughput. The RIA is more sensitive than the EIA (10 microgram/L vs. 35 Ug/L. Both systems have excellent precision (X = 86 ug/L, C.V.RIA = C.V.EIA = 4.6%). Both the EIA and RIA demonstrate good accuracy with recovery of between 97-98% of added thyroxine. The FIA has an apparent sensitivity between that of the RIA and EIA (25 microgram/L), but a precision consistently lower than the other two systems (C.V. = 7.4%, X = 86 microgram/L). Patients' results by RIA compared well with those from EIA )r = 0.91, P less than 0.05), but a poor correlation was found with FIA (r = 0.865, P greater than 0.05). Although not fully automated, the EIA performed on the Abbott ABA-100 analyzer has a sample throughput equal to the automated RIA system (Micromedic, Concept 4).