Accuracy and precision of five analog radioimmunoassays for free thyroxin compared

We compared the precision of free thyroxin (FT4) measurements by kits involving analog RIA and the use of antibody-coated tubes (Becton Dickinson and Coat-A-Count), magnetic separation (Amerlex Magnetic and Corning Magic), or centrifugation of antibodies linked to solid beads (Amerlex). Results of kits with magnetic separation were the most reproducible. Amerlex, Amerlex Magnetic, and Becton Dickinson kits gave values comparing best with those obtained by direct equilibrium dialysis. Coat-A-Count and Corning Magic results differed significantly from dialysis values, both for patients' samples and kit standards. The kits had equal diagnostic efficiency in patients with suspected thyroid disease. On measurement of FT4 some patients were reclassified from "subclinical thyroid disease" to "overt disease." Most patients with triiodothyronine thyrotoxicosis had increased FT4. Several kit values were low for pregnant women and patients with nonthyroidal illness but the Amerlex and Amerlex Magnetic assays had fewer low results. The Amerlex Magnetic FT4 assay gave the best precision, agreement with the reference method, and diagnostic efficiency.