Development and Evaluation of a Time-Resolved Immunofluorimetric Assay for Thyrotropin

Summary: The assay described in this article is based on microtitre plate technology; it employs an europium label. The streptavidin-biotin system has been used and all components are commercially available. The lower detection limit of the assay is below 0.003 mU/1; the standards are made up in newborn calf serum. Correlation with a commercially available immunoluminometric assay (Berilux — Behringwerke) was excellent (r = 0.92, n = 201 data pairs, range covered 0 —10mU/l). The regression line using a double logarithmic transformation was: (log y) = 0.91 (log x) — 0.08. The assay precision at the clinical limits of decision — i. e. between hyper- and euthyroidism (0.2 mU/1) and eu- and hypothyroidism (4mU/l) — was acceptable. The median coefficient of variation was 1.93 % in the range 0.02—1 mU/1 and 2.11 % in the range 1.0 — 3.5 mU/1, with both values being determined from precision profiles using 214 and 188 data pairs respectively. Inter-assay coefficients of variation determined in over 30 consecutive assays were under 6 % in the range 1.3 — 20 mU/1. From 201 sera measured in both assays, 194 sera gave clinically identical values; 8 sera gave clinically discrepant values. The assay has a large dynamic range covering a concentration range of above 5 decades, with the count ratio between the 100 mU/1 standard and the zero standard being in excess of 4150: 1. A high dose hook effec