CRITICAL-EVALUATION OF FULLY AUTOMATED ENZYME IMMUNOASSAYS FOR FREE-THYROXINE AND THYROTROPIN

The technical and diagnostic performance of fully automated immunoassa ys for free thyroxine and thyrotropin using streptavidin/biotin techno logy (Enzymun-Test(R)) were examined. The between-assay precision for free thyroxine was 10.4%, 5.4%, 2.5%, 2.3%, 1.1% and 1.8% at 3.02, 6.2 7, 17.2, 21.9, 25.6, 42.7 pmol/l; and for thyrotropin was 14.2%, 4.7%, 2.9%, 2.8%, 3.2%, 4.5% at 0.12, 0.46, 1.03, 2.05, 4.8, 12.7 mU/l. The functional detection limit of the assay was 0.09 mU/l. Results for th e free thyroxine method correlated well with the IMx (r = 0.91) and th e equilibrium dialysis (r = 0.95) assay. Results for the thyrotropin m ethod correlated well with the Tandem(R)-TSH (r = 0.99) and the IMx (r = 0.99) assays. The euthyroid reference range was 11-23 pmol/l and 0. 5-3.9 mU/l for free thyroxine and thyrotropin respectively. The free t hyroxine assay was not influenced by changes in albumin or thyroxine b inding globulin concentration but showed increases at oleic acid conce ntrations > 4 mmol/l. Spuriously elevated free thyroxine concentration were found in 4 patients, due to assay interference by antibodies in the serum. In a follow up study of 46 patients with non-thyroidal illn ess, serial measurements showed fluctuating free thyroxine and thyrotr opin concentrations with abnormal results occurring in 34%. In a hospi tal setting, a wider range of free thyroxine (10-28 pmol/l) and thyrot ropin (0.22-5.9 mU/l) concentration may be observed in patients who ar e clinically euthyroid. Abnormal thyroid function tests were however t ransient and follow up resolved most diagnostic problems.