The effects of common matrices for assay standards on performance of 'ultra sensitive' immunometric assays for TSH. Report of a joint WHO/IFCC collaborative study

Critical evaluation of fully automated enzyme immunoassays for free thyroxine and thyrotropin

The technical and diagnostic performance of fully automated immunoassays for free thyroxine and thyrotropin using streptavidin/biotin technology (Enzymun-Test) 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, 6.27, 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 the free thyroxine method correlated well with the IMx (r = 0.91) and the equilibrium dialysis (r = 0.95) assay. Results for the thyrotropin method correlated well with the Tandem-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 thyroxine assay was not influenced by changes in albumin or thyroxine binding globulin concentration but showed increases at oleic acid concentrations > 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 illness, serial measurements showed fluctuating free thyroxine and thyrotropin concentrations with abnormal results occurring in 34%. In a hospital setting, a wider range of free thyroxine (10-28 pmol/l) and thyrotropin (0.22-5.9 mU/l) concentration may be observed in patients who are clinically euthyroid. Abnormal thyroid function tests were however transient and follow up resolved most diagnostic problems.