Laboratory and diagnostic performance of a coated well immunoradiometric assay for serum thyrotrophin.

A new, coated well immunoradiometric assay (IRMA) for thyrotrophin (TSH) in serum has been evaluated with a view to its use as a first-line test of thyroid function. The Amerwell TSH IRMA is simple, rapid to perform (2.5 h) and the assay sensitivity was 0.07 mU/L with a working range (intra-assay CV less than 10%) of 0.3-100 mU/L. The mean inter-assay CV was 6.6% for TSH concentrations of 0.30-30.7 mU/L. The method compared favourably with an in-house TSH radioimmunoassay and an alternative commercial IRMA. In consecutive referrals to a thyroid clinic all patients with overt hyperthyroidism (n = 103) had undetectable TSH concentrations and in those with subclinical hyperthyroidism (n = 14). TSH was undetectable in 10 and below the reference range in four. The 95% confidence interval for 63 euthyroid serum samples was 0.36-4.3 mU/L. All hypothyroid patients (n = 20) had increased TSH concentrations. TSH concentrations in pregnancy did not differ significantly from euthyroid TSH values. From 1916 routine tests, 13 undetectable TSH values were found in which thyroid hormone levels were normal and the patients had no known thyroid disorder. The assay appears suitable as a first-line test of thyroid function, but further assessment in a routine laboratory setting is required.