Four sensitive thyrotropin assays critically evaluated and compared

We compared four sensitive procedures for thyrotropin (TSH)--Corning's Magic-Lite, ElectroNucleonics' Delfia, Baxter's Stratus, Nichols' Allegro--for their ability to completely discriminate TSH concentrations in sera in euthyroidism, hyperthyroidism, and hypothyroidism. We evaluated the analytical and clinical performance of these procedures according to previously published criteria. All procedures we examined fulfilled the criterion stipulating less than 1% overlap between the assay variation at the lower normal range limit and the assay sensitivity. Both measurements were made with an interassay CV not exceeding 10% to 15%. All procedures produced results that correlated well for specimens with TSH concentrations of greater than 0.4 milli-int. unit/L, and all four procedures clearly distinguished hypothyroid from euthyroid subjects. In a hyperthyroid-euthyroid comparison, three of the procedures, the Magic-Lite, Delfia, and Allegro, differentiated the two with 97% accuracy, the Stratus procedure with only 90% accuracy. The procedures appeared to differ even more in the measurement of TSH in serum of patients undergoing suppressive treatment with thyroid hormones and in hospitalized nonthyroidally ill patients. The observed differences among procedures were thought to be related in part to a matrix serum effect, which is accentuated in samples from hospitalized patients.