Comparison of the analytical and clinical performances of three thyrotropin immunoassay kits

We first compared the analytical performances in terms of precision of two different generation thyrotropin (TSH) assays: Amerlite TSH-60 Assay (K2-TSH) versus Berilux hTSH (B3-TSH) and Kodak Amerlite TSH-30 Ultrasensitive Assay (K3-TSH). Then, we compared the clinical performances in 69 thyrotropin-suppressed patients with thyroid cancer and in 17 other patients referred for newly diagnosed hyperthyroidism. All the patients were given 200 micrograms of Protirelin IV for TRH testing. At the analytical level, the functional detection limit (FDL) was 0.006, 0.017, and 0.04 mU/l for B3-, K3-, and K2-TSH, respectively. At the clinical level of the 17 hyperthyroid patients, 52.9% displayed a positive TRH test with B3, 5.9% with K3, and 11.8% with K2; besides, 29.4% had a basal TSH detectable value with B3, 5.9% with K3, and 11.8% with K2. Among the patients receiving suppressive therapy: 1) 95.6%, 49.3%, and 50.7% showed a detectable TSH response to TRH, with B3-TSH, K2-TSH, and K3-TSH, respectively, and 2) only 5.8% had undetectable basal TSH values with Berilux hTSH, versus 84% with K2 and 89.8% with K3. Considering our findings, we first conclude that third generation TSH assays (having a functional sensitivity limit between 0.01 and 0.02 mU/l) can be useful for monitoring patients on thyroxine suppressive therapy, so as to distinguish partial from more complete thyrotropin suppression. Secondly, even though K3 has a FDL consistent with a third generation TSH assay, it appears less clinically sensitive than B3. Yet, no current assay can thoroughly ascertain a state of overtreatment. Finally, it is important to routinely determine the FDL, which can vary from one kit to the other, within one generation of TSH assays. Indeed, B3 with a FDL at 0.006 mU/l is more useful for monitoring LT4-suppressed patients for thyroid cancer than K3 whose FDL is 0.017 mU/l.