Redefining functional sensitivity of thyroglobulin assay on Immulite platform: implications in thyroid cancer management.

Circulating thyroglobulin (Tg) measurement after thyrotropin stimulation is a pivotal tool in the management of patients affected by differentiated thyroid carcinoma. The Tg assay on Immulite platform has a declared functional sensitivity of 0.9 ng/mL and it is widely used in clinical practice and research on thyroid carcinoma. Recently, thyroglobulin measured during thyroxine treatment was found to be adequate for thyroid carcinoma follow-up if functional sensitivity at 0.2-0.3 ng/mL was reached by assay methods. Thus, the present study was then undertaken to evaluate the imprecision of Immulite Tg assay at very low concentrations. The detection limit was calculated on zero calibrator and Tg-free pooled sera replicates and was 0.14 and 0.16 ng/mL, respectively. Different serum pools with Tg concentrations ranging from 0.16 to 2.50 ng/mL were assayed according to the National Academy of Clinical Biochemistry guidelines to estimate functional sensitivity. By interpolating the imprecision profile with a coefficient of variation of 20%, the functional sensitivity was 0.36 ng/mL. The Immulite-Tg assay was directly compared with a high-sensitive immunoradiometric Tg assay in sera from 93 patients with thyroid carcinomas. No significant differences in sensitivity and specificity were observed by using the newly defined functional sensitivity. In conclusion, the Immulite Tg assay showed a lower functional sensitivity than expected and performed comparably to high-sensitive immunoradiometric assay in patients with thyroid carcinoma.