Clinical relevance of highly sensitive Tg assay in monitoring patients treated for differentiated thyroid cancer

Serum thyroglobulin (Tg) represents a highly specific biomarker for detecting residual thyroid tissue/recurrence/metastases after treatment for differentiated thyroid cancer (DTC). We evaluated the clinical impact of a highly sensitive Tg assay during routine follow-up of DTC patients. Tg values were measured by a highly sensitive Tg assay during L-T4 suppressive therapy and after recombinant human thyrotropin (rh-TSH) stimulation and were compared with those obtained by using a routinely employed Tg assay. One hundred and sixty consecutive DTC-treated patients (papillary carcinoma n = 124, follicular carcinoma n = 36) were studied. Measured variables included neck ultrasonography, I-131 whole body scanning, and Tg assayed by Immulite (Diagnostic Products Corporation, Los Angeles, CA) and by the highly sensitive Access assay (Beckman Coulter, Brea, CA). During L-T4 therapy, measurable Tg was found in only two patients (1% of total) by Immulite and in 23 patients (14% of total) by Access assay. Using the institutional cut-off of 2 mu g/l after rh-TSH, a negative response was associated with undetectable Immulite Tg during L-T4 therapy in all patients (negative predictive value, NPV, 100%) and in 137 out of 152 patients with Access assay (NPV 90%). Measurable Tg during L-T4 therapy was found in 17% of positive patients with Immulite and in 100% of patients with Access, respectively. The use of a highly sensitive Tg assay may represent a useful diagnostic tool for improving the interpretation of Tg results during monitoring of DTC-treated patients for the early detection of recurrence and for optimizing the use of the more expensive rh-TSH test.