High-sensitivity human thyroglobulin (hTG) immunoradiometric assay in the follow-up of patients with differentiated thyroid cancer

Circulating human thyroglobulin (hTG) measurement has a pivotal role in the management of patients affected by differentiated thyroid cancer (DTC). Generally, hTG serum concentration less than 1 ng/ml is considered a marker of complete remission after total thyroid ablation. Recently, highsensitivity immunoradiometric assays (IRMA) have been developed to detect very low hTG serum concentrations. The present study was undertaken to test a newly developed highsensitivity hTG IRMA and to evaluate its diagnostic performance and reproducibility in the followup of patients affected by DTC. We retrospectively selected 156 patients without signs of recurrence and 39 patients with DTC recurrence. Serum samples were collected during Lthyroxine (T4) suppressive therapy (ONT4) and 4 weeks after T4 withdrawal (OFFT4), and hTG was measured by a specific highsensitivity IRMA (DYNOtest Tgplus, BRAHMS Diagnostica GmbH, Berlin, Germany). Sera showing the presence of antibodies against hTG (AbhTG) or hTGrecovery less than 80% were excluded from the study. The receiver operator characteristic (ROC) curve analysis was performed to select the best cutoff levels, and diagnostic performance of the marker was evaluated. By using ONT4 cutoff level of 0.2 ng/ml and OFFT4 cutoff level of 0.5 ng/ml we obtained a sensitivity/specificity/accuracy profile of 0.92/0.98/0.97 and 0.97/0.98/0.98, respectively. We found falsenegative results in three (12%) and one (4%) out of 24 patients with cervical recurrence by using 0.2 and 0.5 ng/ml cutoff levels, respectively. However, we found falsenegative results in 13 (54%) and six (25%) patients when 1.0 ng/ml cutoff level was used. Finally, DYNOtest Tgplus showed a very satisfactory intra and interassay reproducibility in the very low hTG concentration range. Based on our data, we conclude that DYNOtest Tgplus assay is effective and accurate in evaluation of patients with DTC.