Measuring thyroglobulin in patients with thyroglobulin autoantibodies: evaluation of the clinical impact of BRAHMS Kryptor® Tg-minirecovery test in a large series of patients with differentiated thyroid carcinoma.

Background The present study was undertaken to evaluate the clinical impact of a thyroglobulin (Tg) minirecovery test (Tg-mRec) in a large series of differentiated thyroid carcinoma (DTC) patients treated and monitored homogeneously in a tertiary referral center. Methods Included were 1120 serum samples from 798 DTC patients. Tg, Tg autoantibodies (TgAb) and Tg-mrec measurements were performed on the automated Kryptor® platform and results compared to the corresponding clinical status. Results Among included samples 228 (20%) were TgAb-positive (TgAb+) and 892 (80%) TgAb-negative (TgAb-), respectively. Tg cutoff points were settled at 0.31 μg/L and 0.15 μg/L for TgAb- and TgAb+ patients, respectively, by ROC curve analysis. The diagnostic performance of serum Tg was reduced in TgAb+ compared to TgAb- patients, however, 87% of TgAb+ patients with recurrent disease and, particularly, all patients with distant metastases were correctly detected by adopting an optimized Tg cutoff for TgAb+ patients. A disturbed recovery was found in only 1% of TgAb- patients and in these cases no clinically relevant information was added by the Tg-mRec. Among TgAb+ patients with undetectable Tg and undisturbed Tg-mRec, no one had recurrent disease. However, a falsely undetectable Tg was demonstrated in two patients with recurrent disease who next to increased TgAb also had a disturbed Tg-mRec test. Conclusions There is no additional clinical benefit from performing Tg-mRec in most patients. It can however be considered in TgAb+ patients with undetectable Tg levels as it may help differentiate between patients with true negative and false negative Tg levels in the presence of such antibodies.