Diagnostic performance of a new highly sensitive thyroglobulin immunoassay

The determination of serum thyroglobulin (Tg) is commonly used for detecting the presence of residual thyroid tissue or cancer recurrence in patients treated for differentiated thyroid cancer (DTC). The aim of the study was to evaluate the performance characteristics of a recently introduced fully automated chemiluminescent immunoassay, based on four monoclonal antibodies and which produces results in 40 min. Analytical sensitivity (0.01 mug/l) was computed from 20 replicates of the zero calibrator and of the 'Tg-free' sample pool. Functional sensitivity (0.1 mug/l at 20 coefficients of variation percent) was determined from the imprecision profile obtained by assaying ten serum pools. The reliability of the measurements in the low concentration range (Tg<1 mu g/l) has been checked by progressive dilution with the 'Tg-free' serum of a sample pool at 5.27 mu g/l; measured values were very close to the expected values (recovery 100-133%). Cut-off at the 99th percentile in DTC stage I 'disease-free' treated patients (n=53) was 0.16 mu g/l. Tg measurement in basal conditions during L-thyroxine suppression therapy and 5 days after recombinant human TSH stimulation was performed in 22 patients with DTC. In 80% of patients with basal Tg<0.1 mug/l (12/15), Tg rernained <0.1 mu g/l after stimulation, and in all of these Tg was <1 mug/l. Our results have indicated the optimal analytical and clinical performance of this Tg immunoassay and encourage further studies on larger populations of patients with DTC.