Abstract
Thyroglobulin (Tg) is a proven tumor marker in the follow-up and post-operative management of patients with differentiated thyroid cancer (DTC). All assays for serum thyroglobulin (s-Tg) are based on immunoassays, however, the assay technique has a bearing on the variations seen in the estimations. We studied this using four in-house developed radioimmunoassays (RIA) and immunoradiometric assays (IRMA). Limit of detection, working range, recovery, dilution test, precision profiles and method comparison were evaluated. All four methods were used for the estimation of s-Tg in DTC patients and also compared for their performance using commercially available Tg IRMA kits from DiaSorin and Izotop. The s-Tg values measured by six different immunoassays showed very significant inter-method correlation (0.84-0.99, p < 0.001). However, among the in-house developed assays; the coated tube IRMA showed a better sensitivity and precision at the lower concentration range and hence, is preferable for the routine measurement of s-Tg in patients negative for Tg autoantibodies (TgAb). Although the second generation IRMAs offer practical benefits of having higher sensitivity, shorter turn-around time and convenience of automation, they, unfortunately, also have higher tendency for interference from both TgAb and heterophilic antibodies, if present in the sample. On the contrary, RIA is less prone to such interference and, hence, can be used in patients with TgAb. In order to effectively use this test, it is important that nuclear medicine physicians and endocrinologists understand these intrinsic technical limitations encountered during s-Tg measurement.