Technical evaluation of a new immunoradiometric and a new immunoluminometric assay for thyroglobulin

After removal of differentiated thyroid carcinoma (DTC), serum thyroglobulin (Tg) can indicate persistent or recurrent disease. We describe two novel two-step assays designed to measure low Tg concentrations.

We evaluated prototypes of the new IRMA, DYNOtest Tg-pluS, and the new immunoluminometric assay (ILMA), LUMItest) Tg-pluS. In the first step, a high-salt incubation buffer leads to dissociation of Tg-Tg antibody complexes in serum and is intended to reduce nonspecific interference and interference of potential Tg autoantibodies in the system. We studied recovery of human Tg (from thyroid glands) added to horse serum. We also studied 58 patients with DTC in whom Tg values under thyroid-stimulating hormone (TSH) suppression and TSH stimulation (without thyroxine) were available.

The detection limits were 0.04 microg/L Tg for the IRMA and 0.02 microg/L for the ILMA. Intraassay imprecision (CV) was <10% over the range of the calibration curve in both assays. The day-to-day CV was <20% at 0.2 microg/L for the IRMA and at 0.06 microg/L for the ILMA. No high-dose hook effect was seen with up to 200 000 microg/L added Tg or in dilutions of 12 patient sera with Tg values of 307-38 880 microg/L. Mean recovery of 50 microg Tg/L was 96% in those patients. Among 77 samples with Tg antibody values of 65.2-8150 kilounits/L, recovery by the IRMA was disturbed in 7 cases (9%) and by the ILMA in 9 cases (12%). Tg increased as measured in both assays in 50 of 58 patients after thyroxine withdrawal.

The new assays have improved precision for Tg <1 microg/L, and even low measured Tg concentrations respond physiologically to thyroxine withdrawal. The assays are free of a high-dose hook effect up to a Tg concentration of at least 38 000 microg/L and may further reduce Tg antibody interference.