Technical Evaluation of a New Immunoradiometric and a New Immunoluminometric Assay for Thyroglobulin

Abstract

Background: 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. Methods: 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. Results: The detection limits were 0.04 μg/L Tg for the IRMA and 0.02 μg/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 μg/L for the IRMA and at 0.06 μg/L for the ILMA. No high-dose hook effect was seen with up to 200 000 μg/L added Tg or in dilutions of 12 patient sera with Tg values of 307–38 880 μg/L. Mean recovery of 50 μg 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. Conclusions: The new assays have improved precision for Tg <1 μg/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 μg/L and may further reduce Tg antibody interference.

Incidence and implications of negative serum thyroglobulin but positive I‐131 whole‐body scans in patients with well‐differentiated thyroid cancer prepared with rhTSH or thyroid hormone withdrawal

“…The hook effect is a feature common to all immunoassays that can occur in the presence of very high antigen levels (10–10 000 times the upper limit of the assay range) resulting in the binding capacity of antibody for the solid support being exceeded resulting in a false‐negative result 16 . The hook effect may potentially affect Tg and TgAb measurements, however in the case of Tg measurements in our study, this is felt very unlikely as the postablative and diagnostic I‐131 patients would not be expected to have particularly high Tg levels in the post‐thyroidectomy/ablative I‐131 setting.…”

Thyroglobulin measurement before rhTSH‐aided 131I ablation in detecting metastases from differentiated thyroid carcinoma

“…8 Stimulated Tg values higher than 0·2 ng/ml measured 12 months after thyroid ablation were considered positive for persisting/relapsing disease. 9,10 The presence of anti-Tg antibodies (AbTg) was screened by a specific radioimmunoassay (DYNOtest® anti-TGn, BRAHMS Diagnostica GmbH) and by recovery test with a specific Tg-recovery buffer provided by the producer. Sera showing AbTg levels > 60 U/ml and/or recover < 80% or > 120% were excluded from the study.…”

Clinical relevance of highly sensitive Tg assay in monitoring patients treated for differentiated thyroid cancer

“…[6][7][8][9] Recently, several highly sensitive Tg assays have been developed, yet their real clinical benefit still remains undetermined. [10][11][12][13][14][15] The aim of the present study was to evaluate the clinical impact of Tg measured by a highly sensitive (functional sensitivity 0·1 µ g/l) automated Tg assay in a group of 160 DTC-treated patients, submitted to routine follow-up that included neck ultrasonography (US), 131 I whole body scanning, Tg measurement during L-thyroxine (L-T4) suppressive therapy, and after rh-TSH stimulation. The results were compared to those obtained with a routinely employed Tg assay, with a functional sensitivity that satisfies all the criteria required by the NACB guidelines.…”