Rapid One-Step Modified Technique in the Evaluation of Thyroglobulin by the Immunoradiometric Assay (IRMA) for Monitoring Differentiated Thyroid Carcinoma Patients

Objective: The aim of this study was to develop a modified one-step technique for measuring serum thyroglobulin (Tg), which is currently used as a tumor marker for differentiated thyroid carcinoma (DTC) by using the immunoradiometric assay (IRMA) of the THYROGLOBULINE IRMA kit as compared with the standard method. Methods: The surplus serum specimens from 111 DTC patients who had been treated and followed up at the Division of Nuclear Medicine, Department of Radiology and 105 healthy donors at the Department of Transfusion Medicine, Faculty of Medicine, Siriraj Hospital were included. The technique for serum Tg measurement was optimized from the two-step IRMA standard kits to the one-step modified method. Results: The one-step modified technique results in a decrease of turnaround time (TAT) from two days of both one-step and two-step gold standard kits to only two hours. The optimal volume of TgAb- 125 I was found to be the same that of the kit’s technique. The optimal time for incubation was decreased from overnight at 17-18°C in the standard method to 90 minutes in a shaking water bath at 37°C. The analytical and functional sensitivities of the modified technique were 0.28 and 1.0 ng/ml, respectively. The coefficients of variation (%CV) of both intra-assay and inter-assay precision were 0.87-4.80% and 2.87-9.75%, respectively. The accuracies of the recovery test and dilution test were 92.74-101.82% and 101.27-109.16%, respectively. No cross-reaction presented between the anti-thyroglobulin antibodies and thyroid analogue compounds. The assays working range of Tg concentration under 10% CV of precision profile was 1.59-500 ng/ml. The correlation coefficient (r) between the results of the modified technique and the two-step standard method were excellent: r = 0.99; y = 1.349 x -2.421; P <0.001; n = 111. The reference range of Tg concentration among the euthyroid healthy subjects was 0.37-20.32 ng/ml  (5 th -95 th percentile). Conclusion: The modified technique is simple, rapid, sensitive and reliable for monitoring the serum Tg level in DTC patients not only under stimulation but also during suppression therapy.