A NEW IMMUNORADIOMETRIC ASSAY OF THYROGLOBULIN IN THE FOLLOW-UP OF DIFFERENTIATED THYROID-CANCER

A total of 277 patients suffering from differentiated thyroid carcinoma with individual follow-up periods of up to five years had been investigated. More than 1,000 sera were collected. The present paper reports on the results of the parallel serum Tg determinations by means of a recently introduced Tg-IRMA system in comparison with the previously used, well established Tg-RIA method. The intra-and the interassay variation of the IRMA were found at 37% and 46%, respectively, for 4 ng Tg/ml, and 3% and 6%, respectively, for Tg values above 40 ng/ml. Several effects that could interfere with the serum Tg determination (freezing or repeated freezing and thawing of sera, hemolysis or lipemia or dilution of sera, preanalytical use of serum separating tubes) were examined. Although statistically significant in some instances, at least, all of the above mentioned effects were without any practical relevance for the clinical routine use of the IRMA. In patients being in complete remission and on complete TSH suppressive thyroid hormone treatment but having no residual thyroid tissue (n = 70), the level of clinical significance was 1 ng Tg/ml for the IRMA system, resulting in a sensitivity of 99% and a specificity of 90%, respectively. For the RIA system, however, the level of clinical significance was found to be 10 ng Tg/ml with a sensitivity of 89% and a specificity of 95%. We could additionally define the grey-zone for the practical use of the IRMA (1-3 ng Tg/ml). The IRMA system is significantly more sensitive and detects more sera correctly positive than the Tg-RIA. Obviously, the IRMA is significantly less interfered by TgAb's than the RIA.