Parallel measurements of serum antithyroglobulin (anti-Tg) antibody by competitive binding radioassay and tanned red cell (TRC) agglutination were performed in subjects with and without thyroid disorders. Radioassays were carried out using a partially purified preparation of anti-Tg antibody obtained by affinity chromatography using human thyroglobulin (Tg) coupled to Sepharose 4B. Two-thirds (67.2%) of control subjects had undetectable antibody levels (<20 U/ml) by this method and only 3.1% had concentrations of ≥60 U/ml. Abnormally elevated levels (>60 U/ml) were found in the majority of the patients with Hashimoto's thyroiditis (88.9%) or idiopathic myxedema (69.3%), in half of those with untreated (54.8%) or treated (47.1%) Graves' disease, and only in a minority of those with pituitary hypothyroidism (11.1%) or non toxic goiter (9.5%). A number of patients with toxic adenoma (37.5%) or thyroid carcinoma (27.2%) also had increased antibody concentrations by radioassay. The percentage of TRC-positive sera in thyroid carcinoma (2.3%), toxic adenoma (6.3%), and in untreated (21.6%) or treated (30.8%) Graves' disease was definitely lower than that of abnormal values observed in the radioassay. This could not merely be explained by a greater sensitivity of the latter procedure, since such a discrepancy was not observed in other groups. Direct comparison of parallel tests on a total of 786 sera revealed a highly significant correlation (r = 0.66, P < 0.001) between the two methods, but clearly elevated values by radioassay were found in several TRC-negative samples. The most important discrepancies between the two methods were found in sera of patients with metastatic differentiated thyroid carcinoma; discrepancies of intermediate magnitude were frequently found in Graves' disease, while little discrepancy was observed in patients with a toxic adenoma. Experiments performed on representative sera indicated that, unlike circulating anti-Tg antibody, the substance which caused a positive radioassay response without producing TRC agglutination was not associated with the IgG fraction and could not be removed by immunoadsorption with Tg-Sepharose 4B. Furthermore, addition of Tg to the radioassay system produced a dose-dependent inhibition of tracer binding. A significant inhibition of tracer binding was observed with as little as 4.8 ng of Tg (corresponding to a serum concentration of 48 ng/ml). Moreover, determination of serum Tg by radioimmunoassay (RIA) on 180 TRC-negative sera showed that the concentration of Tg increased progressively with the increase in the apparent anti-Tg antibody level as assessed by radioassay. Evidence has been provided that increased serum Tg levels may produce false positive results in measurements of anti- Tg antibody by competitive binding radioassay. Caution should be exercised in the interpretation of the data obtained by this method.
Interference of serum thyroglobulin in the radioassay for serum antithyroglobulin antibodies
MARIOTTI, STEFANO;
1977-01-01
Abstract
Parallel measurements of serum antithyroglobulin (anti-Tg) antibody by competitive binding radioassay and tanned red cell (TRC) agglutination were performed in subjects with and without thyroid disorders. Radioassays were carried out using a partially purified preparation of anti-Tg antibody obtained by affinity chromatography using human thyroglobulin (Tg) coupled to Sepharose 4B. Two-thirds (67.2%) of control subjects had undetectable antibody levels (<20 U/ml) by this method and only 3.1% had concentrations of ≥60 U/ml. Abnormally elevated levels (>60 U/ml) were found in the majority of the patients with Hashimoto's thyroiditis (88.9%) or idiopathic myxedema (69.3%), in half of those with untreated (54.8%) or treated (47.1%) Graves' disease, and only in a minority of those with pituitary hypothyroidism (11.1%) or non toxic goiter (9.5%). A number of patients with toxic adenoma (37.5%) or thyroid carcinoma (27.2%) also had increased antibody concentrations by radioassay. The percentage of TRC-positive sera in thyroid carcinoma (2.3%), toxic adenoma (6.3%), and in untreated (21.6%) or treated (30.8%) Graves' disease was definitely lower than that of abnormal values observed in the radioassay. This could not merely be explained by a greater sensitivity of the latter procedure, since such a discrepancy was not observed in other groups. Direct comparison of parallel tests on a total of 786 sera revealed a highly significant correlation (r = 0.66, P < 0.001) between the two methods, but clearly elevated values by radioassay were found in several TRC-negative samples. The most important discrepancies between the two methods were found in sera of patients with metastatic differentiated thyroid carcinoma; discrepancies of intermediate magnitude were frequently found in Graves' disease, while little discrepancy was observed in patients with a toxic adenoma. Experiments performed on representative sera indicated that, unlike circulating anti-Tg antibody, the substance which caused a positive radioassay response without producing TRC agglutination was not associated with the IgG fraction and could not be removed by immunoadsorption with Tg-Sepharose 4B. Furthermore, addition of Tg to the radioassay system produced a dose-dependent inhibition of tracer binding. A significant inhibition of tracer binding was observed with as little as 4.8 ng of Tg (corresponding to a serum concentration of 48 ng/ml). Moreover, determination of serum Tg by radioimmunoassay (RIA) on 180 TRC-negative sera showed that the concentration of Tg increased progressively with the increase in the apparent anti-Tg antibody level as assessed by radioassay. Evidence has been provided that increased serum Tg levels may produce false positive results in measurements of anti- Tg antibody by competitive binding radioassay. Caution should be exercised in the interpretation of the data obtained by this method.
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