Methodological approach and diagnostic usefulness of a new assay for anti-thyroid peroxidase autoantibodies

Although thyroid microsomal antibodies (anti-MAb) have been recently proven to be directly to thyroid peroxidase (TPO), current methods for MAb detection still employ unpurified microsomal fractions. The authors have therefore developped and evaluated a specific radioimmunoassay (RIA) for autoantibodies to TPO (anti-TPO Ab) based on competitive inhibition of ¹²⁵I-TPO to an anti-TPO monoclonal antibody coated on plastic microtiter wells (RIA-1) or tubes (RIA-2). Preliminary experiments showed that both assays were able to specifically detect anti-TPO Ab, while negative results were obtained with normal sera and with sera containing other organ- and nonorgan-specific autoantibodies including anti-thyroglobulin antibodies (anti-Tg Ab). No significant difference in sensitivity, specificity and reproducibility was found between RIA-1 and RIA-2, and the results obtained with the two techniques were therefore pooled together. Anti-TPO Ab were then assayed in 110 normal controls and in 441 patients with different autoimmune (AITD) or nonautoimmune (N-AITD) thyroid diseases and compared to anti-M Ab as assessed by passive hemagglutination (PH). Positive anti-TPO Ab were observed in 4/110 (3.6 p.cent) normal controls, 88/117 (80 p.cent) patients with Graves' disease, 122/123 with Hashimoto's thyroiditis or idiopathic myxedema and 21/201 (10.4 p.cent) with miscellaneous N-AITD. A highly significant positive correlation (r=0.91, p<0.0001) was found between anti-M Ab by PH and anti-TPO Ab by RIA; discrepant results were limited to sera with negative or low (1/100-1/400) anti-M Ab titers. Analysis of these discrepant data suggested higher autoimmune disease-specificity of anti-TPO Ab RIA tests when compared to anti-M Ab by PH. Moreover, absorption studies showed that interference of anti-Tg Ab was responsible for anti-M Ab positive tests in 8 selected anti-TPO Ab negative/anti-M Ab positive sera from AIDT patients. In conclusion, a new RIA for TPO Ab has been developed, employing tracer amounts of purified antigen and anti-TPO monoclonal antibody. This assay is sensitive, easy to perform and more specific than PH for anti-MAb. These characteristics should allow its diffusion in the clinical routine.