PASSIVE HEMAGGLUTINATION (TRC) INHIBITOR IN THYROTOXIC SERUM

The tanned red cell haemagglutination test system is widely used in the screening of thyroid and other auto‐immune diseases. We report a phenomenon which casts some doubt on the reliability of the technique. Anti‐thyroglobulin antibody (anti‐Tg) was measured by passive haemagglutination (TRC) and a double‐antibody precipitation technique (DAT) in serum samples taken serially from thirty‐three patients undergoing an 18‐month course of treatment with carbimazole and T3 for thyrotoxicosis. Anti‐Tg was undetectable by both techniques throughout treatment in 21% of the patients, and there was good concordance between results of TRC and DAT in most of the remainder. However, in four (29%) of the 14 Tg antibody‐positive patients, the TRC titre intermittently and abruptly fell to undetectable levels for long periods while the DAT remained unchanged. The hypothesis that a haemagglutination inhibitor was the cause of this phenomenon was tested by mixing increasing dilutions of a TRC‐negative/DAT‐positive serum with similar dilutions of a TRC‐positive/DAT‐positive serum removed from the same patient at another time, and also with dilutions of a known strongly positive TRC control serum. In both cases, the undiluted TRC‐negative/DAT‐positive serum completely inhibited agglutination by the TRC‐positive serum at all dilutions. The inhibition was immunologically non‐specific since the samples which inhibited the anti‐Tg system also inhibited agglutination in the anti‐thyroid microsomal antibody and anti‐DNA systems. It is suggested that some serum specimens from thyrotoxic patients inhibit the second‐order reaction of haemagglutination, but not the first‐order reaction of antigen binding. The practical significance is the demonstration of false negative haemagglutination results in patients known to have high levels of Tg antibody. Copyright © 1979, Wiley Blackwell. All rights reserved