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[Immunological analysis of pseudo-positive reaction in healthy adults at anti-streptolysin O measurement by latex agglutination method].

Streptolysin O (SLO) is a toxic immunogenic protein produced by Streptococcus pyogenes (S. pyogenes). The latex agglutination photometric immunoassay with latex coated by SLO (latex agglutination method) has been most widely used for determination of antibody to SLO (ASO). We measured ASO levels by latex agglutination method in serum specimens collected from 159 healthy individuals and eight patients with S. pyogenes, who had a positive S. pyogenes culture from throat swabs. A significant frequency(about 15%) had positive ASO levels (> 200 unit/ml) in healthy individuals aged < 20 (47 individuals) and 20-29 (80 individuals), respectively, however, none of 30 or over (32 individuals) were positive. The SLO specimen purchased from Sigma Chemical Co. showed at least three protein bands on SDS-PAGE; one was considered SLO protein because it had a molecular weight of 64 kD and of the remaining proteins, one had a molecular weight higher and the other had a molecular weight lower than 64 kD. Serum antibodies among the healthy adults to Sigma-SLO specimen consisted of IgG class in the majority, with little IgM class. Immunoblotting analysis revealed that serum antibodies of the patients recognized the 64-kD protein (SLO), and serum antibodies of the healthy adults recognized different proteins from SLO. It appeared that many of the healthy adults with a positive ASO level had antibodies different from ASO, resulting in pseudo-positive ASO values when ASO was measured by latex agglutination method using SLO specimen containing non-specific proteins. Improvement of commercial kits using this method is required.