DIAGNOSIS OF ENTERIC FEVER BY INHIBITION ASSAY USING PEROXIDASE‐LABELLED MONOCLONAL ANTIBODY AND SALMONELLA TYPHI LIPOPOLYSACCHARIDE

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Abstract
Summary: An enzyme‐linked immunoassay was developed for diagnosing enteric fever. The test measured the inhibition of binding between a labelled, monoclonal IgM antibody and the insolubilized antigen, Salmonella typhi lipopolysaccharide (LPS). Good discrimination was seen between 32 proven typhoid cases (88·0 ± 4·4% inhibition) and non‐typhoid cases. The latter consisted of 27 febrile patients bacteriologically and serologically (Widal test) found to be negative for typhoid (26·3 ± 10·8% inhibition), 46 patients screened for syphilis (VDRL test) but found negative (31·2 ± 13·3% inhibition), and 27 healthy blood donors (44·6 ± 13·9% inhibition). The test also efficiently delected all 5 known typhoid carriers (90·6 ± 3·4% inhibition). The antibody binds antigen 9 in the LPS; however, this reaction was inhibited by antibodies directed against both this antigen and an adjacent antigen, 12. Anti‐12 antibodies presumably inhibit by steric hindrance and their importance in the test is discussed. Thus, the assay potentially detects (only) those systemic infections caused by salmonellae that possess antigen 9 or 12 (viz. S. typhi and S. paratyphi A).