Vi serology in screening of typhoid carriers: improved specificity by detection of Vi antibodies by counterimmunoelectrophoresis

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Abstract
SUMMARY: A purified soluble Vi antigen was used in counterimmunoelectrophoresis (CIE), passive haemagglutination (HA) and solid phase radio-immunoassay (SPRIA) for detection of serum Vi antibody. Serum Vi antibody was detected in 13 of 14 chronic typhoid carriers by both CIE and HA. SPRIA studies showed that Vi antibodies in sera of these carriers were mainly of the IgG class. Successful treatment with amoxycillin in one carrier, who initially showed a positive Vi test, resulted in a reversion in 6 months to seronegativity as measured by Vi CIE. However, Vi antibody was also detected in 10 sera (3%) from 329 control subjects by HA but not by CIE. When the CIE assay was applied to 1030 serum specimens obtained from hospitalized patients, it was able to detect one typhoid carrier, but one (0·1%) non-carrier also gave a positive Vi CIE test. It is thus concluded that Vi CIE is more specific and no less sensitive than Vi HA for the detection of the typhoid carrier state. The finding by SPRIA that the anti-Vi antibodies present in typhoid carriers' sera were mainly of the IgG class further justified the use of CIE as a precipitation test for their detection.