Haemagglutination system for the simultaneous detection of LPS and anti LPS antibodies of S.typhi

In view of the limitations of Widal test for the diagnosis of typhoid, haemagglutination test using sensitized sheep red blood cells was designed at our laboratory. The test gave only 1.8% positivity at 1:40 dilution among 500 normal persons. Eighteen of the 30 culture proven cases gave the HA test positive while the positivity was 62.5% among suspected cases of typhoid. Thus, the anti LPS haemagglutination test showed a sensitivity of 60% and specificity of 98.2%. The positive predictive value and negative predictive value were 66.66% and 96.7% respectively. The haemagglutination inhibition test was also developed for the detection of LPS antigen of S. typhi in the serum samples of typhoid cases and could detect 1.5625 micrograms/ mL of S. typhi LPS antigen but failed to detect LPS in the sera of bacteriologically proven cases of typhoid. However, it could detect the growth of simulated blood cultures of S. typhi within one hour of inoculation and did not give any cross reactions with other bacterial cultures. The data suggest that the haemagglutination test could be a good adjunct for Widal test and the haemagglutination inhibition test could help the early detection of S. typhi in culture.