Vi-specific latex agglutination for early and rapid detection of Salmonella serotype typhi in blood cultures

Latex particles coated with rabbit antisera against Salmonella serotype typhi (S. typhi) Vi and O (STO) antigens were used in slide agglutination tests for the rapid identification of S. typhi in blood culture broths as soon as Gram-negative bacilli (GNB) were detected in them. Among 231 consecutive blood cultures showing GNB tested for Vi, and a subset of 163 tested for STO, by latex agglutination (LA), 125 and 32, respectively, were positive. The GNB in 127 blood cultures were confirmed by conventional methods as S. typhi, 125 (98.4%) of which had been identified by the Vi LA test. In the subset of 163, 81 grew S. typhi, of which only 32 (39.5%) had been identified by the STO LA tests. Thus, the sensitivity of the Vi and STO LA tests was 98.4% and 39.5%, respectively, whereas the specificity was 100% for both tests. Of the S. typhi isolates, 38 (30.4%) were detected by the Vi LA test on day 2 and 73 (58.4%) on day 3, day 1 being the date of inoculation of the blood culture broths. Thus, the Vi LA test is suitable for the early and rapid confirmation of S. typhi in blood culture.