Diagnostic value of indirect hemagglutination in the seroepidemiology of Shigella infections

To evaluate the usefulness of the indirect hemagglutination (HA) test in the epidemiological investigation of shigellosis, single serum specimens were tested from 50 patients with Shigella dysenteriae 1 (Shiga bacillus) infections, 103 asymptomatic contacts of these cases, 267 adult and 100 student control, and serum specimens collected during two outbreaks caused by S. sonnei and one outbreak due to S. flexneri 6. In patients with S. dysenteriae 1, 74% demonstrated titers of ≥ 1:40, with 50% showing titers of ≥ 1:60, whereas in the controls 10.4% had titers of ≥ 1:40 and only 0.3% had titers of ≥ 1:60. IHA titers in serum specimens collected from patients with S. sonnei and S. flexneri 6 were too low to be considered diagnostic for individual patients, but were useful in analysis of group results. Groups of ill individuals yielded titers significantly higher than non ill groups; however, titers from ill groups were usually less than 1:40. The IHA test for S. dysenteriae 1 antibodies serves as a valuable adjunct to the diagnosis of Shiga bacillus dysentery. In the laboratory, an IHA titer of 1:40 or 1:80 is a 'borderline positive.' Shiga bacillus dysentery is strongly indicated when IHA titers are ≥ 1:60.