Effect of duration of diarrhoea and predictive values of stool leucocytes and red blood cells in the isolation of different serogroups or serotypes of Shigella.

Diarrhoeal stools from 11,358 patients were microscopically examined for the presence of white and red blood cells (WBC and RBC) and cultured for enteric pathogens. Of the 3895 patients who had Shigella as sole pathogens, 2681 (72.3%) showed the presence of both WBC and RBC and the remainder had only WBC (P less than 0.001). The presence of both WBC and RBC was as good a predictor of shigellosis as the presence of greater than 25 WBC per high power field (hpf) of the microscope with or without RBC. However, the best predictor of shigellosis (positive predictive value 85%, negative predictive value 83%) was the presence of greater than 25 WBC/hpf and the presence of RBC regardless of their number. More patients infected with S. dysenteriae 1, S. flexneri and S. boydii shed both WBC and RBC than those infected with S. sonnei, most of whom shed WBC only (P less than 0.001). A greater number of patients infected with S. dysenteriae 1 shed more WBC and RBC than those infected with S. flexneri (P less than 0.001). The same trend was found when patients infected with S. flexneri were compared with those infected with S. boydii (though the difference was not statistically significant) and when patients infected with S. boydii were compared with patients infected with S. sonnei (P less than 0.001). There was a progressive decline in the isolation rate of Shigella as the duration of dysentery, before stool culture, increased.