Comparative Evaluation of Five Widal Test Kits Used in Kenya for the Serological Diagnosis of Typhoid Fever

Abstract

Typhoid fever, caused by Salmonella typhi (S. typhi), is currently one of the most serious human diseases in Kenya, sometimes occurring in epidemic proportions, particularly in schools. It is essentially an enteric disease, which frequently becomes septicemic and if left untreated can cause death in 10-20% of the infected population. The disease is usually diagnosed by two methods: (1) isolation of the causative bacterium from suitable samples such as blood during the first week of illness and rose spots on the skin, feces and urine during the second to third week of illness, and (2) serologically by demonstrating a four-fold or greater rise in agglutinins to the ‘O' antigen of S typhi in patients, using a standardized, stained S. typhi antigen, in a test commonly referred to as the ‘Widal test'. In Kenya, there are a number of commercial Widal test kits currently in use. However, there is no documentation on their respective sensitivity and specificity in the diagnosis of typhoid fever. This study was therefore designed to compare sensitivity and specificity of the various Widal test kits in the detection of an active typhoid fever infection. This study compared five Widal test kits, using antisera against S typhi and S. typhimurium ‘O' and ‘H' antigens, experimentally produced in rabbits over a four-week period. S. typhimurium antiserum was used to check for the specificity of the Widal test kits for S. typhi agglutinins as well as cross reactivity with agglutinins against other Salmonella serotypes. All the five Widal test kits used in this study showed 100% sensitivity to S. typhi agglutinins, and Widal test kit E was distinctly more sensitive than the other 4, as it was associated with higher titres. The ‘O' antibody titres displayed by all the test kits were significantly less than the ‘H' antibody titres throughout the experimental period. Two test kits, D and E, picked ‘O' and ‘H' antibody titres greater than the standard cut-off titre of 1:160, whereas three test kits, A, B and C, picked the standard cut-off titre at one week post inoculation with respective S. typhi antigens. However, all test kits picked antibody titres greater than the standard cut-off titre at 2 weeks post inoculation, and at 4 weeks post inoculation, all kits picked an antibody titre greater than 1:2560.