Performance Of Widal Test And Stool Culture As Diagnostic Methods For Salmonella Typhi Infection In Chuka General Hospital, Tharaka Nithi County

ABSTRACT 

Typhoid fever is one of the infectious human diseases. Outbreaks of typhoid fever caused by Salmonella typhi remains a serious health problem worldwide. There are a number of tests available presently, from molecular to immunological and biochemical to microbiological. However, misdiagnosis is usually experienced since most health care facilities use only Widal test without confirmation of results with a second test method. This study aimed at evaluating the performance of Widal test and stool culture in the laboratory diagnosis of typhoid fever using blood culture as gold standard. Presenting patients aged between 5 to 82 years with symptoms clinically suspected to be of typhoid fever visiting Chuka General Hospital for a period of eighteen months were recruited for the study. Informed consent of volunteers and guardians were obtained. Serum samples from a total of 126 patients were subjected to Widal agglutination tests. Blood and stool samples from the same individuals were analyzed for typhoid fever infection using blood and stool cultures respectively. Serotyping was performed using agglutination with Salmonella O, H and Vi antisera. In Widal agglutination test, titre values from 1:160 and above were regarded as significant and therefore positive for the Salmonella antigen. Isolation of Salmonella typhi from stool and blood culture indicated an infection. Raw data were entered into Microsoft excel and analyzed using statistical package for social sciences (SPSS). Analysis involved computation of descriptive statistics such as frequencies, means and standard deviations. Comparison of categorical data was made using Chi square or fisher’s exact test, as appropriate. The confidence intervals for sensitivity and specificity were computed using the Wilson’s score method. A P value ≤ 0.05 was considered statistically significant. The sensitivity, specificity, positive predictive value (PPV) and negative predictive value (NPV) for Widal test and stool culture were calculated. The total number of patients positive for typhoid fever based on Widal test were 70 (55.6%). The prevalence of typhoid fever was 42.9% and 15.1% based on the findings from stool and blood culture respectively. Widal test recorded 73.7% sensitivity, 47.7% specificity, 20% positive predictive value and 91.1% negative predictive value. Stool culture showed 84.2% sensitivity, 64.5% specificity, 29.6% positive predictive value and 95.8% negative predictive value. These results demonstrate that Widal test is not very reliable for diagnosis of typhoid fever since false positive and false negative results are common. The low PPV means that Widal test could only be useful for excluding the disease from the population hence health care personnel should not totally depend on this test alone for diagnosis of enteric fever but should use other diagnostic methods to differentiate Salmonella infection from other infection. There is therefore, an urgent need to develop a rapid, highly sensitive and cheap diagnostic tool for diagnosis of typhoid fever.