Evaluation of typhoid assay for rapid diagnosis of typhoid fever.

Typhoid fever remains a major public health risk with more than 13 million cases occurring annually in Asia alone. Delay in diagnosis and institution of appropriate therapy especially with emergence of multi-drug resistant strains of Salmonella typhi, can be associated with significant morbidity and mortality may significantly increase the risk of adverse outcome. Traditionally, the isolation of S. typhi remains the gold standard for diagnosis but, the Widal test continues to be the most asked for test, despite numerous problems in the standardization and interpretation.1 The dot enzyme immunoassay for detection of serum antibodies to S. typhi is rapid, simple and can be interpreted visually.2 The present study was carried out to evaluate the typhidot test in comparison with blood culture and the Widal test in patients admitted in PGIMER, Chandigarh with a clinical diagnosis of enteric fever. In 50 consecutive patients with S. typhi isolated in blood culture, Widal test and dot EIA were performed. A control group of 50 patients with unrelated complaints were included. The Widal test using in-house S. typhi antigens was considered positive in a single serum sample when the S. typhi H antibody titre was ≥ 320 and O antibody titer was ≥160. The Typhidot test (Malaysian Biodiagnostic Research SDN BDH, Kuala Lumpur, Malaysia) which detects IgM and IgG antibodies against S. typhi by using a specific antigen was performed according to manufacturer’s instructions. The Widal test in single serum sample was positive in 30 (60%) blood culture positive patients and was negative in all 50 cases in the control group. IgG and/or IgM was detected by the typhidot test in 48 (96%) blood culture positive patients and 7 controls. Both IgG and IgM could be detected in 15 patients whereas only IgM could be detected in 30 and only IgG in three. Neither IgG nor IgM could be detected in two patients with S. typhi isolation in blood culture. Presence of IgM was a more sensitive indicator, with 45 (90%) being positive for IgM and 18 of 50 (36%) being positive for IgG. Widal test was found to be 60% sensitive and 100% specific whereas Typhidot was 96% sensitive and 86% specific when compared to blood culture. Typhidot had a very high negative predictive value of 95.55% and positive predictive value of 87.27%. The diagnostic value of Widal in single serum samples remains a contentious issue. The sensitivity of the typhidot test is much higher than the Widal test and is sensitive even in fevers of short duration.3 The high NPV (95.55%) of the test even in a highly endemic area is an advantage and can be successfully used even in low-incidence populations. The Typhidot offers advantages of increased sensitivity, rapidity, early diagnosis and simplicity over the Widal test. Culture isolation remains essential especially for antibiotic susceptibility testing and serological tests for the diagnosis of typhoid fever can be used in conjunction with culture.