Rapid and reliable serological diagnosis of enteric fever: comparative sensitivity and specificity of Typhidot and Typhidot-M tests in febrile Malaysian children

Abstract

The Typhidot test, which detects IgM and IgG antibodies to a Salmonella typhi-specific outer membrane protein, is as sensitive as, and more specific than, the Widal test in the diagnosis of enteric fever in Malaysian children. It is easier and quicker to perform. In order to increase diagnostic accuracy in an area of high endemicity, the Typhidot-M test has been developed in which IgG is first removed. This theoretically allows improved detection of IgM, and thus would differentiate new from recent infections. We evaluated both tests in 134 unselected febrile children admitted to the General Hospital Kota Bharu, Malaysia. The children were divided into two groups: (i) those who were blood and/or stool culture positive for S. typhi and/or who had clinical features strongly suggestive of enteric fever (n = 62); and (ii) those who were both culture-negative and had clinical evidence of another diagnosis (n = 72). The sensitivity and specificity of the Typhidot and Typhidot-M tests were identical at 90.3 and 93.1%, respectively. Both tests had comparable sensitivity but greater specificity than those of the Widal test (91.9 and 80.6%, respectively). When used together, a positive result for Typhidot and/or Typhidot-M was more specific than either test alone (95.2%) but specificity was lower (87.5%). We conclude that the Typhidot and Typhidot-M tests have comparatively high diagnostic accuracy, suggesting that IgM can be detected in children who may have a predominant IgG response to S. typhi. Using these tests in combination increases the negative predictive value but at the cost of a lower positive predictive value.