Sensitivity and specificity of typhoid fever rapid antibody tests for laboratory diagnosis at two sub-Saharan African sites

Objective To evaluate three commercial typhoid rapid antibody tests for Salmonella Typhi antibodies in patients suspected of having typhoid fever in Mpumalanga, South Africa, and Moshi, United Republic of Tanzania. Methods The diagnostic accuracy of Cromotest (R) (semiquantitative slide agglutination and single tube Widal test), TUBEX (R) and Typhidot (R) was assessed against that of blood culture. Performance was modelled for scenarios with pretest probabilities of 5% and 50%. Findings In total 92 patients enrolled: 53 (57.6%) from South Africa and 39 (42.4%) from the United Republic of Tanzania. Salmonella Typhi was isolated from the blood of 28 (30.4%) patients. The semiquantitative.slide agglutination and single-tube Widal tests had positive predictive values (PPVs) of 25.0% (95% confidence interval, CI: 0.6-80.6) and 20.0% (95% CI: 2.5-55.6), respectively. The newer typhoid rapid antibody tests had comparable PPVs: TUBEX (R), 54.1% (95% CI: 36.9-70.5); Typhidot (R) IgM, 56.7% (95% CI: 37.4-74.5); and Typhidot (R) IgG, 54.3% (95% CI: 36.6-71.2). For a pretest probability of 5%, PPVs were: TUBEX (R), 11.0% (95% CI: 6.6-17.9); Typhidot (R) IgM, 9.1% (95% CI: 5.8-14.0); and Typhidot (R) IgG, 11.0% (6.3-18.4). For a pretest probability of 50%, PPVs were: TUBEX (R), 70.2% (95% CI: 57.3-80.5); Typhidot (R) IgM, 65.6% (95% CI: 54.0-75.6); and Typhidot (R) IgG, 70.0% (95% CI: 56.0-81.1). Conclusion Semiquantitative slide agglutination and single-tube Widal tests performed poorly. TUBEX (R) and Typhidot (R) may be suitable when pretest probability is high and blood cultures are unavailable, but their performance does not justify deployment in routine care settings in sub-Saharan Africa.