# Diagnostic accuracy of three tests for Salmonella Typhi infection in adult patients presenting to health facilities in an informal urban settlement in Nairobi, Kenya

## TL;DR

This study compares three tests for diagnosing typhoid fever in Nairobi, finding that the Rapid Ab test is the most accurate for use in resource-limited areas.

## Contribution

The study provides new evidence on the diagnostic accuracy of three tests for typhoid fever in a real-world, resource-limited setting.

## Key findings

- The Rapid Ab test had the highest sensitivity and specificity for diagnosing Salmonella Typhi.
- The Rapid Ab test showed the highest negative predictive value among the three tests.
- The Rapid Ab test is suitable as a standalone diagnostic tool in high-burden, resource-limited settings.

## Abstract

Accurate and timely diagnosis of typhoid fever is undermined by the absence of reliable reference test(s). The objective here was to estimate the sensitivity (Se), specificity (Sp), negative and positive predictive values (NPV and PPV) of the Widal, Salmonella Typhi (S. typhi) rapid antigen (AccurateR Salmonella Antigen Test kit (SAT)) and antibody (OnSite Typhoid IgM/IgG Combo Rapid Test (Rapid Ab)) tests for the diagnosis of acute S. typhi infection in Kenya.

This cross-sectional study enrolled patients symptomatic of typhoid fever at three health facilities situated within an informal urban settlement in Nairobi County, Kenya. Using the patients' diagnostic data, a Bayesian latent class model was used to infer the tests' diagnostic accuracy.

The Rapid Ab test recorded the highest Se (94.9% [95% PCI 76.1, 99.8]) compared to Widal (59.5% [95% PCI 48.6, 77.9]) and SAT (54.2% [95% PCI 43.5, 64.8]). The assay's Sp (95.9% [95% PCI 85.0, 99.8]) was statistically similar to Widal (85.9% [95% PCI 80.8, 91.7] but superior to SAT (67.1% [95% PCI 60.7, 73.0]). PPVs for the Rapid Ab and Widal tests were comparable but distinct from SAT. NPV for the Rapid Ab test was the highest.

The Rapid Ab assay lends itself as a suitable standalone diagnostic to guide surveillance, diagnosis and treatment of acute typhoid fever in high-burden, resource-limited settings.

•The Rapid antibody test exhibits high sensitivity and specificity for Salmonella Typhi.•The Rapid antibody assay posts strong negative and positive predictive values.•The assay lends itself to diagnosis and treatment of acute typhoid fever in poor settings.

The Rapid antibody test exhibits high sensitivity and specificity for Salmonella Typhi.

The Rapid antibody assay posts strong negative and positive predictive values.

The assay lends itself to diagnosis and treatment of acute typhoid fever in poor settings.