Evaluation of a point-of-care immunochromatographic assay for enteric fever in Dhaka, Bangladesh: a prospective diagnostic accuracy study

Abstract

Background: There is a shortage of rapid, accurate, and low-cost assays for diagnosing enteric fever. The dual-path platform for typhoid (DPPT) assay had high accuracy in retrospective studies with banked plasma samples. We aimed to evaluate the diagnostic accuracy of the DPPT assay in a prospective study using fingerstick capillary blood.

Methods: For this prospective diagnostic accuracy study, we enrolled children younger than 18 years, who presented at the Bangladesh Shishu Hospital and Institute (Dhaka, Bangladesh) with at least 3 days of fever. We collected blood and nasal swabs, and did the following assays to assess the accuracy of DPPT: blood culture, serological assays for typhoid fever (DPPT assay, Widal, and Test-it), and molecular assays to identify alternative aetiologies (eg, respiratory syncytial virus [RSV], influenza, dengue virus, and Rickettsia spp). The primary outcomes of the study were the sensitivity and specificity of the DPPT assay for diagnosis of enteric fever. We evaluated the accuracy of combined anti-haemolysin E and anti-lipopolysaccharide IgA readings using the DPPT assay in distinguishing culture-confirmed enteric fever from alternative aetiologies using receiver operating characteristic area under the curve (AUC). Given the imperfect reference standard diagnostics for enteric fever, we used Bayesian latent class models, incorporating results from the typhoid and alternative aetiology diagnostics, to estimate the sensitivity and specificity of the DPPT assay. We also did subgroup analyses for the assay across multiple biological variables.

Findings: Between Aug 17, 2021, and July 16, 2022, we enrolled 501 participants who presented with at least 3 days of fever. 223 (45%) of 501 participants were female and 278 (55%) were male. 77 participants had culture-confirmed enteric fever (62 Salmonella enterica serotype Typhi and 15 Salmonella enterica serotype Paratyphi A) and 70 were culture-negative with PCR-confirmed alternative aetiology (34 influenza A or B, 22 dengue virus, seven Rickettsia spp, six RSV, and one participant co-infected with RSV and dengue virus). The AUC for DPPT on fingerstick capillary blood in distinguishing typhoid from alternative aetiologies was 0·969 (95% CI 0·943-0·994). In latent class analysis, the sensitivity of DPPT was 93% (95% credible interval [CrI] 87-97) and specificity was 89% (85-93). The balanced accuracy was higher for DPPT (91%, 95% CrI 87-94) than blood culture (81%, 78-85), Test-it (77%, 74-79), or Widal (70%, 67-73). Assay performance did not vary by sex, age, duration of fever at presentation, antibiotic use before presentation, Salmonella serotype, or sample type.

Interpretation: The point-of-care DPPT assay achieved high diagnostic accuracy for enteric fever in a highly endemic community. This assay has the potential to improve clinical outcomes for enteric fever, allowing rapid diagnosis and treatment, and could facilitate more appropriate antimicrobial use.

Funding: National Institute of Health, Bill & Melinda Gates Foundation, and Child Health Research Foundation.

Figure 1

Study design

Figure 2

Study flowchart RSV=respiratory syncytial virus. ∗One patient was co-infected with RSV and dengue.

Figure 3

Anti-lipopolysaccharide and anti-haemolysin E IgA values measured by DPPT assay according to putative etiology of febrile illness Individuals with typhoid or paratyphoid and another aetiology (n=2) were depicted in the typhoid or paratyphoid group. RSV=respiratory syncytial virus. DPPT=dual-path platform for typhoid.

Figure 4

Correlation of DPPT quantitative values by sample type Pearson’s correlation r values and plot of DPPT anti-lipopolysaccharide and anti-haemolysin E IgA microreader values using plasma, capillary whole blood, and venous whole blood. DPPT=dual-path platform for typhoid.