Evaluation of enrichment method for the detection of Vibrio cholerae O1 using a rapid dipstick test in Bangladesh

Abstract

Background Culturing is generally considered to be the gold standard for detecting Vibrio cholerae in stool, though it is not always feasible in resource-limited settings. The Crystal VC dipstick test allows for rapid stool testing for the diagnosis of cholera in the field. However, previous studies have found low specificities (49%–79%) associated with direct testing of stool for cholera using this kit when compared to culturing. Methods In the present study conducted in Dhaka, Bangladesh in 2013, we compare direct testing using the Crystal VC dipstick test and testing after enrichment for 6-hours in Alkaline Peptone Water (APW) to bacterial culture as the gold standard. Samples positive by dipstick but negative by culture were also tested using PCR. Results Stool was collected from 125 patients. The overall specificities of the direct testing and testing after 6-hour enrichment in APW compared to bacterial culture were 91.8% and 98.4% (p=0.125) respectively, and the sensitivities were 65.6% and 75.0% (p=0.07), respectively. Conclusion The increase in the sensitivity of the Crystal VC kit with the use of the 6 hour enrichment step in APW compared to direct testing was marginally significant. The Crystal VC dipstick was found to have a much higher specificity than previously reported (91–98%). Therefore this method provides a promising screening tool for cholera outbreak surveillance in resource limited settings where elimination of false positive results is critical.

Cholera Rapid Test with Enrichment Step Has Diagnostic Performance Equivalent to Culture

“…This field evaluation of the enriched rapid test for diagnosis of cholera confirmed initial reports suggesting that the APW incubation step ensures high specificity, which improves confidence in the test results and could diminish the risk of false cholera outbreak alerts [13,14]. Incubation in APW for 4–6 hours is also the first step in laboratory methods used for the diagnosis of V .…”

“…Thus, we cannot exclude the possibility that the dilution itself, rather than enrichment in APW, might increase the specificity. However, it seems that dilution alone does not eliminate the problem with false positive O139 results with the direct RDT method, as encountered here (see S1 Appendix), since this issue was also reported after dilution in the sample diluent [13]. In addition, the results from the study in Bangladesh suggests that the use of the sample diluent could reduce the RDT sensitivity to unacceptably low levels.…”

“…Indeed, it should be noted that the initial version of Crystal VC recommended that the test be performed directly on liquid stool, with no diluent, and all but one of the evaluations of Crystal VC published until now used this previous method. Only one study published so far was done with the current version of Crystal VC using the sample diluent buffer and showed the lowest sensitivity reported so far (66%) and high specificity of 92% [13]. Thus, we cannot exclude the possibility that the dilution itself, rather than enrichment in APW, might increase the specificity.…”

“…Both methods are now included in the package insert of Crystal VC: the “initial screening” method on direct stool diluted in a sample processing reagent, and the “confirmation” method on stool incubated in APW for 4 hours. So far, only one study conducted in Bangladesh has evaluated the performance of both methods with the current version of the Crystal VC test, and showed that the enriched “confirmation” method had both higher sensitivity and higher specificity compared to the direct screening method [13]. However, in this study, the performance of the direct test was not consistent with previously published studies, with low sensitivity (65%) and high specificity (92%); the reasons for these discrepancies were not discussed, but could be due to the addition of a step of dilution of the stools in a sample preparation vial.…”

Randomized Controlled Trial of Hospital-Based Hygiene and Water Treatment Intervention (CHoBI7) to Reduce Cholera

“…Suspected cholera patients seeking care at the icddr,b Dhaka Hospital were defined as persons with acute watery diarrhea ( > 3 loose stools during a 24-h period) and moderate to severe dehydration using the World Health Organization definition. These patients were screened for V. cholerae in their feces by using the Crystal VC Rapid Dipstick test (Span Diagnostics, Surat, India) ( 23 , 24 ). All dipstick-positive findings were confirmed by bacterial culture.…”