Stool culture — A definitive diagnosis of cholera is based on isolation of the organism from clinical samples, which also permits a determination of the antibiotic susceptibility profile. V. cholerae can be isolated from stool using selective media such as thiosulfate citrate bile sucrose agar or taurocholate tellurite gelatin agar. Once cultured, V. cholerae can be identified by biochemical tests; serogroup and serotype can be assigned by testing with specific antibodies. In settings such as the United States, where cholera is a sporadic illness, the clinical microbiology laboratory should be informed of a suspicion of cholera so that appropriate selective media can be used. These selective media are not routinely used for stool culture.

The recovery of viable V. cholerae from clinical specimens can be enhanced by enrichment in alkaline peptone water . V. cholerae can also persist in a number of standard transport media, including Cary-Blair media, while being transported to a central laboratory from field settings.

Rapid tests and other tools

●Antigen detection − Several rapid antigen detection-based tests are commercially available for diagnosis of cholera. These include immunochromatographic lateral flow devices (dipsticks), such as Crystal VC, which detect the presence of the O1 or O139 antigen in watery diarrheal stools, and Cholkit, which detects only the O1 antigen. In one meta-analysis including 20 studies representing 8 different commercial rapid tests, the combined sensitivity was 90 percent (95% CI 86-93) and the specificity was 86 percent (95% CI 81-90), relative to bacterial culture (the diagnostic gold standard); there was variation across individual studies.

The sensitivity of culture is diminished among individuals who no longer shed viable bacteria in the stool; negative stool culture may be observed in the setting of antibiotic administration and/or coinfection with lytic bacteriophage. The apparent diminished specificity of antigen-based dipstick tests (bacterial culture used as the diagnostic gold standard) may reflect of the limits of bacterial culture. To account for this, one study evaluated the performance of Cholkit, Crystal VC, microbial culture, and polymerase chain reaction (PCR) using a Bayesian latent class modeling approach; the sensitivity of Cholkit and Crystal VC were 98 percent (95% CI 88-100 percent for both), with specificities of 97 percent (95% CI 89-100 percent) and 98 percent (95% CI 92-100 percent), respectively.

●Molecular tests − Molecular testing for V. cholerae (eg, PCR), including tests that use dried fecal spots, is also feasible; however, thus far practical use of molecular tests has been limited to epidemiologic research and surveillance.

●Darkfield microscopy − Darkfield microscopy of rice-water stools (at 400x magnification) can also be used to evaluate for the presence of highly motile Vibrios, whose shooting star-like motion can be inhibited by the subsequent addition of specific antibodies [52]. Darkfield microscopy is quite specific for V. cholerae but lacks sufficient sensitivity to be used reliably for diagnosis.