How is cholera diagnosed?

Preliminary diagnosis is usually done by a caregiver who takes a history from the patient and observes the characteristic rice-water diarrhea, especially if a local outbreak of cholera has been identified. The diarrhea fluid is often teeming with motile, comma-shaped bacteria (presumptively V. cholerae) that can be seen with a microscope. The definitive diagnosis is made by isolation of the bacteria from diarrhea fluid on a selective medium thiosulfate-citrate-bile salts agar (TCBS). Reagents for serogrouping Vibrio cholerae isolates are available in all state health department laboratories in the U.S. Readers may see terms like serotypes Inaba, Ogawa, and Hikojima to describe V. cholerae; they simply indicate which O antigens (O antigens designated A, B, or C) are found on these strains of V. cholerae. PCR tests have also been developed to detect the genetic material of cholera, but currently they are not as widely used as the immunologic tests based on type-specific antiserum.

Definitive diagnosis helps to distinguish cholera from other diseases caused by other bacterial, protozoal, or viral pathogens that cause dysentery (gastrointestinal inflammation with diarrhea).

From: http://www.medicinenet.com/cholera/page5.htm