Cholera laboratory tests
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Editor-In-Chief: C. Michael Gibson, M.S., M.D. [1]; Associate Editors-In-Chief: Priyamvada Singh, MBBS [2]
Overview
Lab tests are not mandatory for diagnosis and treatment of cholera. In endemic areas when suspected, treatment should be started as early as possible with fluid replacement and antibiotics. In areas where cholera is uncommon, performing lab tests are worthwhile.Tests used for identification of organisms are: Direct microscopic examination of organism, dark field examination, gram staining, culture, antigen, polymerase chain reaction and serotype tests
Laboratory Findings
- Lab tests are not mandatory for diagnosis and treatment of cholera. In endemic areas when suspected, treatment should be started as early as possible with fluid replacement and antibiotics (when indicated). In areas where cholera is uncommon, performing lab tests are worthy.
Stool Examination
- Direct microscopic examination of organism (wet film)
- Dark field examination (helps visualize the motility of the organism)
- Gram staining
Stool Cultue
- Stool and swab collected in the acute stage of the disease are useful specimens for laboratory diagnosis. A number of special media have been employed for the cultivation for Cholera vibrios. They are classified as follows:
Holding or Transport Media
- Venkataraman-ramakrishnan (VR) medium
- Cary-Blair medium: This the most popularly carrying media. This is a buffered solution of sodium chloride, sodium thioglycollate, disodium phosphate and calcium chloride at pH 8.4.
Enrichment Media
- Alkaline peptone water at pH 8.6
- Monsur's taurocholate tellurite peptone water at pH 9.2
Plating Media
- Alkaline bile salt agar: The colonies are very similar to those on Nutrient Agar.
- Monsur's gelatin Tauro cholate trypticase tellurite agar (GTTA) medium: Cholera vibrios produce small translucent colonies with a greyish black centre.
- TCBS medium: This the mostly widely used medium. This medium contains thiosulphate, citrate, bile salts and sucrose. Also in oysters and lobster in some cases. Cholera vibrios produce flat 2-3 mm in diameter, yellow nucleated colonies.
Antigen and Antibody Tests
- Antigen, polymerase chain reaction
- Serotype tests
- Quick method
- Useful in epidemiological studies
- Classic and El Tor both can be identified by using same method
Hemogram
Elevated serum specific gravity, protein and hematocrit occurs due to dehydration caused by excessive fluid loss.
Serum Electrolytes
- Decreased sodium levels due to sodium loss in stool
- Serum potassium may remain normal for longer periods showing body's compensatory mechanism to deal with acidosis
- Elevated blood urea nitrogen and creatinine (prerenal azotemia)
- Decreased pH, decreased bicarbonate (acidosis)