Investigations
1st investigations to order
FBC
Test
Non-specific. Guided by clinical findings and may be part of routine work-up. It can also be used to assess severity of illness.
Might not be available in the field or in resource-limited settings.[76]
Elevated haematocrit may be found in non-anaemic patients as a result of volume depletion and haemoconcentration.
High neutrophil count may be present in severe infection.
Result
may have elevated haematocrit or neutrophil count
serum electrolytes
Test
Non-specific. Guided by clinical findings and may be part of routine work-up. May not be available in the field or in resource-limited settings.[75]
A low serum bicarbonate, with elevated anion gap due to lactic acidosis, may be seen.
Serum potassium may be low, normal, or high. Potassium levels drop with correction of acidosis. Therefore, potassium supplementation is needed in initial intravenous fluids.
Result
possible potassium derangement or acidosis
serum urea and creatinine
Test
May not be available in the field or in resource-limited settings.[75]
Urea and creatinine may be elevated due to volume depletion and prerenal azotaemia.
Result
possible elevated urea and creatinine
serum lactate
Test
May be elevated due to hypoperfusion from impending cardiovascular collapse.
Non-specific. Guided by clinical findings and may be part of routine work-up.
Might not be available in the field or in resource-limited settings.[75]
Result
elevated
ABG
Test
Arterial blood gas assessment is helpful because cholera infection often results in a low serum bicarbonate with accompanying acidosis, as well as elevated lactate with a high anion gap (due to hyperproteinaemia, lactic acid, and hyperphosphataemia).
Non-specific. Guided by clinical findings and may be part of routine work-up.
Might not be available in the field or in resource-limited settings.[75]
Result
acidaemia
electrocardiogram
Test
May be helpful in assessing severity of volume depletion.
Tachycardia is an appropriate response, so bradycardia is an ominous sign of severe illness.
Hypokalaemia may produce a prolonged PR interval and flattened T waves.
Result
sinus tachycardia, bradycardia, prolonged PR, flat T waves
dark-field/phase-contrast microscopy of stool
Test
Microbiology laboratory test to confirm typical bacteria in fresh stool samples from patients with clinical disease.[4][79][80]
The dark-field examination should include viewing the motile bacteria, and then specific antiserum should be added to confirm that the antiserum has arrested their motility.
The hanging drop method of using a light microscope to examine fresh stool is useful in resource-limited settings.
Large quantities of curved bacilli vary in size from 1 to 3 micrometres in length and 0.5 to 0.8 micrometres in diameter, with a single polar flagellum, and shooting star motility; examination of saline suspensions is also possible.
Result
large quantity of curved bacilli
rapid dipstick testing of stool
Test
A study of 101 stool specimens during a cholera outbreak in Guinea Bissau found that the commercially available rapid dipstick test was 97% sensitive and 71% to 76% specific compared with polymerase chain reaction (PCR) (screening for cholera toxin ctxA and biotype-specific tcpA) as the standard. Dipstick testing thus had a positive predictive value of 87% to 89% and negative predictive value of 92% to 93%.[77][78]
It is highly sensitive and moderately specific for Vibrio cholerae O1/O139 in the epidemic setting. There is emerging evidence that the dipstick test may out-perform culture in some circumstances where culture positivity rates are lower than expected.[90]
Local laboratory technicians are able to perform the test correctly.[77][78]
Result
positive
Investigations to consider
Gram stain of stool
Test
Cheap and widely available but not particularly helpful.
Staining with crystal violet might be a more rapid technique but has not found favour.[79]
Result
small, curved gram-negative rods
stool culture of liquid stool, faecal suspension, or rectal swab
Test
Cary-Blair or similar commercially available preservation media for storage of stool samples required for adequate specimen transport to laboratory.
Vibrio cholerae is sucrose and oxidase positive. Sucrose fermentation produces yellow colonies. Thiosulfate citrate bile salts sucrose (TCBS) and Monsur media are commercially available, easy to prepare, require no autoclaving, and are highly differential and selective.
Alkaline peptone water is the enrichment broth of choice, recommended especially in non-acute or chronic cases where numbers of V cholerae are expected to be low and numbers of competing organisms high.[4][79][80][81]
Result
yellow, shiny colonies, 2 to 4 mm diameter; selective growth of V cholerae on TCBS agar
serogroup confirmation using antisera
Test
Non-O1, non-O139 strains are seen in non-epidemic, sporadic cases of diarrhoeal illness.
Biochemical confirmation of isolate usually not required in endemic areas.[79] On thiosulfate citrate bile salts sucrose agar, the colony is inoculated on to heart infusion agar slant for 6 to 24 hours, then slide-based serology using polyvalent O1 or O139 group-specific antisera is performed.[4]
Result
positive for O1 or O139
antibiogram (sensitivities)
Test
Antibiotic susceptibility (antibiogram) is usually determined using the disc-diffusion method, but epsilometer (E)-test strips for rapid susceptibility tests are also used if borderline resistance is suspected.
Result
antibiotic resistance or sensitivity indicated
enzyme-linked immunosorbent assay (ELISA) assay of stool
Test
Used in combination with polymerase chain reaction and culture to detect Vibrio cholerae.
Use of new-generation molecular diagnostic techniques is limited by availability in resource-limited settings. However, these efficient and reliable tests prevent the rapid spread of the disease by early detection of outbreaks of cholera, may be easily employed in central government laboratories, and may come to be part of the next generation of rapid tests.
Result
positive
molecular testing of stool
Test
New-generation molecular tests, such as one-step multiplex polymerase chain reaction (PCR) assays, can detect various toxigenic, pathogenic, and regulatory genes: ompW, ctxB, rfbO1, tcp, zot, rtxC, ace, hlyA, ompU, and toxR. Quadruplex PCR testing for toxigenic Vibrio cholerae O1/O139 is able to simultaneously determine serotype, biotype, and other factors by detecting the ctxA, tcpA El Tor and/or classical, wbe/wbf, and toxR genes.[83]
Efficient and reliable molecular diagnostic techniques allow simultaneous, rapid testing for a range of enteric pathogens, including V cholerae.[86] In low- and middle-income countries, such technology might only be available in central government laboratories.
Result
positive
loop-mediated isothermal amplification (LAMP) assay of stool
Test
Faster and simpler than conventional polymerase chain reaction in detecting cholera toxin-producing Vibrio cholerae, indicating its importance in early diagnosis of cholera in humans.[84][85]
Use of new-generation molecular diagnostic techniques is limited by availability in resource-limited settings. However, these efficient and reliable tests, which prevent the rapid spread of the disease by early detection of outbreaks of cholera, may be easily employed in central government laboratories and may come to be part of the next generation of rapid tests.
Result
positive
chest x-ray and abdominal x-ray
Test
Radiological tests are rarely helpful in the initial stages of managing cholera cases and should not delay rehydration attempts.
Chest and abdominal radiographs are non-specific tools to exclude emergencies involving the small intestine following severe volume depletion, as a result of secretory diarrhoea.
These imaging tests may also be part of the initial routine work-up of a returning traveller with diarrhoea, but in the epidemic setting are rarely helpful.
They can be used to try to rule out possible complications from severe diarrhoea (e.g., small bowel obstruction, ileus, and perforation) or non-infective abdominal pathology (e.g., severe constipation causing overflow diarrhoea).[87] However, attempting to perform these tests may delay intensive rehydration therapy.
Result
typically normal
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