Investigations
1st investigations to order
clinical diagnosis
Test
Base your diagnosis on the patient history and examination. Tests are rarely needed to diagnose the pathogen.
Result
clinical diagnosis
full blood count
Test
Required if considering starting intravenous fluids.[14]
Look for significant deviations from the normal values.
Anaemia may be a sign of a chronic cause of diarrhoea, such as inflammatory bowel disease.[24]
A raised haemoglobin may indicate severe dehydration.
A significantly raised or lowered WBC count can indicate an infection or inflammation.
Look at platelets as an acute-phase reactant to indicate the severity of an infection.
Result
look for any abnormally high or low WBC count, haemoglobin, or platelet count
urea and electrolytes
Test
Electrolytes should be in the normal range. Both high and low values are a cause for concern.
Required if considering starting intravenous fluids.[14]
Intravenous rehydration may need to be tailored depending upon the results. Consider which fluid and the rate of administration.
Ask for serum electrolytes if you suspect serious volume depletion in which there may be hypokalaemia or renal impairment.
Consider bicarbonate assay, calcium, magnesium, and phosphate.
Patients at higher risk of complications, including older people and patients who are immunocompromised or who have comorbidities, may also need serum electrolytes.
Result
urea, creatinine, or ratio may be elevated
creatinine
Test
Required if considering starting intravenous fluids.[14]
Normal values vary depending on age, sex, and existing comorbidities.
An abnormal result may indicate acute kidney injury.
Result
creatinine (or urea-creatinine ratio) may be elevated
Investigations to consider
stool for culture, ova, and parasites
Test
Order if symptoms are atypical and you suspect a bacterial or parasitic aetiology (e.g., bloody diarrhoea, high fever, or tenesmus).
Take a stool sample for microbiological diagnosis when there is:
Stool ova and parasite and WBC detection under microscopy can be an initial test under these circumstances.
A stool sample may help to exclude non-infectious causes of diarrhoea, such as adverse effects of medications, inflammatory bowel disease, coeliac disease, irritable bowel syndrome, endocrinopathy, or secretory tumours.[11]
Result
may identify enteropathogenic bacteria or show parasitic infection
bacterial stool cultures are positive in less than 5% of presentations[25]
stool viral culture
Test
Rarely necessary. Identifies viral pathogens.
Result
may identify a specific virus
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