Content area
Full text
This article forms part of our 'Tests and results' series for 2012, which aims to provide information about common tests that general practitioners order regularly. It considers areas such as indications, what to tell the patient, what the test can and cannot tell you, and interpretation of results.
Keywords
faeces/microbiology; microscopy; bacteria/isolation and purification; diarrhoea
Stool culture is a laboratory test used to determine the aetiology of infective, bacterial diarrhoea. It refers to the inoculation of selective agar plates with faeces and incubation for 1-2 days to detect the presence of pathogenic bacteria within the bowel flora.
More than 90% of cases of acute diarrhoea are caused by infectious agents1 acquired by faecal-oral transmission via direct personal contact or ingestion of contaminated food or water. It is important to have an understanding of the range of pathogens that may cause infectious diarrhoeal syndromes (Table 1-3).
Indications for testing
Most cases of acute diarrhoea are mild and self limiting and no investigation or treatment is necessary. However, some patients should be investigated regardless of the severity of disease: returned travellers, patients in whom diarrhoea has persisted for more than 4-5 days, patients with bloody stools, immunocompromised patients, and in cases where there is suspicion of an outbreak of enteric disease. Admission to hospital is usually only required in cases of significant dehydration, marked toxaemia, persistent vomiting or severe abdominal pain.
In patients with severe symptoms, empirical antibiotic therapy may be appropriate pending the results of laboratory investigations.1
Logical positioning of stool cultures relative to other related investigations
A thorough clinical history and examination is essential before requesting stool culture. Nonbacterial causes of diarrhoea (eg. viral or parasitic) should be considered in the differential diagnosis. Viruses are more common in children (particularly rotavirus and adenovirus) and are usually self limiting. Norovirus is also an important cause of community acquired diarrhoea. Nucleic acid amplification testing or antigen detection assays are available for detection of viruses in the faeces, but are rarely indicated in the outpatient setting. Testing for parasites (eg. Giardia, Entamoeba) should be prompted by specific epidemiological risk factors (eg. travel, immunocompromised host). For parasites, diagnostic options include stool microscopy for ova, cysts and parasites and antigen detection assays.
Clostridium difficile is an increasingly recognised cause...