Investigations
1st investigations to order
stool microscopy for WBC and red blood cells (RBC)
Test
Helps to differentiate invasive or inflammatory from non-invasive disease.
Stool microscopy for WBCs and RBCs should be done in patients presenting with blood in stool, fever, suspected invasive pathogens (such as Escherichia coli O157:H7), when other diagnoses are considered (such as inflammatory bowel disease, ischaemic or infectious colitis), and with prolonged symptoms (3 days or more).
Dark-field microscopy can be included to identify Vibrio cholerae if suspected.
Result
present/positive in invasive or inflammatory diarrhoea
stool culture
Test
Guidelines recommend stool testing for Salmonella, Shigella, Campylobacter, Yersinia, Clostridioides difficile, Shiga toxin-producing E coli, and Entamoeba in people with diarrhoea accompanied by fever, bloody or mucoid stools, severe abdominal cramping or tenderness, or signs of sepsis.[40] Testing for additional organisms may be considered depending on the clinical scenario.[3] Testing for Yersinia enterocolitica should be arranged in people with persistent abdominal pain and in people with fever at epidemiological risk for yersiniosis.[40] In individuals with addition with large volume rice water stools or either exposure to salty or brackish waters, consumption of raw or undercooked shellfish, or travel to cholera-endemic regions within 3 days prior to onset of diarrhoea, test stool specimens for Vibrio species.[40]
Stool serological testing and toxin testing can help diagnose the type of Shiga-toxin producing bacteria, and also which toxin is being produced. Testing for Yersinia typically involves serological testing with a repeated level 2 weeks later, but local infectious disease consultation is recommended.
Clostridioides difficile (formerly known as Clostridium difficile) toxin test may be included to exclude C difficile diarrhoea.
Some centres are moving to polymerase chain reaction sequencing to detect a number of bacterial, viral, and parasitic infections.
If symptoms persist and the pathogen is isolated, specific treatment should be initiated.
Result
isolation of specific pathogen
stool ova and parasite (O&P) test
Test
Microscopic examination of the stool for ova and parasites (including trematode eggs and amoeba). Performed on all stool samples.
Some centres are moving to polymerase chain reaction sequencing to detect a number of bacterial, viral, and parasitic infections. This is particularly important to separate pathogenic Entamoeba histolytica from non-pathogenic Entamoeba dispar.
Result
detection of specific ova and/or parasite
polymerase chain reaction (PCR) of stool
Test
Used to detect a number of bacterial, viral, and parasitic intestinal pathogens (e.g., Campylobacter, Salmonella, and Shiga toxin-producing E coli O157, Giardia, Cryptosporidium).[3][44][45]
Useful in detecting viruses, such as norovirus, and in differentiating pathogenic from non-pathogenic Entamoeba species.[3][46]
Result
positive for organism
FBC with differential
Test
Indicated in patients presenting with blood in stool, fever, suspected invasive pathogens (e.g., Shiga toxin-producing E coli [e.g., O157:H7]) and extra-gastrointestinal manifestations.
Helps to assess the inflammatory response and the degree of haemoconcentration. May detect evidence of haemolytic uraemic syndrome when Shiga toxin-producing E coli (e.g., O157:H7) is suspected.
Result
high WBC with most inflammatory/invasive pathogen associated diarrhoea; low WBC are associated with typhoid fever and some viruses; anaemia; high Hb and haematocrit could reflect haemoconcentration
serum creatinine and electrolytes
Test
Should be done to rule out electrolyte abnormalities and renal dysfunction in all patients with evidence of moderate or severe dehydration, and in those with severe vomiting or diarrhoea or symptoms without improvement after 24 hours.
Serum electrolyte assessment and urea and creatinine levels help to assess the inflammatory response and the degree of dehydration. Haemolytic uraemic syndrome suspected when uraemia present.
Result
hypokalaemic metabolic acidosis secondary to diarrhoea; hyper- or hyponatraemia and elevated blood urea nitrogen secondary to dehydration
C-reactive protein
Test
This helps distinguish infectious from inflammatory colitis, though in acute severe ulcerative colitis it may also be raised.
Result
elevated
Investigations to consider
botulinum toxin detection test
Test
If a patient has symptoms/signs of botulism, serum, stool, gastric secretions, or food samples should be sent for toxin identification/confirmation.[49] Do not await test results before administering botulinum antitoxin if the patient is symptomatic and botulism is suspected.[49]
Result
botulinum toxin
blood culture
Test
Blood culture is performed to exclude bacteraemia if the patient is febrile (e.g., temperature 38.5°C [>101°F]) and there are signs of sepsis (tachycardia, hypotension, poor capillary refill, tachypnea, acute mental confusion, decreased urine output). Also recommended for immunocompromised people, people with signs of systemic infection, and when enteric fever is suspected.[40] Signs of sepsis may be difficult to differentiate from signs of severe dehydration.
Result
positive; isolation of specific pathogen with severe invasive infections or superimposed secondary infections
serum lipase or amylase
LFTs
Test
Helps to distinguish food poisoning from acute cholecystitis or acute hepatitis.
Result
elevated in patients with wild mushroom toxicity or invasive pathogens associated with systemic illness, particularly Salmonella and Campylobacter ; hypoalbuminaemia secondary to malnourishment or as an acute-phase reactant
hepatitis A IgM antibodies
Test
Performed when history or occupation (daycare, nursing) suggestive of exposure to hepatitis A or if LFTs abnormal.
Result
positive antihepatitis A IgM Ab
hepatitis E IgM antibodies
Test
Performed when history suggestive of exposure to hepatitis E or if LFTs abnormal.
Result
positive antihepatitis E IgM Ab
acute abdominal series
Test
Flat and upright abdominal radiographs should be obtained urgently if the patient experiences severe pain or obstructive symptoms, or if perforation is suspected.
Result
dilated loops with air-fluid level secondary to ileus; free air is consistent with perforation
sigmoidoscopy/colonoscopy
Test
Sigmoidoscopy is considered in patients with bloody diarrhoea in whom no enteric pathogen has been identified, or the bloody diarrhoea persists or increases in severity, and in those patients whose clinical picture and tests results are incompatible with a diagnosis of foodborne illness.
Can be useful in diagnosing inflammatory bowel disease, antibiotic-associated diarrhoea, shigellosis, and amoebic dysentery. A colonoscopy should be reserved for those in whom sigmoidoscopy does not yield a diagnosis, or in patients with persistent symptoms or are not responding to initial treatment. Colonoscopy is more expensive, requires full preparation and sedations, and should be performed in a special setting (endoscopy suite).
Result
inflammatory changes: erythema, mucosal granularity and ulceration, friability
oesophagogastroduodenoscopy
Test
Oesophagogastroduodenoscopy with duodenal aspirate and/or biopsy is considered in immunocompromised patients, patients receiving chemotherapy, and patients with persistent, severe symptoms lasting more than 5 days, and those not responding well to initial treatment.
Result
inflammatory changes in the small intestines or flattening of villi
biopsy
Test
Considered when performing endoscopy. Rarely helpful, but may distinguish inflammatory bowel disease from acute infectious enteritis or colitis by the presence of crypt architectural changes such as crypt branching or sparsity. However, these features take several weeks to develop and are not likely to be present in an infectious colitis.
Electron microscopy is helpful when intracellular parasites (Cryptosporidium or Cyclospora cayetanensis) are suspected.
Oesophagogastroduodenoscopy with both D1 and quadrantic D2 biopsies may be used to exclude coeliac disease if suspected.
Result
histological features of infectious enteritis are non-specific inflammatory changes; occasionally, pathogens are identified and stained
string test (entero-test)
Test
Considered if stool microscopy fails to diagnose Giardia and antigen test is negative or unavailable. A capsule with a string in it is swallowed, with the free end of the string taped to the patient’s cheek. It is allowed to dissolve and the string passes into the duodenum where it is left for 4 to 6 hours before being removed. The string can then be examined for trophozoites.
Result
presence of trophozoites
duodenal aspirate
Test
Considered when performing endoscopy in immunocompromised patients, patients receiving chemotherapy, and patients with persistent symptoms or not responding well to initial treatment.
May be used to diagnose Giardia, Strongyloides, Cystoisospora, or microsporidia infection when other tests have failed to reveal the diagnosis.[40]
Result
positive (bacterial overgrowth) when count >10⁶ m/L
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