More Than One Test
When your doctor requests a "stool test," they may be ordering one or several different investigations — each measuring something distinct. Understanding what each test actually detects helps you interpret results accurately and ask informed questions.
Fecal Calprotectin
Measures neutrophilic inflammation in the gut. Normal (below 50 micrograms per gram) essentially excludes active IBD. Elevated levels (above 200) indicate significant intestinal inflammation warranting further investigation. Borderline results (50 to 200) are common and may reflect mild inflammation, NSAID use, or recent infection. This is the single most useful non-invasive stool marker for distinguishing inflammatory from functional conditions.
Fecal Immunochemical Test (FIT)
Detects haemoglobin (blood) in stool. Used for colorectal cancer screening and detecting significant colonic bleeding. A positive FIT does not diagnose cancer — it identifies individuals who need colonoscopy for further evaluation. False positives occur with haemorrhoids, anal fissures, and menstrual contamination. False negatives occur because not all cancers and polyps bleed, and bleeding can be intermittent.
Stool Culture and Microscopy
Identifies specific bacterial, parasitic, or protozoal pathogens causing acute infectious diarrhoea. Common targets include Campylobacter, Salmonella, Shigella, Giardia, and Cryptosporidium. A negative culture does not exclude infection — it means the specific organisms tested for were not detected. Viral causes of gastroenteritis (norovirus, rotavirus) require separate molecular testing.
Fecal Elastase
Measures pancreatic exocrine function. Low levels (below 200 micrograms per gram) suggest pancreatic insufficiency — the pancreas is not producing enough digestive enzymes. This test is relevant for patients with chronic diarrhoea, steatorrhoea (fatty stools), or conditions associated with pancreatic disease (chronic pancreatitis, cystic fibrosis, diabetes). It does not measure gut inflammation or microbial composition.
Stool PCR Panels
Modern multiplex PCR panels can simultaneously detect 15 to 20 or more pathogens (bacteria, viruses, parasites) from a single stool sample. While comprehensive, they detect genetic material rather than viable organisms — meaning they can remain positive after infection has resolved (particularly for C. difficile, where toxin testing is preferred over PCR to distinguish colonisation from active disease).
Commercial Microbiome Tests
As discussed in other entries, commercial 16S rRNA stool microbiome tests are marketed as health assessments but lack clinical validation, standardised reference ranges, and professional society endorsement. They describe your microbial composition but cannot diagnose disease or reliably guide treatment.
The Golden Rule
No stool test is a diagnosis on its own. Each result must be interpreted in the context of your symptoms, medical history, medications, and the clinical question your doctor is trying to answer. A test ordered for the wrong reason, or interpreted without context, can mislead more than inform.