Author: Dr Henry Wei. Sources checked 15 October 2026.
Investigate persistent watery diarrhoea
I ask about stool consistency, nocturnal symptoms, weight loss, medicines and previous surgery before choosing tests. Microscopic colitis and bile acid diarrhoea are important causes of persistent watery stools.
1. Define the pattern and check urgency
Chronic diarrhoea usually means at least four weeks of symptoms. Ask about stool consistency, blood, urgency, nocturnal stools, incontinence, pain and weight. Review metformin, laxatives, magnesium, antibiotics, travel, water exposure and surgery.
Dehydration, haemodynamic compromise, severe pain, substantial bleeding or systemic illness needs urgent assessment. Weight loss, anaemia, progressive or nocturnal symptoms warrant investigation.
2. Order targeted blood and stool tests
Consider FBC, ferritin, electrolytes, renal function, CRP, albumin and coeliac serology while eating gluten. Add thyroid tests when indicated.
Choose stool microbiology from the exposure and antibiotic history. Calprotectin helps assess possible IBD; FIT may be relevant when investigating possible colorectal cancer.
3. Look for treatable causes
| Clue | Consider | Next step |
|---|---|---|
| Watery or nocturnal diarrhoea | Microscopic colitis | Colonoscopy with right and left colon biopsies, even if the mucosa looks normal. |
| Ileal disease/resection or cholecystectomy | Bile acid diarrhoea | Discuss diagnostic assessment or a supervised treatment trial. |
| Greasy stools, weight loss or pancreatic disease | Pancreatic insufficiency | Faecal elastase on a formed or semi-solid specimen; assess nutrition and the pancreas. |
| Bleeding, raised inflammatory markers or calprotectin | IBD or infection | Assess severity, investigate infection and arrange gastroenterology review as appropriate. |
4. Support nutrition and arrange review
Treat dehydration and electrolyte loss while investigating. Complete coeliac testing before gluten avoidance. Assess bloody diarrhoea, fever or possible severe colitis before using antimotility medicines.
Refer persistent unexplained symptoms, malabsorption or suspected inflammation. After appropriate assessment, recurrent pain or discomfort with altered bowel habit may support IBS-D; diarrhoea without that pattern may fit functional diarrhoea.
Clinical pearls
- Microscopic colitis can have a normal colonoscopic appearance and normal calprotectin.
- Watery stool can dilute faecal elastase and give a misleadingly low result.
- Loose leakage may be overflow around retained stool, particularly with frailty or opioids.
Diagrams for patient discussions
A useful marker, interpreted with your symptoms
What is measured?
Calprotectin is a protein linked to inflammatory white blood cells. The laboratory measures it in poo. It is a clue about inflammation, not a diagnosis of Crohn’s disease, ulcerative colitis or cancer.
What is measured?
Calprotectin is a protein linked to inflammatory white blood cells. The laboratory measures it in poo. It is a clue about inflammation, not a diagnosis of Crohn’s disease, ulcerative colitis or cancer.
Why can it rise?
IBD is one possible cause. Infection, some medicines and other bowel conditions can also raise the result. I interpret the number with your symptoms and other tests.
How is a result interpreted?
The laboratory flag and a referral threshold are different. A lower result makes active inflammation less likely in the right setting, while a value above about 250 µg/g makes active IBD more likely. Neither result is diagnostic, and warning symptoms need assessment regardless of the number.
What happens next?
Review symptoms, the calprotectin trend, infection and medicines. Persistent elevation or concerning symptoms may need gastroenterology assessment; selected intermediate results can be repeated after temporary causes have resolved.
Simplified illustrations, not to scale. These show an example, not your own anatomy or test result.
Read every diagram explanation
What is measured?
Calprotectin is a protein linked to inflammatory white blood cells. The laboratory measures it in poo. It is a clue about inflammation, not a diagnosis of Crohn’s disease, ulcerative colitis or cancer.
Why can it rise?
IBD is one possible cause. Infection, some medicines and other bowel conditions can also raise the result. I interpret the number with your symptoms and other tests.
How is a result interpreted?
The laboratory flag and a referral threshold are different. A lower result makes active inflammation less likely in the right setting, while a value above about 250 µg/g makes active IBD more likely. Neither result is diagnostic, and warning symptoms need assessment regardless of the number.
What happens next?
Review symptoms, the calprotectin trend, infection and medicines. Persistent elevation or concerning symptoms may need gastroenterology assessment; selected intermediate results can be repeated after temporary causes have resolved.
Information to share with your patient
Related clinician articles
References
- bpacnz: inflammatory bowel disease, updated May 2026
- bpacnz: coeliac investigation and management
- Health NZ Waikato laboratory: calprotectin and referral cautions
- Pathlab: stool microbiology clinical details guide (2024)
- BSG: investigation of chronic diarrhoea (2018)
- UEG and European Microscopic Colitis Group guideline (2021)
- Canadian Association of Gastroenterology: bile acid diarrhoea guideline (2020)
- AGA: assessment of pancreatic insufficiency (2023)
- Auckland LabPLUS calprotectin guide, now Northern Region Pathology Network
- Rome Foundation: RomeV diagnostic criteria, sectionC1 IBS