Microscopic colitis — diagnosis, budesonide, and drug-trigger review

Microscopic colitis is a common and commonly missed cause of chronic non-bloody watery diarrhoea in Australia. It produces 5–10 or more loose stools per day, often with nocturnal urgency and faecal incontinence, in a patient whose colonoscopy looks entirely normal. The diagnosis is made only when biopsies are taken — and taken from the right and left colon, not just the rectum, where the characteristic histological changes are often absent. Average diagnostic delay in published cohorts is 2–5 years, during which patients are typically labelled as having irritable bowel syndrome and prescribed antispasmodics that do not work. The condition is as common as ulcerative colitis in epidemiological terms — now estimated at 10–25 cases per 100,000 per year in Australia — and is particularly prevalent in women over 60. It is eminently treatable once diagnosed, carries no excess cancer risk, and has a clear drug-trigger review as its first clinical intervention.

A. Core clinical — the AU general-practice framework

Classification and histology

Microscopic colitis is an umbrella term for two histological subtypes that are clinically identical:

• Collagenous colitis (CC) — thickened subepithelial collagen band ≥10 μm (normal below 5 μm), confirmed on Masson trichrome staining
• Lymphocytic colitis (LC) — increased intraepithelial lymphocytes ≥20 per 100 surface enterocytes (normal below 5)
• Incomplete microscopic colitis (MCi) — collagen band 5–10 μm or IEL count 10–20; UEG/EMCG 2021 now recognises this as a separate entity treated identically

Demographics: female-to-male ratio approximately 3:1, particularly for collagenous colitis; peak age 60–70 years; but approximately 25% of cases present under 45 years. Australian incidence has tripled since the 1990s, paralleling international data.

Coexisting conditions in ~30%: coeliac disease (5–7% — always co-test for coeliac at diagnosis), autoimmune thyroid disease, Sjögren syndrome, type 1 diabetes, rheumatoid arthritis.

Drug triggers — review the medication list first

A major subset of microscopic colitis is drug-induced. Always review the medication list before escalating to pharmacotherapy. Stopping the offending drug induces remission in a meaningful minority without corticosteroids (Bonderup et al. APT 2018):

Drug class	Specific agents most implicated
Proton pump inhibitors	Omeprazole, esomeprazole, pantoprazole, lansoprazole — strongest evidence
NSAIDs	Ibuprofen, naproxen, diclofenac, indomethacin
SSRIs	Sertraline (highest risk), paroxetine
Statins	Simvastatin, atorvastatin
ACE inhibitors	Ramipril, perindopril
Antiplatelet agents	Clopidogrel, ticlopidine
Ranitidine	TGA-cancelled 2020 — no longer prescribed but may appear in patient histories

When the offending drug must continue (clopidogrel post-coronary stent, SSRI for severe depression), the risk-benefit decision is made case by case — proceed to pharmacotherapy without prolonged drug withdrawal.

History

• Stool frequency — typically 5–10+ loose/watery stools per day; nocturnal stools in ~50% (distinguishing from IBS-D, which rarely causes nocturnal waking)
• Stool character — watery, non-bloody; the presence of blood should prompt reassessment for IBD or malignancy
• Urgency and faecal incontinence — common and underreported; ask directly
• Duration — typically months to years; relapsing-remitting course in many
• Medication history — start date of PPIs, NSAIDs, SSRIs, statins relative to symptom onset; drug-induced colitis may begin weeks to months after drug initiation
• Smoking — current smokers have significantly worse disease activity and poorer budesonide response (Khalili 2024); smoking cessation is a modifiable intervention
• Red flags requiring re-evaluation: bloody stool, significant weight loss, anaemia, raised faecal calprotectin >250 μg/g — these suggest IBD or malignancy

Diagnosis — colonoscopy with targeted biopsies

The macroscopic appearance is normal. Occasionally, subtle mucosal erythema or “cat-scratch” linear ulcers are seen in collagenous colitis, but the diagnosis cannot be made endoscopically — biopsies are required.

Biopsy instructions to include on the referral:

“Request random biopsies from right colon (≥2 specimens), transverse colon (≥2 specimens), and left colon (≥2 specimens), even if the mucosa appears macroscopically normal. Seeking assessment for microscopic colitis.”

Rectal biopsy alone misses up to 40% of collagenous colitis because the collagen band is thinnest in the distal colon. Missing the diagnosis leads to another 1–2 years of misdiagnosis as IBS-D.

Coeliac serology (anti-TTG IgA plus total IgA) at the time of colonoscopy referral — coeliac disease coexists in 5–7%.

Faecal calprotectin — useful to differentiate from IBD (calprotectin >250 μg/g suggests macroscopic bowel inflammation); not on MBS, out-of-pocket approximately $60–80. Normal or mildly elevated in microscopic colitis.

Differential diagnosis

Differential	Key discriminating feature	Investigation
IBS-D	Younger; no nocturnal stools; normal histology	Normal colonoscopy + biopsies
Coeliac disease	Positive TTG serology; duodenal villous atrophy; iron deficiency	Anti-TTG IgA; duodenal biopsy
Inflammatory bowel disease (UC, Crohn)	Bloody stool; macroscopic colitis at colonoscopy; calprotectin >250	Colonoscopy + biopsies
C. difficile infection	Recent antibiotic use; positive stool toxin or PCR	Stool C. difficile PCR
Bile acid diarrhoea	Post-cholecystectomy; post-terminal ileal surgery; watery and urgency	Empirical cholestyramine trial
Lactose or fructose intolerance	Postprandial timing; bloating	Breath test; exclusion trial
Hyperthyroidism	Suppressed TSH; weight loss with preserved appetite	TSH

B. Evidence — budesonide as first-line induction

The AGA 2016 Microscopic Colitis Clinical Guideline and UEG/EMCG 2021 European Guideline both give budesonide a strong recommendation as first-line pharmacotherapy for moderate-to-severe microscopic colitis.

Induction evidence: multiple placebo-controlled RCTs show pooled clinical remission at 6–8 weeks of approximately 80% with budesonide 9 mg/day, versus approximately 30% with placebo. The number needed to treat is approximately 2. Budesonide’s high topical anti-inflammatory potency combined with 85–90% first-pass hepatic metabolism produces effective mucosal action with minimal systemic corticosteroid effects.

Maintenance evidence: Miehlke et al. Gastroenterology 2014 RCT showed that low-dose budesonide (6 mg/day or 3 mg/day) reduces relapse from approximately 80% to approximately 25% in patients with chronic-active or relapsing disease.

What does not work for induction:

• Mesalamine — no benefit over placebo; both AGA and UEG recommend against (AGA 2016)
• Prednisolone — inferior to budesonide with more systemic adverse effects
• Methotrexate — explicitly not recommended in UEG/EMCG 2021
• Probiotics — no RCT evidence

C. Management ladder

Step 1 — Modifiable triggers: Stop the offending drug if clinically safe. Replace PPI with H2 blocker (famotidine) where possible. Switch NSAID analgesic to paracetamol. If SSRI is essential, consider switching to an agent with lower microscopic colitis association. Integrate smoking cessation support — NPS MedicineWise and eTG both recommend this as a modifiable risk factor.

Step 2 — Symptomatic relief for mild disease: Loperamide 2–4 mg as needed, up to 16 mg/day per UEG/EMCG 2021 (conditional recommendation). Cholestyramine 4 g twice daily is an alternative where bile acid diarrhoea overlap is suspected.

Step 3 — Induction of remission (moderate-to-severe): Budesonide MMX (Cortiment) 9 mg orally once daily in the morning for 6–8 weeks. Budesonide-EC (Entocort) 3 mg capsules, taken as 3 capsules once daily, is an equivalent formulation. Counsel about potential adverse effects: glucose, blood pressure, bone density on prolonged or cumulative use.

Step 4 — Maintenance for relapsing or chronic-active disease: After 6–8 weeks induction, taper to the lowest effective dose — typically 3 mg/day or 6 mg alternate days. Loperamide and cholestyramine can be used as steroid-sparing adjuncts. Review annually; FBC, electrolytes, glucose, and bone mineral density assessment if cumulative budesonide exceeds 3 months per year.

Step 5 — Refractory disease: Gastroenterology referral for patients who do not respond to or relapse repeatedly despite adequate budesonide. Biologics (infliximab, adalimumab, vedolizumab) are reported in case series with modest response rates of 30–60% but are not PBS-listed for microscopic colitis.

D. Australian operations

The PBS reality for budesonide

This is the most practically important detail for Australian prescribers: budesonide MMX (Cortiment 9 mg) carries PBS Authority approval for ulcerative colitis only, and budesonide-EC (Entocort 3 mg capsules) is PBS Authority for Crohn disease only. For microscopic colitis, both require private prescription — approximately $200 per month. Patients should be counselled about this cost at the time of diagnosis.

Alternatives to manage cost: eTG notes that some specialist gastroenterology prescriptions under exceptional circumstances can attract PBS authority under off-label provisions. Where cost is prohibitive, loperamide plus drug-trigger modification and lifestyle change represents a lower-cost initial approach for mild disease.

MBS items for microscopic colitis

• MBS item 32222 — diagnostic colonoscopy with biopsy
• MBS item 30473 — gastroscopy (if combined duodenal biopsy for coeliac assessment)
• Faecal calprotectin is not on MBS — approximately $60–80 out-of-pocket
• MBS item 965 (GPCCMP preparation) and item 967 (review) — chronic relapsing microscopic colitis qualifies; use for dietitian and continence-nurse access via the Team Care Arrangement

Gastroenterology referral pathways

Refer to gastroenterology when:

• Diagnostic colonoscopy is required and GP does not have access to gastroscopy services
• Diagnosis is uncertain or biopsies are inconclusive
• No response to adequate budesonide trial (8 weeks at 9 mg/day)
• Multiple relapses on maintenance therapy
• Weight loss, anaemia, or calprotectin >250 μg/g — re-evaluate for IBD or malignancy

The Gastroenterological Society of Australia (GESA) provides patient-facing educational resources.

E. Special populations

Women over 60. This is the highest-risk demographic — collagenous colitis has a female-to-male ratio of approximately 4:1 in this age group, and PPIs (taken for reflux or as gastroprotection with NSAIDs prescribed for osteoarthritis) are the commonest identifiable drug trigger. Reviewing the PPI indication — whether it remains clinically necessary — is part of every microscopic colitis management plan.

Patients with autoimmune disease. The coexistence of coeliac disease (5–7%), autoimmune thyroid disease, Sjögren syndrome, and type 1 diabetes with microscopic colitis means a systematic autoimmune review is warranted at diagnosis. Untreated coeliac disease perpetuates intestinal inflammation and does not respond to budesonide — treating coeliac disease first (or simultaneously) is important.

Patients on clopidogrel, SSRI, or other non-stoppable drugs. Where the offending drug cannot be stopped — antiplatelet therapy after coronary stenting, or SSRI for severe or recurrent depression — proceed to budesonide without prolonged withdrawal. Document the risk-benefit discussion. Annual review of the drug indication is reasonable.

Older adults with frailty or polypharmacy. Budesonide’s systemic effects are substantially lower than prednisolone but not zero — monitor blood pressure, blood glucose, and bone density in patients receiving repeated or prolonged courses. A bone-protective plan (calcium, vitamin D, bisphosphonate where indicated) applies when cumulative budesonide exposure exceeds 3 months per year.

Patients with significant faecal incontinence. Microscopic colitis-related faecal incontinence is common and underreported, and has significant quality-of-life and employment consequences. Refer to a continence physiotherapist as part of the Team Care Arrangement when incontinence is present; this can proceed alongside pharmacotherapy.

When to escalate

Same day or emergency review:

• Severe dehydration — especially in older patients or those with comorbidities
• Electrolyte derangement (hypokalaemia, hyponatraemia)
• Suspected toxic colitis (very rare in microscopic colitis)

Routine gastroenterology referral:

• Diagnostic colonoscopy not yet done, or biopsies inadequate
• No response to 8 weeks of budesonide 9 mg/day
• Multiple relapses despite adequate maintenance therapy

Urgent reassessment or expedited gastroenterology review:

• Blood in stool (reconsider IBD or malignancy)
• Weight loss >5% in 6 months
• Anaemia
• Faecal calprotectin >250 μg/g

What this article is and is not

This is general health information drawn from the AGA 2016 Microscopic Colitis Guideline, UEG/EMCG 2021 European Guideline, Therapeutic Guidelines (eTG), Australian Medicines Handbook, and the published microscopic colitis trial literature. It is not personal medical advice and does not create a doctor–patient relationship. Management decisions involve information this article cannot include — including the PBS and cost considerations that require discussion with your GP.

For patient resources: GESA patient information, HealthDirect — diarrhoea, Better Health Channel — diarrhoea, NPS MedicineWise.

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Sources cited

1. AGA Clinical Guidelines — Microscopic Colitis. Gastroenterology 2016
2. UEG/EMCG European Microscopic Colitis Guidelines. UEG Journal 2021
3. Khalili — Microscopic Colitis Update. Clin Gastroenterol Hepatol 2024
4. Therapeutic Guidelines (eTG) — Gastrointestinal
5. Australian Medicines Handbook (AMH)
6. Bonderup OK et al. — Drug discontinuation in microscopic colitis. APT 2018
7. Miehlke S et al. — Budesonide maintenance in collagenous colitis. Gastroenterology 2014
8. NPS MedicineWise
9. GESA — Patient resources
10. HealthDirect — Diarrhoea
11. Better Health Channel — Diarrhoea
12. PBS — Budesonide (Cortiment, Entocort)
13. MBS Item 32222 — Colonoscopy