Microscopic colitis gets missed for a straightforward reason: it is invisible. The colon looks entirely normal during colonoscopy. There is no redness, no ulceration, no structural abnormality that a gastroenterologist can see with the naked eye. The inflammation exists only at the cellular level, detectable only when tissue samples are placed under a microscope. This creates a diagnostic gap that sends a substantial number of patients home with an IBS-D diagnosis when they actually have a specific inflammatory condition with a highly effective targeted treatment. The gap is not theoretical. Studies show that microscopic colitis accounts for 10-15% of chronic watery diarrhea cases evaluated by colonoscopy, making it one of the most common diagnoses that gets missed when biopsies are not taken.
Why does microscopic colitis get misdiagnosed as IBS?
The misdiagnosis happens at a specific point in the clinical workflow: the moment a gastroenterologist looks at a normal-appearing colon and decides whether to take biopsies. In many clinical settings, biopsies are taken only when visible abnormalities are present. If the mucosa looks pink, smooth, and healthy, the colonoscopy is reported as normal, the patient is reassured, and the diagnosis defaults to IBS-D based on symptom criteria. This practice is not unreasonable in most contexts. Biopsies add time to the procedure, require pathology processing, and increase costs. For the majority of patients with normal-appearing colons, biopsies will indeed be unremarkable. But for the 10-15% with microscopic colitis, skipping biopsies means skipping the diagnosis entirely.
Several factors compound the problem. The Rome IV criteria used to diagnose IBS do not require biopsy confirmation that microscopic colitis has been excluded. Many gastroenterologists do not routinely biopsy normal-appearing colons during colonoscopy for chronic diarrhea, particularly in younger patients or those without alarm features. And the symptoms of microscopic colitis overlap so completely with IBS-D that nothing in the clinical presentation forces the question.
The biopsy gap in clinical practice
Guidelines from the European Microscopic Colitis Group and the American Gastroenterological Association recommend random biopsies of normal-appearing colonic mucosa in patients undergoing colonoscopy for chronic watery diarrhea. Despite these recommendations, adherence varies widely. A study of colonoscopy practices found that random biopsies for chronic diarrhea are taken in only 30-50% of cases where they are indicated, depending on the institution and the individual endoscopist's practice patterns.
The result is a diagnostic lottery. Whether your microscopic colitis gets detected depends partly on which gastroenterologist performs your colonoscopy and whether that individual routinely biopsies normal-appearing mucosa in the setting of chronic diarrhea. Patients who happen to see an endoscopist who follows the biopsy recommendation get diagnosed. Those who do not may carry an IBS-D label indefinitely.
âšī¸If you are scheduled for a colonoscopy to investigate chronic diarrhea, ask your gastroenterologist before the procedure whether they plan to take random biopsies even if the colon looks normal. This is a reasonable and evidence-based request that could prevent a missed diagnosis.
Drug-induced microscopic colitis: the medication connection
One of the most underappreciated aspects of microscopic colitis is its association with commonly prescribed medications. Multiple drug classes have been linked to the development of microscopic colitis, and this connection is frequently overlooked during IBS evaluations.
- Proton pump inhibitors (PPIs). Omeprazole, lansoprazole, esomeprazole, and other PPIs are among the most commonly implicated medications. A systematic review by Beaugerie and Pardi (2012) identified PPIs as having a strong association with microscopic colitis. Given that PPIs are among the most prescribed medications globally, this represents a large at-risk population.
- Nonsteroidal anti-inflammatory drugs (NSAIDs). Ibuprofen, naproxen, aspirin, and other NSAIDs are well-documented triggers. The mechanism is thought to involve disruption of the mucosal barrier and altered immune regulation in the colon.
- Selective serotonin reuptake inhibitors (SSRIs). Sertraline and other SSRIs have been associated with microscopic colitis in case reports and observational studies. Because SSRIs are sometimes prescribed for IBS itself (for gut-brain axis modulation), there is an ironic scenario where treating presumed IBS with an SSRI could actually be causing or worsening an undiagnosed microscopic colitis.
- Other implicated medications. Statins, H2 receptor antagonists (ranitidine, famotidine), acarbose, ticlopidine, and checkpoint inhibitor immunotherapies have all been associated with microscopic colitis in published literature.
Drug-induced microscopic colitis has an important clinical implication: in many cases, discontinuing the offending medication can lead to resolution of the colitis without requiring budesonide therapy. This makes medication review an essential step in any patient with chronic diarrhea, whether the working diagnosis is IBS or microscopic colitis. The temporal relationship matters. If diarrhea began or worsened within weeks to months of starting a new medication, drug-induced microscopic colitis should be considered.
Why budesonide response is diagnostically important
Budesonide is an oral corticosteroid that works locally in the gut with limited systemic absorption. It suppresses the mucosal inflammation that defines microscopic colitis. The response rate is striking: more than 80% of microscopic colitis patients achieve clinical remission (defined as fewer than 3 stools per day without watery stools) within 6-8 weeks of starting budesonide at 9 mg daily.
This response rate is so high and so specific to microscopic colitis that the treatment outcome itself provides diagnostic information. IBS-D does not respond to budesonide in any meaningful way because there is no mucosal inflammation for the drug to suppress. If a patient with chronic watery diarrhea improves dramatically on budesonide, the clinical picture strongly supports microscopic colitis regardless of whether biopsies have been obtained. Some clinicians use this as a therapeutic trial when clinical suspicion is high but histological confirmation is pending or equivocal.
Conversely, if budesonide produces no improvement, microscopic colitis becomes much less likely, and the IBS-D diagnosis gains support. This bidirectional diagnostic value makes budesonide response one of the most useful clinical data points in distinguishing these two conditions.
Clinical scenarios where the misdiagnosis happens
- The normal colonoscopy without biopsies. A patient with chronic diarrhea undergoes colonoscopy. The colon looks normal. No biopsies are taken. The report reads 'normal examination.' The patient is diagnosed with IBS-D. Microscopic colitis was never tested for.
- The PPI patient. A patient on long-term omeprazole for reflux develops chronic diarrhea. The diarrhea is attributed to IBS or a medication side effect. Neither the clinician nor the patient connects the PPI to a possible inflammatory trigger. Biopsy is not pursued.
- The younger patient. A 35-year-old woman with chronic diarrhea is considered too young for microscopic colitis (typical demographic is women over 50). Biopsies are not taken because the clinical suspicion is low. However, microscopic colitis occurs across all adult age groups.
- The partial diet responder. A patient improves somewhat on a low-FODMAP diet and is considered to have confirmed IBS-D. The partial improvement is consistent with both IBS-D and microscopic colitis (reduced dietary irritants may modestly reduce colitis symptoms). The underlying inflammation remains undiagnosed and untreated.
What to ask your doctor
If you have been diagnosed with IBS-D and are not improving, or if you have chronic watery diarrhea that has been attributed to IBS, several questions can help clarify whether microscopic colitis has been properly excluded.
- "Were random biopsies taken during my colonoscopy, even though the colon looked normal?" If the answer is no, microscopic colitis has not been ruled out.
- "Could any of my medications be causing microscopic colitis?" Specifically mention PPIs, NSAIDs, and SSRIs if you are taking them.
- "Would a budesonide trial be appropriate to see if this is an inflammatory process rather than IBS?" This is a reasonable question when clinical suspicion exists.
- "Should I have a repeat colonoscopy with random biopsies to specifically evaluate for microscopic colitis?" If your original colonoscopy did not include biopsies and your symptoms persist, this is an evidence-based next step.
Frequently Asked Questions
How often is microscopic colitis misdiagnosed as IBS?
Precise misdiagnosis rates are difficult to establish because the diagnosis depends on biopsies that are not always taken. However, given that microscopic colitis accounts for 10-15% of chronic watery diarrhea cases and that random biopsies are only performed in 30-50% of appropriate colonoscopies, a substantial number of cases are likely missed and mislabeled as IBS-D.
Can stopping a medication cure microscopic colitis?
In drug-induced cases, discontinuing the offending medication can lead to complete resolution of the colitis. This has been documented with PPIs, NSAIDs, and SSRIs among others. Resolution typically occurs within weeks to months of stopping the drug. However, not all cases are drug-induced, and those with idiopathic microscopic colitis usually require budesonide therapy.
Is a budesonide trial safe to try without a biopsy-confirmed diagnosis?
Budesonide is generally well-tolerated for short-term use. Some clinicians will prescribe a therapeutic trial when clinical suspicion for microscopic colitis is high but biopsy is pending or has not been performed. However, biopsy remains the gold standard for diagnosis, and a budesonide trial should not permanently replace histological confirmation. Discuss the approach with your gastroenterologist.
Can microscopic colitis come and go, mimicking IBS flares?
Yes. Microscopic colitis can follow a relapsing-remitting course, with periods of active diarrhea alternating with periods of relative calm. This pattern can closely mimic IBS flares, further complicating the differential diagnosis. The key difference is that microscopic colitis flares respond predictably to budesonide, while IBS flares do not.
Should I request biopsies if I am having a colonoscopy for any reason?
If you have chronic diarrhea, requesting random biopsies during any scheduled colonoscopy is reasonable and evidence-based. If your colonoscopy is for a different indication (cancer screening, polyp surveillance) and you do not have diarrhea, random biopsies for microscopic colitis are not typically indicated.
â ī¸This article is for educational purposes only and is not a substitute for professional medical advice, diagnosis, or treatment. Always consult a qualified healthcare provider with questions about a medical condition.