When to Go to the ER for a UC Flare (Red Flags + What They’ll Check)
Last Updated Jan 15, 2026
Ulcerative colitis (UC) flares can range from uncomfortable to dangerous. Deciding whether an ulcerative colitis flare needs the ER can feel stressful, especially when symptoms change quickly. This guide shares common UC flare emergency symptoms that many clinicians treat as “red flags,” plus what emergency teams often check and simple ways to describe severity. This is general education, not medical advice, and it cannot replace care from a licensed clinician.
Red flags that often warrant ER evaluation during a UC flare
A UC flare may need urgent evaluation when there are signs the body is struggling with bleeding, dehydration, infection, or a serious complication. Severe flares can include frequent bowel movements plus whole body symptoms like fever and a fast or irregular heartbeat, which can be a sign the illness is affecting more than the colon. [1]
Common red flags in an ulcerative colitis flare ER situation include severe bleeding UC (large amounts of blood, passing clots, or blood with weakness or shortness of breath), dehydration ER UC concerns (very little urination, dizziness, fainting, or inability to keep fluids down), and fever UC flare symptoms (especially when paired with worsening diarrhea or belly pain). Persistent belly pain, ongoing diarrhea that does not improve, and an unexplained fever lasting more than a day or two are also warning signs that should be assessed by a healthcare professional. [2]
Some complications are uncommon but serious. One example is toxic megacolon, a severe complication linked with UC that can include belly swelling or distension, fever, rapid heart rate, constipation, and dehydration, and it requires immediate treatment. [3] Another reason emergency teams may act quickly is concern for problems like perforation (a hole in the colon) or severe blood loss, which are considered emergencies. [4]
What the ER will check, what triage looks for, and how to communicate severity clearly
In the ER, triage usually focuses first on safety: vital signs (temperature, blood pressure, oxygen level, and high heart rate UC concerns), mental alertness, visible blood loss, and signs of dehydration. From there, the team typically tries to answer a key question: is this “just” inflammation from UC, or is there also infection, anemia from blood loss, electrolyte imbalance, or a complication that needs immediate treatment.
Testing often includes bloodwork (for anemia, inflammation, kidney function, and electrolytes), stool testing to look for infection, and sometimes imaging if a complication is suspected. In hospital evaluations for severe UC, baseline tests commonly include a complete blood count, C-reactive protein (a marker of inflammation), electrolytes and other chemistry tests, stool cultures including a Clostridioides difficile (C. difficile) test, and imaging such as an abdominal X-ray or computed tomography (CT). [5] Separately, professional guidelines recommend stool testing to rule out C. difficile when ulcerative colitis is suspected, since infection can mimic or worsen a flare. [6]
Clear communication can help the team understand urgency. Helpful details to share include: bowel movement count in the last 24 hours, how much blood is present and how often, highest temperature measured and when, heart rate trends (from a monitor if available), ability to drink and urinate, vomiting, belly swelling, current UC medications and last doses, recent antibiotics, and any prior hospitalizations for UC.