Introduction: Who Should Undergo Diagnostics

If you or someone you know is receiving cancer treatment with immune checkpoint inhibitors—medications that help the immune system fight cancer—it’s important to be aware of potential digestive problems. Immune-mediated enterocolitis should be suspected in anyone being treated with these powerful drugs who develops new bowel symptoms, particularly diarrhea or abdominal pain.^[1]

The timing of symptoms matters. Most cases of immune-mediated enterocolitis appear around 6 to 8 weeks after starting treatment with immune checkpoint inhibitors, though symptoms can show up as early as the first week or as late as six months after beginning therapy.^[3][5] This means you need to remain watchful throughout your cancer treatment and beyond.

You should seek diagnostic testing if you experience an increase in bowel movements compared to your normal pattern. For example, if you usually have one bowel movement daily but suddenly find yourself going five times a day, this warrants attention.^[5] Other warning signs include diarrhea that may contain mucus or blood, abdominal pain, cramping, bloating, fever, nausea, vomiting, or a persistent feeling of needing to empty your bowels even when there’s nothing to pass.^[2][4]

People with certain risk factors may need to be especially vigilant. If you have a history of inflammatory bowel disease, such as Crohn’s disease or ulcerative colitis, your risk of developing enterocolitis while on immune checkpoint inhibitors can be significantly higher—as much as 42% even with lower-risk treatments.^[6] The type of cancer treatment also matters, as combination therapies using different types of checkpoint inhibitors carry higher risks than single-drug treatments.

Classic Diagnostic Methods

When immune-mediated enterocolitis is suspected, doctors use a combination of approaches to confirm the diagnosis and rule out other causes of bowel inflammation. The diagnostic process begins with a careful review of your medical history, focusing on when symptoms started, their severity, and what cancer treatments you’ve been receiving. This history helps doctors distinguish immune-mediated enterocolitis from other similar conditions.^[6]

Laboratory Testing

Blood tests are typically among the first diagnostic steps. Your doctor will likely order a complete blood cell count and a comprehensive metabolic panel to check your overall health status. These basic tests can reveal signs of inflammation or complications but aren’t specific to immune-mediated enterocolitis.^[5] More specialized blood tests may include measuring C-reactive protein (a protein that increases when inflammation is present in the body) and erythrocyte sedimentation rate (which measures how quickly red blood cells settle at the bottom of a test tube—a faster rate suggests inflammation).^[5]

Stool studies are essential to rule out infections that could be causing your symptoms. Before diagnosing immune-mediated enterocolitis, doctors must exclude common bacterial infections, viruses, parasites, and particularly Clostridioides difficile (commonly called C. diff), a bacterium that causes severe diarrhea.^[3][7] Your doctor may also test for lactoferrin or fecal calprotectin, which are proteins found in stool that indicate intestinal inflammation.^[7]

Endoscopic Examination

A colonoscopy is the most important diagnostic tool for confirming immune-mediated enterocolitis. During this procedure, a flexible tube with a camera is inserted through the rectum to view the inside of your large intestine and part of your small intestine. This allows doctors to see inflammation directly and determine how severe it is.^[3][4]

The appearance of the intestinal lining during colonoscopy can vary considerably. In some cases, doctors observe ulcers—open sores in the intestinal wall—along with redness, swelling, tissue that bleeds easily when touched, and areas covered with inflammatory material.^[6] These findings can closely resemble inflammatory bowel disease, making it challenging to distinguish between the two conditions based on appearance alone.

Not all cases of immune-mediated enterocolitis show dramatic changes during colonoscopy. Some patients have what’s called microscopic colitis, where the intestinal lining looks completely normal to the naked eye, but inflammation is visible when tissue samples are examined under a microscope.^[6][11] This is why taking biopsies—small tissue samples—during colonoscopy is standard practice, even when the intestines appear relatively normal.

The tissue samples collected during colonoscopy are sent to a laboratory where they’re stained and examined under a microscope. This histologic examination reveals the specific type and pattern of inflammation, helping to confirm the diagnosis and rule out other conditions. In immune-mediated enterocolitis, pathologists may see various patterns of inflammation, including those consistent with lymphocytic or collagenous colitis.^[6]

Imaging Studies

In certain situations, your doctor may order imaging tests such as a computed tomography (CT) scan of the abdomen. These scans create detailed cross-sectional images of your abdomen and can help identify complications such as bowel perforation (a hole in the intestinal wall) or severe swelling.^[4] Imaging is particularly useful when symptoms are severe or when there’s concern about complications that might require urgent intervention.

Symptom Severity Grading

Once immune-mediated enterocolitis is confirmed, doctors categorize the severity using a standardized system called the National Cancer Institute’s Common Terminology Criteria for Adverse Events. This system divides cases into grades from 1 to 5 based on how many bowel movements you’re having per day compared to your baseline, whether there’s blood in the stool, and how much the symptoms interfere with daily activities.^[3][4]

Grade 1 represents mild symptoms with less than four extra bowel movements per day. Grade 2 involves four to six extra movements daily or blood in the stool. Grade 3 indicates seven or more extra movements, significant bleeding, or inability to care for yourself. Higher grades are associated with life-threatening complications or death.^[3] This grading system not only helps with diagnosis but also guides treatment decisions.

Diagnostics for Clinical Trial Qualification

When patients with cancer are being considered for clinical trials involving immune checkpoint inhibitors, specific diagnostic criteria help determine eligibility and monitor for side effects like immune-mediated enterocolitis. These standardized tests ensure patient safety and allow researchers to collect consistent data across different study sites.

Before enrolling in clinical trials using immune checkpoint inhibitors, patients typically undergo baseline gastrointestinal assessments. These initial evaluations establish a starting point for comparison should symptoms develop later. Baseline testing often includes standard blood work, stool tests to document the absence of infections, and sometimes questionnaires about bowel habits and frequency of bowel movements.^[3]

Clinical trials employ the same grading system used in routine practice—the National Cancer Institute’s Common Terminology Criteria for Adverse Events—to classify the severity of any enterocolitis that develops. This standardized approach allows researchers to compare safety data across different trials and treatment regimens.^[3][4] Trial participants who develop grade 2 or higher gastrointestinal symptoms typically require colonoscopy with biopsy to confirm the diagnosis and assess severity before treatment modifications can be made.

In some clinical trials, patients with pre-existing inflammatory bowel disease may face additional screening requirements or even exclusion from participation, given their higher risk of experiencing enterocolitis flare-ups. When these patients are included, trials may require more frequent monitoring with stool inflammatory markers and more rigorous baseline endoscopic evaluations.^[6]

Ongoing monitoring throughout clinical trials includes regular symptom assessments, often through patient-reported outcome measures and scheduled laboratory testing. Participants are educated about warning signs and instructed to report changes in bowel habits immediately, as early detection of immune-mediated enterocolitis is crucial for preventing serious complications and maintaining patient safety while preserving the potential benefits of cancer treatment.^[5]

Some research protocols incorporate investigational diagnostic approaches to better understand immune-mediated enterocolitis. These may include analysis of the gut microbiota—the community of bacteria living in the intestines—or measurement of specific immune markers in blood or stool samples. While these tests are primarily for research purposes, they may eventually lead to better ways of predicting who will develop enterocolitis and how severe it might become.^[2]