A Negative CT Is Not Enough: Avoiding Missed Diagnosis of Colorectal Cancer

Summary

Colonoscopy remains the definitive diagnostic test for colorectal cancer in symptomatic patients. Failure to refer a symptomatic patient for colonoscopy is a common cause of malpractice claims, often due to a combination of cognitive bias, system failures, and misunderstanding of the limitations of CT imaging.

A 65-year-old patient presented to primary care with fatigue, vague abdominal discomfort, and intermittent rectal bleeding. The patient’s history included a routine screening colonoscopy at age 50 that was negative other than for diverticulosis and internal hemorrhoids and multitarget stool DNA testing (Cologuard®) at age 60, which was negative. Laboratory studies revealed mild iron-deficiency anemia. A CT scan of the abdomen and pelvis was obtained and interpreted as normal except for diverticulosis. The patient was treated conservatively for presumed hemorrhoidal bleeding. Nine months later, the patient presented to the emergency department with symptoms of a bowel obstruction. Colonoscopy revealed a near-obstructing sigmoid adenocarcinoma. Pathology demonstrated stage III colorectal cancer.

This scenario represents a common pathway leading to malpractice claims involving gastrointestinal disease: failure to refer a symptomatic patient for colonoscopy after reassuring imaging. The error is rarely a lack of clinical knowledge. More commonly, it reflects a combination of cognitive bias, system failures, and misunderstanding of the limitations of CT imaging.

The Core Problem: CT Does Not Exclude Colorectal Cancer

Routine contrast-enhanced CT scans are frequently ordered for abdominal pain, anemia, and other nonspecific gastrointestinal symptoms. When interpreted as “normal,” they often create a false sense of reassurance for both physicians and patients. However, routine CT imaging is not designed to evaluate colonic mucosal disease and should not be considered sufficiently sensitive to exclude colorectal neoplasia, particularly early-stage, flat, or non-obstructing lesions.^1,2

Colonoscopy remains the diagnostic gold standard because it allows direct visualization of the colonic mucosa, detection of small lesions, tissue biopsy, and therapeutic intervention.^3,4

Why These Misses Occur

The following cognitive errors contribute to missed colorectal cancer diagnoses.

Anchoring bias. Once an imaging study appears reassuring, clinicians often anchor to a benign explanation such as hemorrhoids, diverticulosis, or irritable bowel syndrome. Future symptoms are then interpreted through that diagnostic framework rather than prompting reconsideration of malignancy. A previously negative multitarget stool DNA test may further reinforce this bias. Although stool DNA testing has high sensitivity for established colorectal cancer, sensitivity is lower for stage I cancers and substantially lower for advanced adenomas, making it fundamentally a cancer detection test rather than a cancer prevention test.^5,6,7

Attribution errors. Common benign conditions are frequently blamed for concerning findings:

• Hemorrhoids for rectal bleeding.
• Anticoagulation for iron-deficiency anemia.
• Irritable bowel syndrome for chronic abdominal pain.
• Dietary changes for unintended weight loss.

While these diagnoses are common, they remain diagnoses of exclusion in adults over age 45 with persistent alarm symptoms.^3,8

Overreliance on imaging. Many clinicians subconsciously treat CT imaging as a screening examination for colorectal cancer. It is not. CT evaluates structural abnormalities and complications but cannot reliably exclude mucosal neoplasia.^1,2

Diffusion of responsibility. The primary care physician assumes gastroenterology will arrange follow up. Gastroenterology assumes the patient will be re-referred if symptoms persist. The patient believes “the scan ruled out cancer.” The diagnostic loop never closes.

Symptoms That Require Colonoscopy Regardless of Imaging

The following findings warrant direct endoscopic evaluation even when CT imaging is normal:

• Iron-deficiency anemia in adults.
• Hematochezia or unexplained rectal bleeding.
• Positive fecal immunochemical test (FIT).
• Positive multitarget stool DNA test.
• Persistent change in bowel habits lasting longer than four to six weeks.
• Unintentional weight loss.
• Persistent abdominal pain without an alternative explanation.

Guidelines from the American College of Gastroenterology, the U.S. Preventive Services Task Force, the National Comprehensive Cancer Network, and the American Gastroenterological Association consistently emphasize that symptomatic patients require diagnostic evaluation rather than screening, with colonoscopy serving as the preferred diagnostic examination.^3,4,8,9

A Common Failure: Referral Never Happens

In malpractice reviews, many missed colorectal cancers are not due to procedural complications or lesions overlooked during colonoscopy. They occur because colonoscopy was never performed.

The typical sequence is predictable:

1. Alarm symptoms develop.
2. CT scan is ordered.
3. CT findings are interpreted as benign.
4. Symptoms are attributed to a common condition.
5. No gastroenterology referral is placed.
6. The patient is reassured.
7. Cancer is diagnosed months or years later.

The key failure is not diagnostic skill—it is premature diagnostic closure.¹⁰

Documentation That Reduces Liability

Clear communication and documentation substantially reduce medicolegal risk. Documentation should specifically include statements such as:

• CT imaging does not exclude colorectal neoplasia.
• Colonoscopy is recommended to evaluate symptoms.
• The possibility of malignancy was discussed with the patient.
• A specific follow-up timeframe was provided.

From a legal perspective, the absence of documentation is frequently interpreted as the absence of clinical concern.

Practical Prevention Strategies

For referring clinicians:

• Do not use CT as a rule-out test for colorectal cancer.
• Refer patients with alarm symptoms directly for colonoscopy.
• Avoid attributing iron-deficiency anemia solely to hemorrhoids without endoscopic evaluation.
• Clearly document follow-up plans.
• Remember that multitarget stool DNA testing has important limitations for advanced adenomas and should never replace diagnostic colonoscopy in symptomatic patients.^5,7

For gastroenterologists:

• Clearly state in consultation notes that CT cannot exclude mucosal disease.
• Track incomplete referrals whenever feasible.
• Educate referring clinicians regarding alarm symptoms.
• Reinforce that negative stool-based screening tests do not exclude colorectal cancer in symptomatic patients.

For health systems:

• Implement tracking systems for iron-deficiency anemia.
• Flag positive FIT and multitarget stool DNA results until colonoscopy is completed.
• Use electronic medical record alerts for unresolved diagnostic recommendations.

Closing the Loop

Most delayed colorectal cancer diagnoses are preventable. They occur when a reassuring test substitutes for definitive evaluation and when responsibility for follow up becomes ambiguous. A negative CT scan should not end the evaluation of concerning gastrointestinal symptoms. Instead, it should often increase the urgency of obtaining a colonoscopy when alarm features are present.

The most important preventive step is remarkably simple: When alarm symptoms exist, ensure colonoscopy is performed—and that the recommendation is clearly documented.

Missed colorectal cancers are rarely failures of technology. They are failures of diagnostic closure and represent one of the most preventable serious diagnostic errors in outpatient medicine.

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References

1. Johnson CD, et al. Accuracy of CT colonography for detection of large adenomas and cancers. N Engl J Med. 2008;359(12):1207-1217. https://www.nejm.org/doi/full/10.1056/NEJMoa0800996
2. Pickhardt PJ, et al. Computed tomographic virtual colonoscopy to screen for colorectal neoplasia in asymptomatic adults. N Engl J Med. 2003;349:2191-2200. https://www.nejm.org/doi/full/10.1056/NEJMoa031618
3. Shaukat A, et al. ACG Clinical Guidelines: Colorectal Cancer Screening 2021. Am J Gastroenterol. 2021;116:458-479. https://doi.org/10.14309/ajg.0000000000001122
4. National Comprehensive Cancer Network. NCCN Clinical Practice Guidelines in Oncology: Colorectal Cancer Screening. Version: 2.2026. https://www.nccn.org/guidelines/category_1
5. Imperiale TF, et al. Multitarget stool DNA testing for colorectal-cancer screening. N Engl J Med. 2014;370:1287-1297. https://www.nejm.org/doi/full/10.1056/NEJMoa1311194
6. Imperiale TF, et al. Next-generation multitarget stool DNA test for colorectal cancer screening. N Engl J Med. 2024;390:873-884. https://www.nejm.org/doi/full/10.1056/NEJMoa2310336
7. Exact Sciences. Cologuard Prescribing Information. https://www.cologuardhcp.com/resources
8. U.S. Preventive Services Task Force. Screening for Colorectal Cancer: Recommendation Statement. JAMA. 2021;325(19):1965-1977. https://www.uspreventiveservicestaskforce.org/uspstf/recommendation/colorectal-cancer-screening
9. Ko CW, et al. AGA Clinical Practice Guidelines on the Gastrointestinal Evaluation of Iron Deficiency Anemia. Gastroenterology. 2020;159:1085-1094. https://www.gastrojournal.org/article/S0016-5085(20)34847-2/fulltext
10. Croskerry P. The importance of cognitive errors in diagnosis and strategies to minimize them. Acad Med. 2003;78(8):775-780.  https://doi.org/10.1097/00001888-200308000-00003

Our thanks to J. Casey Chapman, MD. Dr. Chapman is the Chief Medical Officer of The Specialty Alliance for gastroenterology and urology specialists, a part of Cardinal Health. Dr. Chapman is a practicing gastroenterologist with Gastroenterology Associates in Baton Rouge, Louisiana. He focuses his practice and research on inflammatory bowel disease and is the medical director of the Crohn’s and Colitis Center and Inflammatory Bowel Disease Integrated Practice Unit at Baton Rouge General. He has a particular interest in establishing clinical workflows that enhance patient outcomes and experiences by leveraging quality and outcomes-based data.

The opinions expressed here do not necessarily reflect the views of The Doctors Company. We provide a platform for diverse perspectives and healthcare information, and the opinions expressed are solely those of the author.

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