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In A Nutshell
- Four Signs, One Standout: A 2023 study of insurance records linked rectal bleeding, iron deficiency anemia, diarrhea and abdominal pain with later colorectal cancer diagnosed before 50, with rectal bleeding showing by far the strongest link.
- Common Signs, Modest Prediction: These complaints are frequent in the general population, and the count of signs only modestly separated future patients from people without the disease, so the list cannot work as a self-test.
- Symptoms Need A Visit, Not A Birthday: The CDC recommends routine screening starting at 45 for people without symptoms, but bleeding, low iron or abdominal and bowel problems that persist should be checked by a clinician at any age.
A little blood on the toilet paper. A stomachache that keeps coming back. A blood test that shows low iron. For most adults in their 30s and 40s, none of these sounds like cancer, and most of the time it isn’t.
But researchers studying colorectal cancer in people under 50 keep finding the same few complaints in patients’ medical histories, sometimes long before diagnosis. A large 2023 study of insurance records linked four of them to a later diagnosis: rectal bleeding, iron deficiency anemia, diarrhea and abdominal pain. Some appeared in billing records as early as two years before diagnosis. It is tempting to read that as a simple list of warning signs. What the “two years” actually measures, and how much any single sign means on its own, is more complicated.
How Was The Two-Year Window Actually Measured?
Through insurance claims, not a countdown from the moment a symptom first appeared.
The study, published in the Journal of the National Cancer Institute, came from researchers at Washington University School of Medicine in St. Louis and the University of California San Diego. It used a case-control design, in which scientists start with people who already have a disease and look backward, comparing their records with similar people who don’t. The team compared 5,075 commercially insured U.S. adults diagnosed with colorectal cancer before age 50 against 22,378 people without it, matched by age, sex, region, length of insurance enrollment and prescription coverage. The records covered 2006 through 2015, and the cancer patients averaged about 43 years old.
Of 17 signs and symptoms the team checked, these four stood out when recorded between three months and two years before diagnosis. The number of different signs a person had began to predict a later diagnosis starting 18 months beforehand. Because each date marks a billing claim, the records cannot show how long someone lived with a problem before seeing a doctor.
Only 19.3% of patients had one of the four signs first recorded during that window, while 49.3% first had one recorded within the final three months. For the earlier group, the median gap between those records and diagnosis was 8.7 months. The links were stronger for rectal cancer than for colon cancer, and stronger for younger patients.
Do All Four Signs Carry The Same Weight?
No, and the differences were large. After accounting for factors such as other health conditions, obesity, smoking and how often people saw doctors, rectal bleeding was tied to about 5.1 times the odds of a later diagnosis. Iron deficiency anemia, a shortage of healthy red blood cells caused by low iron, came next at about 2.1 times the odds, followed by diarrhea at 1.4 times and abdominal pain at 1.3 times.
Those numbers compare odds between the cancer group and the comparison group. They are not the chance that a person with a given symptom has cancer. Abdominal pain was the most common sign among cancer patients, recorded for 11.6% of them, yet it had the weakest link, partly because it also turned up in 7.7% of the comparison group.
Having more than one sign strengthened the association. One sign corresponded to 1.94 times the odds, two to 3.59 times and three or more to 6.52 times. Still, when researchers tested how well the simple symptom count sorted future patients from everyone else, it scored 0.579 on a scale where 0.5 is a coin flip and 1.0 is perfect.
An editorial published alongside the study raised a related practical problem: so many adults have abdominal pain, rectal bleeding, bowel changes or low iron that giving all of them a colonoscopy isn’t realistic.
What Does The Wider Research Show?
It adds weight to the concern without creating a universal rule. A 2024 systematic review and meta-analysis in JAMA Network Open, which pooled results from earlier studies, drew on 81 studies covering more than 24.9 million patients younger than 50.
Among people ultimately diagnosed with early-onset colorectal cancer, the review estimated that 45% first came to medical attention with rectal bleeding, 40% with abdominal pain and 27% with changes in bowel habits. Across 23 studies, the average time from when symptoms were first reported or recorded to diagnosis was 6.4 months; across 16 studies, the median was four months.
The evidence base has clear weak spots. Most of the included studies, 76 of 81, captured patients at a single point in time rather than following them forward. The studies differed so much in who they analyzed that the reviewers could not combine them into one estimate of how strongly each symptom is linked to cancer risk, and studies measured the delay to diagnosis in different ways.
When Should Someone Seek An Evaluation?
When symptoms persist, return or occur together, regardless of age. The American Cancer Society reports that colorectal cancer rates among people younger than 50 rose 2.9% per year from 2013 through 2022, even as overall rates fell by about 1% per year.
The CDC recommends routine screening beginning at 45 for most people, and it separates screening, which is for people without symptoms, from diagnostic testing, which looks for the cause of symptoms. Its symptom guidance lists blood in or on the stool, abdominal pain that doesn’t go away, changes in bowel habits and low iron in the blood, among other signs, and advises talking with a doctor about any of them. The agency also notes that other conditions can cause the same problems and that colorectal cancer doesn’t always cause symptoms.
A brief stomachache or a single bout of diarrhea is not a cancer warning. A problem that doesn’t go away is a reason for a checkup now, not at the next screening birthday.
Disclaimer: This article summarizes published research and public health guidance for general information only and is not medical advice. It combines a case-control study of US insurance claims, a systematic review and meta-analysis of mostly cross-sectional studies, and statistics and recommendations from the American Cancer Society and the CDC. None of these sources measured the chance that any one person with a given symptom has cancer, and the claims study recorded when a sign was billed, not when it began. Abdominal pain, diarrhea, rectal bleeding and anemia have many causes other than cancer. Anyone with symptoms that persist, return or occur together, or with questions about screening, should talk with a health care professional.
