Stanford scientists say colorblindness may hide a deadly bladder cancer warning
Seeing blood in urine is often the first clue that something is wrong. For many people, noticing this symptom prompts a medical visit that can lead to an early diagnosis of bladder cancer. However, for people with colorblindness, detecting that warning sign may be far more difficult. Because many forms of color vision deficiency make it hard to perceive red tones, blood in urine can go unnoticed.
Researchers from Stanford Medicine and collaborating institutions report that missing this early symptom may have serious consequences. After analyzing health records, the team found that people who had both bladder cancer and colorblindness faced a 52% higher mortality rate over 20 years compared with bladder cancer patients who have normal vision.
The researchers believe that people with colorblindness may delay seeking care because they fail to recognize blood in their urine. As a result, the disease may be diagnosed at a later stage, when treatment is more difficult and survival rates are lower.
“I’m hopeful that this study raises some awareness, not only for patients with colorblindness, but for our colleagues who see these patients,” said Ehsan Rahimy, MD, adjunct clinical associate professor of ophthalmology and senior author of the study published in Nature Health.
The study’s lead author is Mustafa Fattah, a medical student at Columbia University Vagelos College of Physicians and Surgeons.
Colorblindness and Cancer Risk
Colorblindness, also known as color vision deficiency, is more common than many people realize. About 1 in 12 men and 1 in 200 women are affected. The most widespread types interfere with distinguishing between red and green shades. This can create everyday challenges such as interpreting traffic lights, coordinating clothing, or judging whether meat is fully cooked.
Bladder cancer also occurs much more frequently in men, who develop the disease about four times as often as women. In 2025, roughly 85,000 Americans were expected to receive a bladder cancer diagnosis.
Earlier reports and smaller studies had already suggested that colorblindness might delay the diagnosis of certain diseases. If someone cannot easily recognize blood in stool or urine, conditions such as colorectal cancer or bladder cancer could be detected later. A 2009 study involving 200 men with bladder cancer found that those with color vision deficiency were often diagnosed at more advanced and invasive stages than those with typical vision.
Another experiment conducted in 2001 asked participants to identify which images of saliva, urine, and stool contained blood. People with normal vision correctly identified the samples 99% of the time, while participants with colorblindness were correct only 70% of the time.
These earlier findings prompted Rahimy and his colleagues to investigate whether colorblindness might ultimately affect survival in people diagnosed with bladder cancer or colorectal cancer.
Searching Millions of Health Records
To explore the question, researchers used a large research platform called TriNetX. The system compiles real time electronic health records from around the world and contains about 275 million de identified patient records.
Because the database is so large, scientists can identify groups of patients who share unusual combinations of conditions using diagnostic codes.
“The power in this type of study is the ability to curate a particular population of interest — in this case, patients who are colorblind who develop bladder cancer or colorectal cancer,” Rahimy said. “It’s unusual to have that combination, but when you’re casting a net in an ocean’s worth of data, you have a better shot at capturing a rare fish.”
From roughly 100 million U.S. patient records, the researchers identified 135 people diagnosed with both colorblindness and bladder cancer, along with 187 patients who had both colorblindness and colorectal cancer.
For each group, the team created a comparable control group of patients with the same cancer diagnosis and similar demographic and health characteristics but with normal vision.
Among bladder cancer patients, those with colorblindness had a lower chance of survival than those with normal vision. Over a 20 year period, the overall mortality risk was 52% higher for the colorblind group. (The mortality risk includes deaths from all causes.)
“That was our working hypothesis, based on the previous studies,” Rahimy said.
Why Colorectal Cancer Showed a Different Pattern
The researchers expected to observe a similar pattern among people with colorectal cancer. Instead, they found no statistically significant difference in survival between patients with and without colorblindness.
One reason may be that colorectal cancer typically presents with several early symptoms. Rahimy noted that “Blood in the stool is not the chief symptom or the most common symptom that these patients present with.”
Studies show that nearly two thirds of colorectal cancer patients first report abdominal pain, and more than half notice changes in bowel habits. By contrast, between 80% and 90% of bladder cancer patients initially notice blood in their urine without any pain.
Routine screening also plays an important role. Colorectal cancer screening is widely recommended for most people between the ages of 45 and 75, which reduces reliance on noticing blood in stool as the first sign of trouble.
“There’s much more focus on catching colorectal cancer at an early age and much more public awareness,” Rahimy said.
Why the Risk May Be Even Higher
The researchers caution that the mortality difference they observed could actually be underestimated. The study relied on standard diagnostic codes called ICD-10 codes that are recorded in electronic health records.
Many people with colorblindness never receive a formal diagnosis, which means they would be classified as having normal vision in the database.
“Most people with color vision deficiency are typically functioning fine. They don’t have any other vision issues. Many affected individuals may not even know they have it,” Rahimy said.
Raising Awareness for Patients and Doctors
The findings point to the need for additional research to better understand how color vision deficiency affects disease detection.
“This is a 30,000-foot view. When we’re seeing certain trends and things that warrant further investigation, they deserve their own more in-depth analyses or studies,” Rahimy said.
The results have already prompted conversations among medical specialists. Rahimy said urologists and gastroenterologists, including a colleague who is colorblind, told him they had never considered colorblindness as a possible factor in cancer diagnosis. Some clinicians said they might start including questions about colorblindness in screening questionnaires.
“If this study raises awareness and people read this and casually pass it along, I think it’s done its job,” Rahimy said.
For people with color vision deficiency, the findings highlight the importance of routine health checks. Doctors recommend getting a urine test during annual checkups, and some individuals may want to ask a partner or family member for help monitoring changes.
“If you don’t trust yourself to know that there’s a change in the color of your urine, it could be worth having a partner or somebody you live with periodically checking it for blood, just to make sure,” Rahimy said.
A researcher from Beaumont Health contributed to the study.
The study received funding from the National Institutes of Health (grant P30-EY026877) and Research to Prevent Blindness, Inc.
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