A new Stanford Medicine study suggests that colorblindness may obscure one of the earliest warning signs of bladder cancer with serious consequences.
For many people, the first hint of bladder cancer is a visible one. Blood in the urine is the most common early sign and often the reason patients seek medical care. But that signal is not equally obvious to everyone. For people with colorblindness, spotting blood can be difficult, increasing the likelihood that an important warning sign is overlooked.
New research from Stanford Medicine suggests that this missed cue may have long-term consequences. After analyzing large sets of electronic health records, researchers found that bladder cancer patients with colorblindness had a 52% higher risk of death over a 20-year period compared with patients who had normal color vision.
The findings point to delayed diagnosis as a likely explanation. When blood in the urine goes unnoticed, people may wait longer to seek care, allowing the cancer to progress before treatment begins. Bladder cancer outcomes are strongly tied to how early the disease is detected.
“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 January 15 in Nature Health.
The study was led by Mustafa Fattah, a medical student at Columbia University Vagelos College of Physicians and Surgeons.
Not seeing red
Colorblindness, also known as color vision deficiency, affects far more people than is often assumed, with about 1 in 12 men and 1 in 200 women affected. The most common forms make it harder to distinguish red from green, which can complicate everyday activities and, as this study suggests, the recognition of certain medical symptoms.
That overlap is particularly relevant for bladder cancer, which disproportionately affects men and occurs about four times more often in males than females. In 2025, an estimated 85,000 Americans were diagnosed with the disease.
Previous research had already raised concerns about this connection. Smaller studies and case reports suggested that people with color vision deficiency may be diagnosed with bladder or colorectal cancer at later stages because they fail to notice blood in urine or stool. In one 2009 study of 200 men with bladder cancer, patients with colorblindness were more likely to have advanced and invasive disease at diagnosis.
Earlier experimental work also showed how large the perceptual gap can be. In a 2001 study, participants were shown images of saliva, urine, and stool and asked to identify which samples contained blood. People with normal vision answered correctly nearly all the time, while those with colorblindness missed the presence of blood in about one out of every three cases.
Taken together, these findings prompted Rahimy’s team to ask a broader question: whether differences in color perception could translate into measurable differences in survival.
Locating an unusual subset
The researchers took advantage of a research platform called TriNetX, which aggregates real-time electronic health records from around the world, providing some 275 million de-identified patient records.
The massive patient pool allows researchers to find patients with unusual sets of characteristics based on 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.”
Starting with roughly 100 million patient records from the United States, the researchers found 135 patients diagnosed with both colorblindness and bladder cancer and 187 patients diagnosed with both colorblindness and colorectal cancer.
For each group, the platform allowed researchers to curate a well-matched control group with the same cancer diagnosis, similar demographics, and other health characteristics, but with normal vision.
They found that among people diagnosed with bladder cancer, those who were colorblind indeed had lower survival probability than those with normal vision. Over 20 years, those who were colorblind had a 52% higher overall mortality risk. (The mortality risk includes deaths from all causes.)
“That was our working hypothesis, based on the previous studies,” Rahimy said.
Different for colorectal cancer
The researchers expected a similar effect among people with colorectal cancer, but they found no statistically significant difference in survival among people with or without colorblindness.
Colorectal cancer often has other early symptoms, Rahimy said. “Blood in the stool is not the chief symptom or the most common symptom that these patients present with.”
A study of colorectal cancer found that nearly two-thirds of patients initially complain of abdominal pain and over half notice a change in stool habits. In contrast, 80% to 90% of patients with bladder cancer first notice blood in their urine without pain.
Moreover, the prevalence of colorectal cancer screening — recommended for most people from ages 45 to 75 — makes noticing blood in the stool less essential for a timely diagnosis.
“There’s much more focus on catching colorectal cancer at an early age and much more public awareness,” Rahimy said.
The new study — which relies on standard diagnostic codes, known as ICD-10 codes, entered into electronic health records — may be undercounting the deaths among those with colorblindness and bladder cancer. Many colorblind people never receive a formal diagnosis, meaning they would be assumed to have normal vision in the study.
“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.
Awareness is the goal
The new findings highlight the need for a closer look. “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.
He has already heard from urologists and gastroenterologists — including a colleague who is colorblind — that they had never considered colorblindness as a factor in cancer diagnosis. Some said they may begin to ask about colorblindness on 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 new findings are all the more reason to get a urine test at every annual checkup and, perhaps, to ask a favor of a loved one.
“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.
Reference: “Impact of colour vision deficiency on bladder and colorectal cancer survival” by Mustafa Fattah, Amer F. Alsoudi, Prithvi Mruthyunjaya and Ehsan Rahimy, 15 January 2026, Nature Health.
DOI: 10.1038/s44360-025-00032-7
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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