Recognising the sight of blood in urine, the most common first sign of bladder cancer, is often the impetus that leads people to a diagnosis. But for those with colourblindness, who usually have difficulty seeing red, that warning sign is more likely to go down the toilet unnoticed.
Now, a study by Stanford Medicine researchers and collaborators has found that missing this early sign can have serious health consequences. In an analysis of health records, they discovered that people with bladder cancer who are also colourblind have a 52% higher mortality rate over 20 years than people with bladder cancer and normal vision.
The findings suggest that people with colourblindness are more likely to delay seeking care for bladder cancer because they fail to recognise blood in their urine, and that catching the cancer at later stages leads to worse outcomes.
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“I’m hopeful that this study raises some awareness, not only for patients with colourblindness, 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 Jan. 15 in Nature Health.
The lead author of the study is Mustafa Fattah, a medical student at Columbia University Vagelos College of Physicians and Surgeons.
Not seeing red
Colourblindness — also known as colour vision deficiency — is more prevalent than people realise, affecting about 1 in 12 men and 1 in 200 women. The most common forms make it difficult to distinguish red and green, which presents everyday challenges such as deciphering traffic lights, matching outfits, and judging the rawness of meat.
Bladder cancer is also more common among males, affecting men about four times as often as women. In 2025, an estimated 85,000 Americans were diagnosed with bladder cancer.
Previous case reports and small-scale studies hinted that colourblindness may delay the diagnosis of conditions such as colorectal cancer and bladder cancer because patients fail to notice blood in their stool and urine. A 2009 study of 200 male patients with bladder cancer, for example, found that those with colour vision deficiency tended to be diagnosed at a more advanced and invasive stage than peers with normal vision.
In a 2001 study, researchers showed photos of saliva, urine, and stool to participants who were colourblind or had normal vision and asked them to identify which ones contained blood. While participants with normal vision correctly identified the bloody substances 99% of the time, those who were colorblind were correct only 70% of the time.
Intrigued by these earlier findings, Rahimy’s team set out to determine whether colourblindness ultimately led to worse outcomes for people with bladder cancer and colorectal cancer.
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 colourblind 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 colourblindness and bladder cancer and 187 patients diagnosed with both colourblindness and colorectal cancer.
For each group, the platform allowed researchers to curate a well-matched control group with the same cancer diagnosis, similar demographics,s 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 colourblindness.
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 colourblindness and bladder cancer. Many colourblind people never receive a formal diagnosis, meaning they would be assumed to have normal vision in the study.
“Most people with colour 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 colour 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 colour 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.
Source: Stanford Medicine
