Seeing red in urine can be the first sign that something is wrong. Many people go to the doctor when they notice this and get an early bladder‑cancer diagnosis.
People who are colorblind often cannot see red tones. This makes it harder for them to spot blood in urine, so the warning may be missed.
Scientists from Stanford Medicine looked at millions of health records. They found that patients who had both bladder cancer and colorblindness were 52 % more likely to die within 20 years than patients with normal vision.
The researchers think the higher death rate is because colorblind patients delay seeing a doctor. They do not notice the blood, so the cancer is found at a later, harder‑to‑treat stage.
What Is Colorblindness?
Colorblindness, or color‑vision deficiency, affects about 1 in 12 men and 1 in 200 women. The most common forms make it difficult to tell red from green. This can cause everyday problems, like reading traffic lights or judging if meat is cooked.
Bladder cancer is also more common in men—about four times as many men as women get it. In the United States, roughly 85,000 new cases were expected in 2025.
Earlier, smaller studies hinted that not seeing blood in stool or urine could delay diagnosis of cancers such as colorectal or bladder cancer. One 2009 study of 200 men with bladder cancer showed that those with color‑vision problems were diagnosed at more advanced stages.
Another test in 2001 asked participants to spot blood in pictures of saliva, urine, and stool. People with normal vision were correct 99 % of the time, while colorblind participants were correct only about 70 %.
These clues led the Stanford team to ask whether colorblindness actually affects survival after a cancer diagnosis.
Mining Millions of Records
The researchers used a platform called TriNetX, which stores anonymized electronic health records from around the world—about 275 million patients.
With such a huge database, they could find rare groups, like people who have both colorblindness and bladder or colorectal cancer.
From roughly 100 million U.S. records, they identified 135 patients with both colorblindness and bladder cancer, and 187 with both colorblindness and colorectal cancer.
Each group was matched to a control group of patients with the same cancer but normal vision.
For bladder‑cancer patients, the colorblind group had a 52 % higher overall death risk over 20 years.
Why Colorectal Cancer Was Different
The team expected a similar result for colorectal cancer but did not see a significant difference.
Colorectal cancer usually shows many early signs, such as abdominal pain or changes in bowel habits. Blood in stool is not the main symptom, and regular screening between ages 45 and 75 catches many cases early.
Because of these extra clues and screening programs, missing the red color in stool matters less for colorectal cancer.
Possible Under‑estimation
The study may actually underestimate the risk. Many people with colorblindness never receive a formal diagnosis, so they are recorded as having normal vision in the database.
Most color‑vision‑deficient people function fine and may not even know they are affected.
What Doctors and Patients Can Do
The findings suggest doctors should ask about color vision when taking medical histories. Some urologists and gastroenterologists said they had never considered it before.
For people who are colorblind, routine health checks become especially important. A simple urine test during an annual exam can catch blood that the patient might not see.
Patients can also ask a family member or roommate to look at their urine color occasionally.
The study was funded by the National Institutes of Health and Research to Prevent Blindness, Inc.