Color blindness hides a key warning sign of bladder cancer

For most people with bladder cancer, the first red flag is literally red: blood in the urine. But for people with color blindness, that warning sign may be easy to miss — and missing it could prove deadly.

An analysis of electronic health records from more than 275 million people worldwide found that individuals who developed bladder cancer had significantly worse survival outcomes if they also had color vision deficiency, researchers report January 15 in Nature Health.

Comparing bladder cancer patients of similar ages, genders and general health, the team found that about half of the 135 color-blind patients analyzed were still alive 10 years after diagnosis compared with roughly three-quarters of 135 matched individuals with normal color vision.

Over the entire 20-year study period, the patients with color blindness faced a 52 percent greater risk of death than otherwise comparable individuals. The probable explanation for the survival gap, says bioengineer Mustafa Fattah of Columbia University, isn’t any inherent difference in cancer biology, but a sensory blind spot. “Impaired perception of red is really the driving force here.”

The first noticeable symptom of bladder cancer is typically painless blood in the urine. If someone can’t easily distinguish red from other colors, they may not recognize that something is wrong and delay seeing a doctor, increasing the chances that the cancer will be more advanced by the time it is diagnosed — and thus harder to treat.

To check whether the same problem affects other cancers, Fattah and his colleagues looked at colorectal cancer, which can cause blood in the stool. Considering 187 patients with color blindness and another 187 without, they found no difference in survival between the two groups.

Presumably, Fattah says, that is because routine screening for colorectal cancer often catches the disease before symptoms appear. Bladder cancer, by contrast, has no recommended screening for people without symptoms, putting more responsibility on patients to notice subtle changes themselves.

Although the data are still too limited to justify changes in screening recommendations, they could prompt clinicians to be more vigilant with color-blind individuals.

“I would keep a low threshold to investigate for bladder cancer for a color-blind patient with urinary symptoms,” says Masahito Jimbo, a family medicine specialist who studies cancer-screening practices at the University of Illinois at Chicago. He was not involved in the study but did write an accompanying commentary for Nature Health.

The potential impact of the findings is sizeable, given that both bladder cancer and color blindness are common, especially among men. Bladder cancer ranks among the most frequently diagnosed cancers in the United States, with about 80,000 new cases each year. Roughly three-quarters of patients are male.

Color blindness also skews heavily male. About 1 in 12 men has some form of color vision deficiency, compared with about 1 in 200 women. Most cases involve difficulty distinguishing reds and greens — exactly the colors needed to spot blood in urine.