Study finds link between colorblindness and death from bladder cancer

In a 2001 study, researchers showed photos of saliva, urine and stool to participants who were colorblind 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 colorblindness 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 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.

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.