Color blindness linked to lower bladder cancer survival, early study hints

Could being colorblind make you less likely to survive bladder cancer? That's the surprising hypothesis that researchers have proposed based on a small study.

The research, published Jan. 15 in the journal Nature Health, examined data from 135 patients with both bladder cancer and color blindness, and compared those patients to 135 patients with only bladder cancer. The data were taken from TriNetX, an international registry of electronic health records of more than 275 million patients.

Among these health records, people who had both color blindness and a bladder cancer diagnosis had shorter survival times than did bladder cancer patients without the vision deficiency. Overall, those with color blindness had a 52% higher risk of dying within 20 years of their bladder cancer diagnosis, compared with the normal-vision group.

The study authors suggested a plausible reason for this observed difference: Color blindness may make it more difficult to spot blood in your urine — an early sign of the cancer — thereby delaying diagnosis.

"Bladder cancer is a bad disease. If you delay your diagnosis, it will make a difference to your prognosis," Dr. Veeru Kasivisvanathan, a urological oncologist and surgeon at University College London who was not involved in the study, told Live Science.

A possible link

Blood in urine is one of the most common early symptoms of bladder cancer, alongside frequent urination; pain or burning during urination; feeling as if you need to urinate even if your bladder isn't full; and urinating frequently during the night.

If anyone spots blood in their urine, they should see their doctor straight away, Kasivisvanathan said. But, as the study authors suggested, being unable to clearly distinguish red from yellow could make it very difficult to spot this early warning sign.

Color blindness, also known as color vision deficiency, is a fairly common condition with one recent study reporting that about 1 in 40 people globally have some form of color vision deficiency. (Those figures are likely approximate, as screening for color vision deficiency is often not routine.) Color vision deficiency tends to be more common in males than in females, per the study.

The results of the new study should be taken with extreme caution, Kasivisvanathan and Shang-ming Zhou, a professor in e-health at the University of Plymouth who wasn't involved in the work, told Live Science. Indeed, the study authors also acknowledged that there are major limitations to their research.

For instance, because color blindness often goes undiagnosed, it's possible that some people with the condition were mistakenly added to the cohort without color blindness in the analysis, potentially muddying the results. The term "color blindness" also encompasses various conditions with different red-perception abilities. Protanopia (red-blindness) should theoretically carry a higher risk than deuteranopia (green-blindness) in this context, but the study cannot differentiate between these subtypes, said Zhou.

In addition, the study was very small, which makes the results less dependable, and makes it difficult to screen for other factors that could explain the difference in prognosis. Lastly, from these data alone, it's not possible to prove that color blindness delayed the diagnosis of the disease; for now, that is just a hypothesis.

"The authors properly frame this as the hypothesis-generating work," Zhou said. "Current evidence is insufficient to recommend routine blood cancer screening in [patients with color vision deficiency], and the absolute risk increase remains unclear," he emphasized.

In short, more research is needed to confirm that color blindness raises the risk of death from bladder cancer, and to evaluate how those patients might be better protected, if that's the case. Still, this is "the right type of [study] design for that type of question," Kasivisvanathan said, adding that while the research is not conclusive, it does open up interesting areas for investigation.

It could be that patients with known risk factors for bladder cancer — such as being a male over the age of 50, smoking, using blood thinners, or having a history of radiotherapy — might benefit from being warned about the potential risk of having undiagnosed color blindness on top of their other risk factors. And perhaps those with known color blindness and cancer risk factors could be encouraged to screen their urine in other ways, such as using test strips, Kasivisvanathan said.

This study also raises questions about other cancers that are associated with blood in bodily fluids in their early stages, such as oral cancers, Zhou added. But for now, more research is needed, all of the experts said.