That's a really big deal, she says, because that means those patients lost critical time on the waiting list for a new kidney.
"The more time you spend on a waitlist, if you're waiting for a kidney, the more likely you are to get a kidney," Mendu says.
Given these existing disparities, Mendu says, kidney specialists should rethink the use of the race correction.
Dr. Neil Powe, a kidney specialist at UCSF, is among those who caution against abandoning race in estimating GFR too quickly. He points out that racial disparities in outcomes for patients with chronic kidney disease existed long before the use of race to calculate kidney function became widespread two decades ago. And in the study from Mendu and her colleagues, he notes that 80% of the Black patients who did have an estimated GFR of 20 or lower with the race correction were not referred for a kidney transplant, either.
Powe says that this suggests the race-based equations "are a smaller part of what's causing disparities in African Americans getting waitlisted [for a kidney transplant]. And those other things that cause disparities could be racism as well."
First, do no harm
The use of race-adjusted algorithms to estimate kidney filtration rates dates back to 1999. It was based on a study that included 1,304 white people and 197 Black people. The researchers found that, on average, Black people in the study had higher kidney filtration rates than white people at the same blood creatinine concentrations, suggesting that the formula then used to estimate GFR was underestimating kidney function in Black people. So the authors introduced a race correction to the formula that better fit the data.
Another much larger study, published in 2009, revised the equation used to estimate GFR but also found that it more accurately reflected measured GFR when it adjusted values for Black patients.
Dr. Vanessa Grubbs, a kidney specialist at UCSF and a longtime critic of using a race-adjusted GFR, says the originators of race adjustments in GFR algorithms were unable to explain why Black people might produce and clear creatinine from their bodies differently than white people do.
"The suggestion that Black people and only Black people are different than every other human on the planet is just ludicrous," she says.
One explanation that is often cited is muscle mass, since creatinine is a breakdown product of muscles. In the 1999 study, the authors observed that three previous studies had found Black people on average have greater muscle mass than white people. But as Grubbs noted in a recent paper, those studies, now decades old, were small and did not measure muscle mass directly. Yet the use of race as a proxy for muscle mass reinforces the notion that "black bodies are biologically different than white ones," she wrote.
Mendu notes that in the 2009 study, the majority of all patients — not just Black ones — had a sizable difference between their measured and estimated GFRs, suggesting that GFR is an imprecise variable.
And she says that race itself is a poor marker of biological difference. "We know that there is more diversity within Black patients than there is genetic diversity between a Black person and a white person," she says. "To say that somebody being Black is somehow a monolithic thing when it comes to genetics, when it comes to ancestry, I think is challenging."
How, she asks, would you apply the race adjustment to someone biracial, like former President Barack Obama?
A movement for change
The debate in kidney medicine comes amid a broader examination of the use of race in clinical diagnostics in the medical community. That reckoning has been brought about in large part by medical students at institutions across the country who have questioned the scientific evidence justifying the use of race in diagnostic formulas and whether that might be perpetuating inequalities.
"From our social science and genetics faculty, we were getting a message loud and clear: Race is a social construct, and it's not a reliable proxy for genetic difference. And then, on the other hand, our clinical faculty were turning around and teaching us that race is being used as a proxy for genetic difference every day in clinical medicine," says Dr. Leo Eisenstein, a second-year medical resident at New York University and Bellevue hospitals.
As a medical student at Harvard University, Eisenstein was part of a coalition of students whose research and lobbying efforts convinced Beth Israel Deaconess Medical Center in Boston to abandon the use of race in GFR in 2017. The students zeroed in on race-adjusted estimated GFR, he says, in part because it seemed to be systematically correcting Black patients to a healthier level in a way that might be less protective.