ALMOST SYSTEMIC, EH?:

Kidneys are unsung heroes of the body. Their miles and miles' worth of delicate meshwork compose a complicated filtration system that metabolizes cellular waste, maintains blood pressure, and regulates acid buildup. When kidneys fail, all these essential processes stop. Water, chemicals, pollutants--everything in urine--get stuck. That can leak into the lungs and cause the body to balloon and swell. It can interrupt communication between cells. It can make a heart stop.

Knowing how well a patient's kidneys can clean their blood is important for countless medical decisions, from diagnosis of kidney disease to proper medication dosing. It's difficult to calculate kidney function quickly--doing so once required patients to store a full day's worth of urine in their refrigerators so physicians could look at what the kidneys had successfully filtered out of the body. In 1999, a major study in the Annals of Internal Medicine painstakingly measured the GFR of more than 1,500 patients to develop a model that could more easily predict how well the kidneys were working using just a rapid blood test and some math; the resulting number is known as the estimated GFR, or eGFR. The equation--which has been formally cited more than 15,000 times in the literature--was quickly folded into medical practice and remains a standard of care today.

The formula focuses on creatinine, a byproduct of muscle activity and one of the many things that the kidney is supposed to remove from blood. Generally, when creatinine levels in a patient's blood are high, it signals that the kidneys are struggling to do their job of filtering toxins; this makes creatinine levels something of a proxy for overall kidney health. But things get more complicated when you consider that people vary in shape and size, have different muscle mass in their bodies, and therefore have different amounts of creatinine in their blood to start with.

Indeed, during development of the mathematical model, authors found that overall, the population of Black patients in their study had higher levels of creatinine in their blood compared with the group of white patients. The authors reasoned this was likely because "on average, black persons have higher muscle mass than white persons." They added a race-based coefficient to correct for what they assumed must be naturally high levels of creatinine in Black patients, recommending that doctors inflate the eGFR of patients by 21 percent "if Black."

This eGFR multiplier can make Black patients with kidney disease appear healthier than they are. It means that Black patients have to reach higher levels of kidney disease before they are considered sick enough to qualify for certain treatments or interventions. The adjustment is the reason Jordan wasn't able to register on the kidney waitlist after his first transplant evaluation years ago.

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