IN A FEW WORDS

The Color of Kidneys

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Over my 30 years as a primary carephysician, I’ve grown accustomed to

reviewing the chemistry panel with my pa-tients, many of whom want to understand theirlaboratory findings and like to keep copies ofthe test results. But as an African Americanphysician with a multiethnic practice, I knowthat trouble is coming when we reach the partof this panel that reports kidney function.Beneath the creatinine results is the now-familiar GFR estimate, with the instruction tomultiply this number by 1.21 if the patient isAfrican American. And this is where myheadaches begin.

My Asian and Latino patients see the modi-fication for African Americans and ask, “Whatabout me?” They worry that they have beenleft out of the equation. My African Americanpatients ask, “Why is it different for AfricanAmericans?” They are not entirely happy thatthe modification of the estimate makes theirkidney function look better. Many of themknow that African Americans have a higherrisk of developing chronic kidney disease thanwhites do. They may have friends or familywho had kidney failure. They have seen thearticles on disparities between whites andAfrican Americans when it comes to variousaspects of kidney disease care, from preven-tive management to access to transplantation.They worry that the value for non–AfricanAmericans is the “real” number and that thevalue for African Americans is increased sothat intervention will be delayed. The suspi-cion in their tone is born of years of experiencewith separate and unequal.

I explain that the eGFR is an estimate, not ameasured value like the rest of the chemistry

panel. That because African Americans do

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have a disproportionate risk of chronic kidneydisease, there is more data for African Ameri-cans than for other ethnic groups. That creati-nine comes from muscle and that there arestudies that show that black people may have agreater muscle mass for a given weight. Usu-ally, at this point, I get a snort of disbelief fromblack folks. Why haven’t we heard about thisbefore?

That was my first reaction, too. We’vedefinitely heard that our bones are denser.Many of my patients and I are old enough toremember the days when our dense boneswere proposed as the reason so few of uslearned to swim. And yes, our bones, onaverage, are denser. But we couldn’t swim, notbecause of our dense bones, but because thepools were segregated. Before that, in the 19thcentury, there was a whole pseudoscience ofcraniometry which “proved” that blacks wereinferior because our skulls were smaller (as itturned out, they weren’t). History has taughtus to be skeptical of reports of racial dif-ferences and wary that they will be usedagainst us.

My more sophisticated patients ask, “Howdid they define African American?” In Louisi-ana, in the last century, one drop of blackblood was the legal definition of black. Recentgenetic ancestry studies suggest that AfricanAmericans, on average, have approximately20% European ancestry, but this proportion ishighly variable on the individual level. Visualcues can be misleading. A nephrologist told methat he assumed that a brown-skinned patientwas African American because his wife was,and overestimated his GFR. Where I practice,mixed-race patients are the rule, not the excep-

tion. I suspect that few clinicians could guess

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President Obama’s ancestry by looking at him.And if they could, which eGFR should theyuse? The answer is not at all clear.

The National Kidney Foundation guide-lines, which recommend that laboratories re-port and that physicians use GFR estimatesrather than serum creatinine to assess kidneyfunction, explain that the eGFR is less accuratefor patients who do not have an averagemuscle mass and standard diet, people overseventy, athletes, the malnourished, the obese,vegetarians, or people with changing kidneyfunction. Those categories include a substan-tial proportion of my patients. Given theselimitations, I wonder if the racial difference isso important that it needs to be included onevery lab slip.

On a broader level, despite the wealth ofpopulation-based information available on ra-cial or ethnic differences in various laboratoryparameters, it is not clear to me how usefulthese are for the individual patient encountersthat make up my day. African Americans tendto have higher creatine kinase levels and lowerneutrophil counts, in general, than white peopledo. (And I hope the cardiologists and hematolo-gists don’t recommend that correction factorsbe printed on every lab slip.) Yet I would notdismiss a high creatine kinase in a blackpatient with muscle pain or a low white cellcount in a black patient with anemia and

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e interpreted in a clinical context. That’s ourob as physicians. Or it used to be.

When the African American correction fac-or first appeared (a surprise to me and myommunity-based colleagues), I asked a blackephrologist what to do if the correction factorncreased the GFR to normal from belowormal. He said, “If you’re worried about aatient’s kidney function, forget the estimate.rder a 24-hour urine.” A reminder that the

linical context is always more important thansingle number.First, do no harm. When we discuss the

sefulness of screening tests for cancer, weonsider the psychological impact of false-ositive tests. Even if the eGFR correctionactor succeeds in achieving greater accuracyor kidney function reporting, it may incurnforeseen costs by singling out a group thatas faced scientific bias in the past. Separatingne ethnic group from the general populationn a laboratory slip has a psychological im-act, an impact that might sometimes out-eigh the benefit of a better estimate.

Toni Martin, MDBerkeley, California

Dr Martin is a general internist who practices in Berkeley.Address for correspondence: PO Box 5296 Berkeley, CA94705. E-mail: tmartinmail@comcast.net.© 2011 by the National Kidney Foundation, Inc.0272-6386/$36.00doi:10.1053/j.ajkd.2011.08.018

thrombocytopenia. All laboratory tests have to

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Am J Kidney Dis. 2011;58(5):xxvii-xxviii