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Tests for Kidney Function

March 3, 2021


“How to tell how well the kidneys are doing? These are the three easy labs to ask your doctor about. And guess what? You don’t need a nephrologist, a kidney doctor, to check these labs. You can just ask your primary doctor for it. Creatinine. This is a marker in the blood that is used to assess how well the kidneys are actually filtering. Normal creatinine is anywhere from 0.6 to about 1.2. The creatinine in the blood is actually a breakdown product, muscle tissue. But it’s used as a marker for kidney filtering. So the more creatinine you have in your blood, the more it accumulates, the worse your kidneys are filtering, generally. This number is dependent on your actual muscle mass, because it’s a breakdown of muscle tissue. So who has higher muscle mass? Well, generally males have higher muscle mass than females. Young people have more muscle mass than older people.

Blacks have higher muscle mass than other races. Athletic people have higher muscle mass than non-athletic people, and taller people have higher muscle mass than shorter people. So these types of people will generally have higher baseline creatinines in their blood. So how do you offset that and actually count for these differences? Well, there are actually two ways to calculate kidney function. Your EGFR and your creatinine clearance. EGFR. Kidney function is actually defined by a calculation called EGFR, your estimated glomerular filtration rate. Consider it like a percentage of your kidney function. In normal kidney function, it’s supposed to be over 90%. Now, one of the main factors that determine your EGFR is your blood creatinine, because it’s used in its calculation. What is fascinating is that blacks generally have a higher blood creatinine than other races. So guess what? When you get your blood test results back, there are actually two calculations for ETFR: one for African-Americans and the other is for non-African Americans.

Now, for whatever reason, the EGFR calculation can sometimes be off. Particularly in the very old, the very young or the very sick. Creatinine clearance. So the gold standard for kidney function is actually something called creatinine clearance. And this is very similar to EGFR. Normal creatinine clearance is greater than 90. But rather than being just a blood test, it’s actually based off a urine collection over a 24 hour period. Yeah, that’s right. You gotta pee into a container over 24 hours. Keep it on ice. After the urine is collected, your blood creatinine is drawn, and your weight and height are measured. This is the most accurate measure of kidney function, but it’s used less frequently because it’s kind of cumbersome to do.”

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