eGFR

Your kidneys filter about a tenth of a litre of blood every minute through millions of tiny sieves, take waste products and surplus water out of it and give the rest back. That filtration rate is the true measure of how well your kidneys work, but measuring it directly takes a complicated test. So it is estimated: from your creatinine, the muscle waste the kidneys have to filter away at a fixed pace, corrected for your age and your sex, which say how much muscle someone has on average. That is the eGFR, the estimated filtration rate, in millilitres per minute for a body of standard size. Because the estimate rests on creatinine, it inherits everything that affects creatinine. Plenty of muscle mass, a hard training session, a meat meal, dehydration and creatine lift creatinine and lower the eGFR without anything wrong with the kidneys; little muscle does the reverse and presents the kidneys as better than they are. And the formula calculates with an average body: in someone much larger or smaller than average, the estimate is off in the same direction. One value is therefore an estimate with a margin; the trend over several measurements is the real number. Your report shows the eGFR in ml/min/1.73m². Healthy is 60 ml/min/1.73m² or higher, and at a young age usually above 90. With the years it falls by itself, about a point a year after your forties, and a value below 60 ml/min/1.73m² that lasts three months or longer is called reduced kidney function.

Kidneys decline silently. They have so much reserve that you notice nothing of losing half, and symptoms only come when little is left. The eGFR makes that decline visible years earlier, and that is the reason it is calculated with every blood test. High blood pressure, a high blood sugar and atherosclerosis are the biggest damagers, and anyone with one of those three follows the eGFR every year. Visible even earlier is protein in the urine, which often precedes a fall in the eGFR by years in diabetes and high blood pressure. For anyone who is healthy, the eGFR is a number to keep high. The higher within the normal range, the more reserve your kidneys have for later. Blood pressure, blood sugar, weight, smoking and salt are the levers, and anti-inflammatory painkillers such as ibuprofen and naproxen are the pitfall: they pinch the blood vessels in the kidney shut, and with regular use, certainly beside blood pressure medicines or in heat and dehydration, they cost filtering capacity. The eGFR also counts for the dosing of many medicines, because what the kidneys have to excrete builds up when they filter less. For anyone who does sport or has a lot of muscle, the eGFR is a number to read with a pinch of salt. A bodybuilder or a creatine user sees the estimate come out ten to twenty points below their true filtration, and a slight older person sees it too high. In both cases cystatin C, a second measure that does not come from the muscles, is the way to know whether the estimate is right.

A low eGFR means your kidneys filter less, or the estimate comes out lower than your true filtration. In order of how often it occurs: reduced kidney function, and then the question belongs with it whether it has been so for three months and whether there is protein in the urine; plenty of muscle mass, which makes more creatinine with healthy kidneys; and dehydration, which lowers it temporarily. After that come a hard training session in the days before, creatine, high blood pressure or diabetes as the damagers in the long term, and medicines that slow the excretion of creatinine or lower filtration itself. Between 45 and 59 ml/min/1.73m² the kidney function is mildly to moderately reduced, between 30 and 44 ml/min/1.73m² moderately to severely, and below 30 ml/min/1.73m² a kidney specialist belongs with it. A high eGFR is no problem in itself, but can present the kidneys as better than they are. Little muscle mass is the most common reason: little creatinine gives a too favourable estimate, especially in older people and after long illness. In pregnancy the kidneys really do filter harder. Rare are hyperfiltration, in which the kidneys temporarily work too hard early in diabetes and that precedes damage, and a severe liver disease, in which the liver no longer makes the raw material of creatinine. A high eGFR in someone with little muscle is a reason to read cystatin C alongside, not to be reassured. Always read the eGFR beside the creatinine it comes from and beside your muscle mass, beside protein in the urine, which responds earlier, beside potassium and sodium, which say whether the kidneys are still doing their job, and with doubt beside cystatin C. The trend over the years says more than one value.