eGFR: normal values and what training does to them

The eGFR value estimates how many millilitres of blood your kidneys filter per minute, scaled to a standard body surface area of 1.73 square metres. The e stands for estimated: nobody measured your filtration. The laboratory takes your creatinine, your age and your sex and puts them into an equation. The common equation is called CKD-EPI, in its 2021 version without a race coefficient; more and more European laboratories also report an EKFC result. Both equations start from the same creatinine and therefore share the same blind spot.

That blind spot is muscle mass. The equation assumes everyone produces creatinine at roughly the same rate. In a trained body that assumption fails: more muscle means more creatinine in the blood, and the equation reads that as less filtration. That is how a healthy kidney can lose a few points on paper.

If you want to know what your kidneys actually do, cystatin C is the fairer measure. That protein comes from almost all body cells and not specifically from muscle tissue, so an eGFR based on cystatin C or on the combined equation leaves muscle mass out of the calculation.

Most of the alarm around this result comes from healthy, trained people, and a value of 78 after a heavy leg week says little. Four things stack up easily in an athlete. Creatine from the supplement cupboard raises the creatinine in your blood without changing filtration. A large meat meal the night before does the same, because meat itself contains creatinine. A hard session causes extra muscle turnover. And a dehydrated draw concentrates the blood. Each of the four lowers the calculated value, and together they can sink a perfectly good kidney below 60 on paper.

That is why the trend counts for more than one number. Measure under comparable conditions, in a quiet week, well hydrated and without a meat meal the night before, and compare with your own earlier values.

A single result under 60 is not a diagnosis. Chronic kidney damage only exists once the value stays under 60 for at least three months or protein is found in the urine. The reverse also holds: a normal eGFR does not prove the kidneys are healthy, because albumin in the urine can show damage while filtration is still normal.

A high result is no performance indicator in athletes. Above 90 the equation is imprecise and many laboratories report only "greater than 90". Sustained high filtration, hyperfiltration, accompanies early diabetes and excess weight and is a risk signal rather than a compliment.

Preferably measure in a quiet week and not right after a competition or a heavy training block. Allow 48 to 72 hours after your last hard session, arrive well hydrated and skip the large meat meal the night before; then you measure your baseline and not your training week.

If you use creatine, mention it at a kidney work-up and discuss with your doctor whether a two-week pause before the draw is worthwhile. Also mention the antibiotic trimethoprim if you take it, because it raises creatinine without touching the kidney.

If the value stays low under clean conditions, repeating is the next step. One value under 60 is not a diagnosis; for that it has to persist for at least three months. If in doubt ask for cystatin C or the combined equation, because that separates muscle mass from real loss of filtration. Discuss any abnormal result with your doctor.