eGFR in muscular men: what the equation gets wrong

The CKD-EPI equation assumes an average build. Your creatinine, age and sex go in. There is no field for muscle mass. That is exactly the problem in a trained body.

Creatinine is a waste product of muscle tissue. More muscle means more production per day. The equation does not know that and assumes a fixed rate. It therefore reads more creatinine as less filtration.

At 100 kilos with a low body fat percentage this adds up. A result of 75 to 80 is then not unusual. The newer EKFC equation does not solve this. It starts from the same creatinine.

Three habits stack up in strength athletes. Creatine supplementation raises creatinine measurably. A protein-rich diet with plenty of meat does the same. A heavy leg session adds extra muscle turnover. All three push the calculated value down without touching the kidney.

There is an honest limit to that. Anabolic steroid use is a different story. It genuinely can burden the kidney, partly through blood pressure. The reassurance that it is just my muscle mass no longer holds there.

A high value also deserves attention. Sustained hyperfiltration accompanies early diabetes and excess weight. That is a risk signal rather than an achievement.

Book the draw at least 48 to 72 hours after a heavy session. Arrive well hydrated and skip the big meat meal the night before. That way you measure your baseline and not your training week.

If you use creatine, consider a two-week pause before a kidney work-up. Discuss that with your doctor and never stop prescribed medication yourself.

If the value stays low under clean conditions, ask for cystatin C. It does not come from muscle tissue and settles the argument. Always have a persistently low result assessed by a doctor.