Creatinine

For adult men, a typical normal serum creatinine range is roughly 0.74 to 1.35 mg/dL, though ranges vary slightly by lab. Because creatinine comes from muscle metabolism, muscular men naturally sit at the higher end of normal — or even a touch above — without any kidney problem. It's best interpreted through eGFR, which factors in age and sex, rather than the raw creatinine value alone.

No, though the names are easily confused. Creatine is a compound found in muscle and taken as a supplement to support strength and power. Creatinine is the waste product left over when creatine and muscle tissue break down, and it's what the lab measures to assess kidney function. They're related but not interchangeable.

Creatine supplementation can raise your blood creatinine modestly because more creatine in the muscle means more creatinine byproduct — but this is a measurement artifact, not kidney damage. In healthy people, standard creatine doses have not been shown to harm the kidneys. The catch is that the higher creatinine can mechanically lower a creatinine-based eGFR and make kidney function look worse than it is.

Ask for a cystatin-C-based eGFR. Cystatin C is a protein filtered by the kidneys that doesn't depend on muscle mass, so it isn't skewed by heavy training or creatine supplementation the way creatinine is. Comparing creatinine-based and cystatin-C-based eGFR gives a fairer picture of true kidney function in muscular men, and your physician can order it if your creatinine looks borderline.