Creatinine: What the Test Shows and Why It Matters for Athletes
Few lab values cause as much unnecessary worry among athletes as creatinine. A muscular lifter gets routine blood work, sees a number flagged above the reference range, and suddenly wonders whether their kidneys are failing. In most cases the explanation is benign — but not always. Our editorial team explains what creatinine actually measures, what skews it in active people, and when a high result deserves a closer look.
What creatinine is
Creatinine is a waste product formed from creatine and phosphocreatine, the energy buffer stored mainly in skeletal muscle. Every day a fairly constant fraction of the body's creatine pool converts non-enzymatically into creatinine, which then enters the bloodstream.
The kidneys filter creatinine freely at the glomerulus, and a small additional amount is secreted by the tubules. Because production is relatively steady and removal depends mostly on filtration, the blood level serves as a convenient, inexpensive proxy for how well the kidneys filter.
In US laboratories serum creatinine is usually reported in milligrams per deciliter (mg/dL). Typical adult reference ranges are roughly 0.7–1.3 mg/dL for men and 0.6–1.1 mg/dL for women, although each lab sets its own interval.
The weak point is obvious once you know the biology: creatinine depends not only on the kidneys but also on how much muscle you have and how much creatine you eat or supplement. That is exactly where athletes differ from the average patient.
From creatinine to eGFR
A creatinine number on its own is rarely used to judge kidney function. Labs convert it into an estimated glomerular filtration rate (eGFR), expressed in mL/min/1.73 m². In the US, the recommended equation since 2021 is the race-free CKD-EPI formula, which uses creatinine, age, and sex.
| eGFR (mL/min/1.73 m²) | KDIGO category | Interpretation |
|---|---|---|
| ≥ 90 | G1 | Normal or high |
| 60–89 | G2 | Mildly decreased |
| 45–59 | G3a | Mildly to moderately decreased |
| 30–44 | G3b | Moderately to severely decreased |
| 15–29 | G4 | Severely decreased |
| < 15 | G5 | Kidney failure |
The equations were built on general populations with average muscle mass. When someone carries far more muscle than average, creatinine is higher for reasons unrelated to the kidneys, and eGFR is consequently underestimated. A heavily muscled athlete can be labeled "G2" or even "G3a" while having perfectly healthy kidneys.
Importantly, chronic kidney disease is not diagnosed from a single eGFR value. KDIGO guidelines require abnormalities persisting for more than three months, and they place equal weight on markers of kidney damage, especially albumin in the urine.
Why athletes often read "high"
Several factors push creatinine upward in trained people, often simultaneously:
- Greater muscle mass: more muscle means a larger creatine pool and more daily creatinine production.
- Creatine supplementation: raises the creatine pool and can lift serum creatinine modestly without harming filtration.
- High meat intake: cooked meat contains creatinine; a large steak a few hours before the draw can raise the value temporarily.
- Intense or prolonged exercise: muscle breakdown and fluid shifts after hard sessions can lift creatinine for a day or more.
- Dehydration: reduced blood volume concentrates creatinine.
Studies comparing elite athletes with sedentary people have repeatedly found higher average creatinine in athletes, particularly in power and strength sports. This is one reason some sports medicine experts argue for athlete-specific interpretation.
The International Society of Sports Nutrition position stand on creatine concluded that, in healthy people, supplementation at recommended doses has not been shown to damage the kidneys — while acknowledging that it can raise serum creatinine and therefore distort eGFR.
How to get a more accurate picture
If a result comes back flagged, the first step is to repeat the test under controlled conditions. Many sports physicians suggest avoiding strenuous exercise for 48–72 hours, skipping large meat meals the evening before, arriving well hydrated, and — if relevant — pausing creatine for a few weeks before retesting.
Cystatin C is a valuable second marker. It is produced by nearly all nucleated cells at a steady rate and is much less influenced by muscle mass. Combined creatinine–cystatin C equations, also updated in 2021, are considered more accurate and are particularly useful when muscle mass is unusual.
A urine albumin-to-creatinine ratio (UACR) is equally important. Healthy kidneys let very little albumin through; persistently elevated albumin points to kidney damage even when eGFR looks acceptable. It is also a cheap, quick test.
Blood pressure, a basic urinalysis, and, where indicated, kidney ultrasound complete the picture. A nephrologist referral is appropriate when abnormalities persist or other risk factors are present.
When a high value is a real warning
Not every high creatinine in an athlete is benign. Rhabdomyolysis — severe muscle breakdown after extreme exertion, especially in heat or with unaccustomed eccentric work — can cause acute kidney injury. Warning signs include severe muscle pain and swelling, weakness, and dark, cola-colored urine. This is a medical emergency.
Frequent use of NSAIDs such as ibuprofen or naproxen, particularly combined with dehydration, is a well-known cause of kidney stress in endurance and contact-sport athletes.
Anabolic-androgenic steroid use is another concern. Case reports and case series have described kidney damage, including focal segmental glomerulosclerosis, in long-term users, along with elevated blood pressure that strains the kidneys over time. Undisclosed use also makes lab interpretation harder for the treating doctor.
Finally, a gradual upward trend across several years of tests, rising blood pressure, or protein in the urine should never be dismissed simply because the person is muscular. Trends matter more than any single number.
Editorial Team Conclusions
Creatinine is a useful but imperfect marker of kidney function, and its weaknesses are amplified in muscular, active people.
A mildly elevated value in a lifter is often explained by muscle mass, creatine use, diet, or recent hard training. Retesting under standardized conditions and adding cystatin C and urine albumin usually clarifies the situation.
At the same time, real kidney problems do occur in athletes, especially with rhabdomyolysis, frequent NSAID use, or anabolic steroid use. Look at trends, not single numbers, and share your full supplement and medication history with your doctor.
Next, consider reading our guides to liver enzymes in strength athletes, to hematocrit monitoring, and to creatine safety myths.
References
- Inker LA, Eneanya ND, Coresh J, et al. New creatinine- and cystatin C–based equations to estimate GFR without race. N Engl J Med. 2021;385(19):1737–1749.
- Levey AS, Stevens LA, Schmid CH, et al. A new equation to estimate glomerular filtration rate. Ann Intern Med. 2009;150(9):604–612.
- Kidney Disease: Improving Global Outcomes (KDIGO) CKD Work Group. KDIGO 2012 clinical practice guideline for the evaluation and management of chronic kidney disease. Kidney Int Suppl. 2013;3(1):1–150.
- Kreider RB, Kalman DS, Antonio J, et al. International Society of Sports Nutrition position stand: safety and efficacy of creatine supplementation in exercise, sport, and medicine. J Int Soc Sports Nutr. 2017;14:18.
- Banfi G, Del Fabbro M. Serum creatinine values in elite athletes competing in 8 different sports: comparison with sedentary people. Clin Chem. 2006;52(2):330–331.
- Kashani K, Rosner MH, Ostermann M. Creatinine: from physiology to clinical application. Eur J Intern Med. 2020;72:9–14.
