Can a creatinine level of 1.57, a Blood Urea Nitrogen (BUN)/Creatinine ratio of 8, and an estimated Glomerular Filtration Rate (eGFR) of 37 in a 61-year-old female with high muscle mass due to strength training and daily creatine (Creatine) intake of 4.5 grams indicate false kidney failure?

Elevated Creatinine in High-Muscle-Mass Athletes: Likely False Indication of Kidney Failure

This patient's laboratory values most likely represent a false indication of kidney failure caused by the combination of high muscle mass from strength training and creatine supplementation, rather than true renal dysfunction. The abnormally low BUN/creatinine ratio of 8 is the critical finding that strongly suggests the elevated creatinine is disproportionate to actual kidney disease 1.

Why These Values Are Likely Falsely Elevated

Muscle Mass Effect on Creatinine

• Serum creatinine is profoundly affected by muscle mass because creatinine is produced from muscle catabolism 2.
• Individuals with higher muscle mass—such as athletes performing strength training—naturally produce more creatinine, which elevates serum levels independent of kidney function 2.
• A creatinine level of 1.2 mg/dL can correspond to a creatinine clearance of 110 mL/min in a young, muscular male athlete but only 40 mL/min in an elderly woman with low muscle mass 2.
• The rate of creatinine production is directly proportional to skeletal muscle mass in stable individuals 2.

Creatine Supplementation Impact

• Creatine supplementation, particularly creatine ethyl ester, can artificially elevate serum creatinine without indicating renal dysfunction 3, 4.
• Creatine monohydrate (the most common form) at 4.5 grams daily can increase creatinine production as it is metabolized to creatinine 5.
• Creatine supplements are safe and do not cause renal disease; reports of kidney damage associated with their use are scanty 5.
• The combination of strength training and creatine supplementation can artificially elevate serum creatinine levels without indicating true renal dysfunction 1.

The BUN/Creatinine Ratio: The Key Diagnostic Clue

• The abnormally low BUN/Cr ratio of 8 strongly suggests that the elevated creatinine is disproportionate to actual renal dysfunction 1.
• In true kidney failure, both BUN and creatinine rise together, typically maintaining a ratio of 10-20:1.
• A ratio of 8 indicates creatinine is disproportionately elevated relative to BUN, which is the expected pattern when increased muscle mass and creatine supplementation drive creatinine production.

Critical Limitations of Standard eGFR Calculations

Why eGFR of 37 May Be Inaccurate

• Serum creatinine should never be used as a standalone marker of renal function and must be interpreted with caution in individuals with altered muscle mass 2.
• Standard eGFR equations (MDRD, CKD-EPI, Cockcroft-Gault) are substantially influenced by muscle mass, making eGFR both a marker of sarcopenia AND kidney function—not kidney function alone 6.
• These formulas were not validated in populations with exceptionally high muscle mass, leading to systematic underestimation of true GFR in muscular individuals 2.
• Among individuals with decreased GFR measured by gold-standard methods, 40% had serum creatinine levels within the normal laboratory range 2.

Recommended Clinical Algorithm

Immediate Actions

1. Discontinue creatine supplementation immediately 1.
2. Ensure adequate hydration, as dehydration can falsely elevate creatinine and reduce GFR 1.
3. Review for other causes of true kidney disease: diabetes, hypertension, nephrotoxic medications (NSAIDs, ACE inhibitors, aminoglycosides), vascular disease, or urinary obstruction 1.

Diagnostic Workup Within 2-4 Weeks

1. Repeat renal function testing after creatine washout (typically 2-4 weeks allows creatinine to normalize if supplementation was the cause) 1, 3.
2. Order cystatin C-based eGFR calculation, which is not influenced by muscle mass and provides more accurate assessment in individuals with altered muscle mass 1, 6.
3. Obtain urinalysis and urine protein-to-creatinine ratio to assess for proteinuria, hematuria, or other signs of intrinsic kidney disease 1.
4. Consider 24-hour urine collection for creatinine clearance, which may be more accurate than estimated equations in this population 2.

Interpretation and Follow-Up

• If cystatin C-based eGFR is >60 mL/min/1.73 m² and urinalysis is normal, true kidney disease is unlikely 1, 6.
• If eGFR remains <45 mL/min/1.73 m² after creatine washout despite normal cystatin C, refer to nephrology for comprehensive evaluation including renal ultrasound and consideration of gold-standard GFR measurement (iothalamate or EDTA clearance) 2, 1.
• If kidney function normalizes after creatine discontinuation, the patient may resume training but should avoid creatine supplementation 1, 5.

Common Pitfalls to Avoid

• Do not assume kidney failure based solely on creatinine-based eGFR in muscular individuals 2, 6.
• Do not overlook the BUN/Cr ratio—it provides critical context that standard eGFR equations miss 1.
• Do not continue creatine supplementation in anyone with creatinine clearance <60 mL/min until true kidney function is clarified 1.
• GFR must decline to approximately half the normal level before serum creatinine rises above the upper limit of normal in individuals with low muscle mass—the inverse is true in high-muscle-mass individuals where creatinine may be elevated despite normal GFR 6.