Can creatinine-based eGFR misestimate true kidney filtration?

The claim that creatinine-based estimated glomerular filtration rate (eGFR) can inaccurately reflect true kidney function is well-supported by clinical evidence. Because serum creatinine is a metabolic byproduct rather than a direct measurement of filtration, its concentration is heavily influenced by factors independent of the kidneys.

Mechanistic drivers of creatinine generation

Serum creatinine is produced through the continuous, non-enzymatic dehydration of creatine and phosphocreatine. Approximately 95% of the body’s creatine is stored in skeletal muscle; consequently, the rate of creatinine generation is primarily a function of total muscle mass.

• Muscle Mass: Individuals with high muscle mass generate more creatinine, which can lead to higher serum levels and a potential underestimation of GFR. Conversely, in females or individuals with sarcopenia, lower generation rates may lead to lower serum creatinine, causing eGFR to overestimate actual kidney function.
• Non-Renal Elimination: While filtration accounts for most clearance, approximately 10–40% of creatinine is eliminated through active tubular secretion via organic cation transporters (OCT2). Factors that inhibit these transporters can raise serum levels without a change in true GFR.

Dietary and exogenous influences

Dietary intake significantly modulates the creatinine pool, independent of renal clearance.

• Cooked Meat Consumption: Ingesting cooked meat provides exogenous creatine and pre-formed creatinine, which can acutely elevate serum levels by as much as 30% for several hours, leading to a transient, false decrease in eGFR.
• Metabolic Variability: High-protein diets and creatine supplementation increase the precursor pool for creatinine, potentially resulting in "pseudo-renal failure" where eGFR suggests impairment despite normal measured filtration.

Clinical accuracy and limitations

Standard equations like CKD-EPI demonstrate a variability of approximately 25% when compared to gold-standard measured GFR (using exogenous markers like iohexol).

• Systematic Bias: In healthy populations or those at physiological extremes, eGFR may deviate significantly from true GFR. For a 30-year-old female, variations in body composition or recent meat intake can shift eGFR results outside the 30% accuracy window (P30) typically expected in clinical settings.

Bottom line

Creatinine-based eGFR is an estimate, not a direct measure, and is frequently skewed by muscle mass and diet. For precise assessment in individuals with atypical muscle profiles or high protein intake, clinicians may consider cystatin C or measured GFR.