How we estimate GFR--a pitfall of using a serum creatinine-based formula

Area of Science:

• Nephrology
• Clinical Biochemistry

Background:

• Chronic kidney disease (CKD) diagnosis relies on estimated glomerular filtration rate (eGFR).
• eGFR calculations are derived from serum creatinine, a marker influenced by muscle mass and diet.
• Increased CKD diagnoses in the UK are often in early stages detected in primary care.

Observation:

• A patient with high muscle mass consuming creatine supplements presented with reduced serum creatinine and eGFR.
• Creatine, a precursor to creatinine, is known to transiently elevate serum creatinine levels.
• Despite discontinuing creatine, serum creatinine remained elevated, impacting eGFR calculations.

Findings:

• Serum creatinine and calculated eGFR can be inaccurately affected by high muscle mass and creatine supplementation.
• 24-hour urinary creatinine clearance confirmed normal renal function, contradicting the eGFR result.
• This case illustrates creatine's impact on serum creatinine and subsequent eGFR estimations.

Implications:

• Awareness of confounding factors like creatine intake and muscle mass is crucial for interpreting eGFR.
• Misinterpretation of eGFR can lead to unnecessary investigations and patient anxiety.
• Accurate assessment of renal function requires considering individual factors beyond standard eGFR calculations.