Muscle mass and estimates of renal function: a longitudinal cohort study

Area of Science:

• Nephrology
• Biomarkers
• Clinical Chemistry

Background:

• Creatinine-based estimation of Glomerular Filtration Rate (GFR) is common but can be confounded by variations in muscle mass.
• Muscle mass significantly influences creatinine levels, potentially leading to inaccurate GFR estimations (eGFRCr).

Observation:

• The 24-hour height-indexed creatinine excretion rate (CER index) was analyzed in relation to GFR estimations.
• Discordant classification occurred when eGFRCr was not <60 mL/min/1.73 m² while muscle mass-independent eGFRCys was <60 mL/min/1.73 m².
• CER index varied by sex and age, with males showing an increase until 45 years and females a consistent decrease.

Findings:

• Low muscle mass (low CER index) in a 70-year-old male led to a significant discrepancy of 24.7 mL/min/1.73 m² between eGFRCr and eGFRCys.
• Discordant classification rates due to low muscle mass were 18.5% in older males and 15.2% in older females.
• Muscle wasting over follow-up showed increasing disagreement between eGFRCr and eGFRCys, highlighting the impact on serial measurements.

Implications:

• Low muscle mass can cause substantial overestimation of GFR when relying solely on creatinine.
• Muscle wasting can lead to spurious overestimation of GFR in repeated measurements.
• Utilizing muscle mass-independent markers like cystatin C is crucial for accurate renal function assessment in individuals with low muscle mass or muscle wasting, also addressing issues with race-based adjustments.