Is an eGFR of 44 mL/min/1.73 m² Appropriate for a Muscular 44-Year-Old Man?
No, an eGFR of 44 mL/min/1.73 m² is not "okay" even in a muscular man—this represents Stage G3b chronic kidney disease with moderately to severely decreased kidney function, but you must confirm this with cystatin C-based eGFR because creatinine-based estimates are systematically inaccurate in patients with extremes of muscle mass. 1
Why Muscle Mass Matters
Creatinine is a breakdown product of muscle metabolism, so highly muscular individuals generate more creatinine at baseline, which artificially lowers the calculated eGFR when using creatinine-based equations (CKD-EPI or MDRD). 2
Extremes of muscle mass can affect eGFR accuracy, making creatinine-based estimates unreliable in this population. 2
The creatinine-based eGFR appears discordant with the clinical picture in approximately 16-20% of individuals with eGFR <60 mL/min/1.73 m², and this discordance is even more pronounced in muscular patients. 1
Mandatory Next Step: Measure Cystatin C
Order a cystatin C-based eGFR immediately to confirm or refute the diagnosis of chronic kidney disease. 1, 3
Cystatin C is not influenced by muscle mass, making it the gold standard confirmatory test when creatinine-based eGFR seems inconsistent with the clinical scenario. 1, 4
If the cystatin C-based eGFR is ≥60 mL/min/1.73 m², the CKD diagnosis is not confirmed, and the low creatinine-based eGFR was likely a false positive due to high muscle mass. 3
If the cystatin C-based eGFR is <60 mL/min/1.73 m², the CKD diagnosis is confirmed, and you must proceed with full Stage G3b management. 3
The combined creatinine-cystatin C equation (eGFRcreat-cys) provides 94.9% accuracy within 30% of measured GFR and should be the definitive value used for clinical decision-making when discordance exists. 3
If CKD Is Confirmed (Cystatin C-Based eGFR <60 mL/min/1.73 m²)
Classification and Risk
An eGFR of 44 mL/min/1.73 m² indicates Stage G3b CKD (eGFR 30-44 mL/min/1.73 m²), representing loss of approximately two-thirds of normal kidney function. 1
This stage is associated with markedly increased cardiovascular disease risk, faster CKD progression, and increased mortality compared to earlier stages. 1
Mandatory Screening for Complications
Screen for blood pressure abnormalities, volume status, electrolyte disturbances, metabolic acidosis, anemia, and mineral bone disease every 6-12 months. 1
Measure serum calcium and phosphorus every 3-6 months, and measure parathyroid hormone (PTH) every 6-12 months in Stage G3b. 1
Measure urine albumin-to-creatinine ratio (UACR) annually to assess for albuminuria. 1
Medication Management
Verify dosing of all medications, as many require adjustment when eGFR <60 mL/min/1.73 m². 1
Strictly avoid NSAIDs, as they reduce renal blood flow and can precipitate acute kidney injury. 1
Use ACE inhibitor or ARB as first-line antihypertensive therapy if albuminuria is present, targeting blood pressure <130/80 mmHg. 1
Dietary Modifications
Limit dietary protein to 0.8 g/kg body weight per day to reduce hyperfiltration injury and slow CKD progression. 1
Restrict sodium to <2 g/day to reduce blood pressure and maximize diuretic effectiveness if needed. 1
Nephrology Referral
Refer to nephrology now, as eGFR <45 mL/min/1.73 m² meets mandatory referral criteria. 1
Earlier referral at eGFR ≈44 mL/min/1.73 m² is appropriate given the high-risk features, even though mandatory referral is generally recommended at eGFR <30 mL/min/1.73 m². 1
Common Pitfall to Avoid
Do not dismiss the low eGFR solely because the patient is muscular—while muscle mass can falsely lower creatinine-based eGFR, you cannot assume this without measuring cystatin C. 1, 2
Approximately 23% of patients with creatinine-based eGFR 45-59 mL/min/1.73 m² actually have normal kidney function when confirmed with cystatin C, but this proportion is lower at eGFR 44 mL/min/1.73 m², making true CKD more likely even in a muscular patient. 3