Understanding GFR When Serum Creatinine Levels Are Equal
Between two patients with identical elevated serum creatinine levels, a woman has better GFR than a man (Answer B).
The Relationship Between Creatinine and GFR
Serum creatinine is an imperfect marker of kidney function because it reflects both GFR and creatinine production from muscle mass. When two patients have the same numerical creatinine value, the patient producing less creatinine endogenously must have a lower GFR to reach that same serum level 1.
Why Women Have Better GFR at the Same Creatinine
- Women produce less creatinine due to lower muscle mass compared to men, so an identical serum creatinine in a woman reflects better kidney clearance than in a man 1, 2
- All validated GFR estimation equations (MDRD, CKD-EPI, Cockcroft-Gault) include sex-specific correction factors, with women receiving a multiplier of 0.742-0.85 to account for this difference 1, 3
- At the same creatinine level, age, and sex, estimated GFR calculations systematically assign higher GFR values to women than to men 2
Why the Other Options Are Incorrect
Urine output (Option A) does not reliably indicate GFR. Patients can maintain normal or even high urine output while having severely reduced GFR through compensatory mechanisms. Urine output reflects volume status and tubular function more than glomerular filtration 1.
Body weight (Option C) has a complex relationship with creatinine. Heavier patients typically have more muscle mass and produce more creatinine, so the same serum creatinine level in a heavier patient could reflect either better GFR (more production requiring more clearance) or worse GFR (more production overwhelming clearance). GFR estimation equations normalize for body surface area precisely because weight alone does not predict GFR 1, 4.
Older age (Option D) is associated with lower GFR. All validated equations subtract age-based coefficients (ranging from 0.203 to 0.943 × age) because GFR physiologically declines with aging 1, 5. At the same creatinine level, an older patient has worse—not better—GFR than a younger patient.
Clinical Application
When interpreting creatinine values:
- Never compare raw creatinine values between patients of different sex, age, or body size without calculating estimated GFR 1
- Use validated equations (CKD-EPI preferred for adults, FAS equations for full age spectrum) that incorporate sex, age, and race to estimate GFR from creatinine 5, 3
- A creatinine of 1.2 mg/dL might represent normal kidney function in a young muscular man but significant kidney disease in an elderly woman with low muscle mass 2
The sex difference is so fundamental that serum creatinine alone is unreliable for assessing kidney function without demographic correction factors 6.