GFR and Creatinine Clearance Are Not the Same
No, GFR and creatinine clearance are distinct measurements of kidney function, though both assess filtration capacity. 1 While creatinine clearance measures the volume of blood plasma cleared of creatinine per unit time (requiring timed urine collection), GFR represents the actual rate at which the kidneys filter blood and is best measured using exogenous filtration markers. 1
Key Distinctions
What Each Measurement Represents
GFR is the gold standard index of kidney function, representing the volume of fluid filtered through the glomeruli per unit time, ideally measured using exogenous markers like inulin, iothalamate, or iohexol. 2
Creatinine clearance specifically measures how efficiently creatinine is cleared from the blood, calculated from timed urine collection and serum creatinine levels. 1
Creatinine clearance systematically overestimates true GFR because creatinine is both filtered by glomeruli and secreted by renal tubules, leading to falsely elevated values. 3
Clinical Practice Implications
For routine kidney function assessment, estimated GFR (eGFR) using serum creatinine-based equations is recommended as the initial test, not creatinine clearance. 2
The National Kidney Foundation guidelines explicitly state that measured creatinine clearance using 24-hour urine collections does not provide more accurate estimates of GFR than prediction equations. 2
In the MDRD study, predicted GFR provided a more accurate estimate of true GFR (measured by urinary clearance of 125I-iothalamate) than measured creatinine clearance. 2
Clinical laboratories should automatically report eGFR alongside serum creatinine concentrations, not creatinine clearance. 4
When to Use Each Method
First-Line Assessment
- Use eGFR from serum creatinine for initial kidney function evaluation in all patients. 2, 1
- This requires only a blood test without the inconvenience and frequent inaccuracy of timed urine collections. 2
Confirmatory Testing
Consider measured creatinine clearance only when: 2, 1
- eGFR is thought to be inaccurate (extremes of muscle mass, dietary intake, body size)
- Measured GFR using exogenous markers is unavailable
- You're already collecting 24-hour urine for other purposes (e.g., albumin excretion rate)
Most Accurate Assessment
Measured GFR using exogenous filtration markers (iohexol, iothalamate, inulin, 51Cr-EDTA, or 99mTc-DTPA) should be used when: 2
- Accurate GFR assessment will impact treatment decisions
- Both eGFR and creatinine clearance are likely inaccurate
- Evaluating living kidney donors (confirmatory test required)
Common Pitfalls
Serum creatinine alone is inadequate because GFR must decline to approximately half the normal level before serum creatinine rises above the upper limit of normal. 2
In elderly patients, serum creatinine may appear normal despite reduced GFR due to age-related decline in muscle mass reducing creatinine generation. 2
eGFR equations have significant limitations at normal-range GFR values and in patients with extremes of muscle mass, making them less reliable for living donor evaluation without confirmatory testing. 2
Creatinine assay standardization is critical—laboratories must calibrate to international standards to ensure accurate eGFR calculation. 2, 4