GFR is Not the Same as Creatinine Clearance
No, glomerular filtration rate (GFR) and creatinine clearance are not the same, although they are related measures of kidney function. According to the Kidney Disease: Improving Global Outcomes (KDIGO) consensus guidelines, these terms should be clearly distinguished when assessing kidney function 1.
Key Differences Between GFR and Creatinine Clearance
Definition and Measurement
- GFR: The actual volume of fluid filtered by the glomeruli per unit time, considered the best overall measure of kidney function 1
- Creatinine Clearance: A method that estimates kidney function based on creatinine measurements, which typically overestimates true GFR 2
Accuracy and Precision
- Creatinine clearance overestimates GFR because creatinine is not only filtered by the glomeruli but also secreted by the renal tubules 3
- This overestimation becomes more pronounced at lower levels of kidney function 2
- Studies have shown that the mean percentage difference between creatinine clearance and measured GFR can be as high as 8% ± 16% 4
Methods of Assessing Kidney Function
Measured GFR (mGFR)
- Gold Standard: Measured using exogenous filtration markers like inulin, iohexol, or radioisotopes 1
- Advantages: Removes the effect of endogenous non-GFR determinants that influence eGFR calculations 1
- Limitations: More complex, expensive, and time-consuming than estimated methods 1
Creatinine Clearance Measurement
- Traditional Method: 24-hour urine collection with serum creatinine measurement 2
- Formula: (Urine creatinine × Urine volume) / (Serum creatinine × Time of collection) 2
- Limitations:
Estimated GFR (eGFR)
- Common Equations:
- MDRD (Modification of Diet in Renal Disease) Study equation
- CKD-EPI (Chronic Kidney Disease Epidemiology Collaboration) equation
- Cockcroft-Gault equation (estimates creatinine clearance, not GFR directly) 2
- Components: Typically include serum creatinine, age, sex, weight, and sometimes race 2
Clinical Implications
When to Use Each Method
- For Routine Assessment: Use eGFR based on serum creatinine and/or cystatin C 1
- For Critical Clinical Decisions: Consider measured GFR using exogenous filtration markers when eGFR is thought to be inaccurate 1
- When mGFR Not Available: Consider timed urine collections for measured creatinine clearance if eGFR is thought to be inaccurate 1
Common Pitfalls
- Relying solely on serum creatinine: Can result in gross and unpredictable overestimates of kidney function 1
- Not accounting for patient factors: Elderly patients may have normal serum creatinine despite significant GFR reduction due to decreased muscle mass 2
- Laboratory variation: Differences in creatinine assay calibration can account for errors in GFR estimates as high as 20% 1
Practical Recommendations
- Use the term "GFR" specifically when referring to glomerular filtration and "kidney function" when discussing the totality of kidney functions 1
- For clinical practice, use validated GFR estimating equations rather than relying on serum creatinine alone 1
- Consider using cystatin C-based eGFR in specific circumstances where creatinine-based estimates may be inaccurate 1
- For drug dosing in elderly patients with declining renal function, the Cockcroft-Gault equation may still be preferred over MDRD-based GFR estimates 5
Remember that accurate assessment of kidney function is crucial for diagnosing chronic kidney disease, monitoring its progression, and calculating appropriate doses of drugs that are cleared by the kidneys.