Where to Have Creatinine Clearance Drawn
Creatinine clearance is not "drawn" at a laboratory—it is calculated using a serum creatinine blood test that can be obtained at any standard clinical laboratory, hospital lab, or outpatient phlebotomy center. 1
Understanding What is Actually Measured
- Serum creatinine is the only blood test required for calculating creatinine clearance using estimation formulas like Cockcroft-Gault or MDRD 1
- The patient needs a simple blood draw at any laboratory that performs basic chemistry panels 1
- No special laboratory or urine collection is needed when using estimation equations, which are now the standard of care 1
Standard Clinical Practice for Assessing Renal Function
Initial Assessment
- Use serum creatinine and an estimating equation for initial assessment of GFR, rather than measured creatinine clearance 1
- The blood draw can be performed at:
- Hospital-based laboratories
- Outpatient reference laboratories (Quest, LabCorp, etc.)
- Physician office laboratories
- Any facility performing basic metabolic panels 1
When Calculation is Sufficient
- For most clinical situations, calculated creatinine clearance using the Cockcroft-Gault formula is adequate and preferred over 24-hour urine collections 1, 2, 3
- The Cockcroft-Gault equation requires only: age, weight, sex, and serum creatinine—all obtainable without urine collection 2, 3
- Studies demonstrate that calculated creatinine clearance is at least as accurate as 24-hour urine collections and substantially easier to obtain 4, 5
When More Accurate Measurement is Needed
Circumstances Requiring Measured GFR
Consider timed urine collections for measured creatinine clearance only if:
- Measured GFR using exogenous filtration markers is not available AND
- Estimated GFR from combined creatinine-cystatin C equations is thought to be inaccurate 1
Specialized Testing Locations
- When more accurate ascertainment of GFR will impact treatment decisions, measure GFR using plasma or urinary clearance of an exogenous filtration marker (such as iothalamate or iohexol) 1
- These specialized measurements require:
- Nephrology clinics with GFR measurement capabilities
- Academic medical centers
- Specialized renal function laboratories 1
Important Clinical Caveats
Common Pitfalls to Avoid
- Never rely on serum creatinine concentration alone to assess kidney function—always use an estimating equation 1, 2
- Serum creatinine significantly underestimates renal insufficiency, particularly in elderly patients with reduced muscle mass 2
- 24-hour urine creatinine clearance collections are prone to inaccuracy due to incomplete urine collection and offer no advantage over calculated estimates in most patients 2, 4, 5
When Formulas May Be Inaccurate
Consider cystatin C-based equations or direct GFR measurement in:
- Extremes of body size (severe obesity or cachexia)
- Extremes of age (very elderly or very young)
- Severe malnutrition
- Diseases of skeletal muscle
- Paraplegia or quadriplegia
- Vegetarian diet
- Rapidly changing kidney function 1, 3
Laboratory Considerations
- Clinical laboratories should implement standardized creatinine assays calibrated to international reference materials to ensure accuracy 1
- The Jaffe method may overestimate serum creatinine by 5-15% compared to enzymatic methods 2, 3
- Enzymatic creatinine assays are preferred, especially in children 1
Practical Algorithm for Clinicians
- Order serum creatinine at any standard laboratory 1
- Calculate creatinine clearance using Cockcroft-Gault formula for medication dosing decisions 2, 3
- Use eGFR from creatinine (MDRD or CKD-EPI equations) for diagnosing and staging chronic kidney disease 1
- Add cystatin C measurement when eGFR from creatinine is less accurate (eGFR 45-59 mL/min without albuminuria, extremes of body composition) 1
- Refer for measured GFR using exogenous markers only when estimation is unreliable and accurate GFR will change management 1