From the Guidelines
Creatinine clearance should not be measured using timed urine collections, as prediction equations provide more accurate estimates of GFR. According to the National Kidney Foundation practice guidelines for chronic kidney disease, measurement of creatinine clearance by using timed urine collections does not provide more accurate estimates of GFR than do prediction equations 1.
Measurement Considerations
- The guidelines recommend obtaining 24-hour urine collections only for special clinical circumstances, as timed urine collections are inconvenient and frequently inaccurate 1.
- Predicted GFR provided a more accurate estimate of GFR than measured creatinine clearance in the MDRD study 1.
- The Cockcroft-Gault formula can be used to estimate creatinine clearance, which requires only a single blood sample and considers the patient's age, weight, gender, and serum creatinine level.
Clinical Implications
- Creatinine clearance is important for assessing kidney function and guiding medication dosing, especially for drugs eliminated by the kidneys.
- The measurement of creatinine clearance works because creatinine is produced at a relatively constant rate by muscle metabolism and is freely filtered by the kidneys without being reabsorbed, making it an ideal marker for glomerular filtration rate.
- However, the use of prediction equations is recommended over timed urine collections for estimating GFR 1.
From the Research
Methods for Measuring Creatinine Clearance
- Creatinine clearance can be measured using a 24-hour urine collection method, which is considered the traditional and most accurate method 2, 3, 4, 5, 6.
- However, this method can be unreliable and inconvenient, particularly in ambulatory patients and outpatients, due to errors in collection, timing of collection, and measurement of urine volume 2.
- Alternative methods have been proposed, including abbreviated urine collection (≤12 hours) 5, and estimation of creatinine clearance using formulas such as Cockcroft-Gault and MDRD 3, 4, 6.
- These formulas can provide a rapid and reliable means of approximation of creatinine clearance, but their accuracy may vary depending on the population and stage of kidney disease 3, 4.
Comparison of Methods
- Studies have compared the accuracy of different methods for measuring creatinine clearance, including 24-hour urine collection, abbreviated urine collection, and estimation using formulas 2, 3, 4, 5, 6.
- The results have shown that abbreviated urine collection can correlate well with 24-hour measured creatinine clearance, but may overestimate creatinine clearance compared to 24-hour urine collection 5.
- The Cockcroft-Gault formula has been shown to be more closely associated with 24-hour urine collection than the MDRD formula in some populations, such as patients with a single kidney 6.
- Cystatin C-based equations have been shown to be more accurate than creatinine-based equations in estimating glomerular filtration rate, particularly in patients with chronic kidney disease 4.
Limitations and Variations
- The accuracy of creatinine clearance measurement can vary depending on the population, stage of kidney disease, and method used 3, 4, 5, 6.
- No single method or formula is optimal for all populations, and the choice of method may depend on the specific clinical context and patient characteristics 3, 4.
- Further studies are needed to evaluate the accuracy of different methods for measuring creatinine clearance in various populations and clinical settings 5.