From the Guidelines
A 24-hour urine collection for creatinine clearance is not recommended as a routine test for assessing kidney function, as it does not provide more accurate estimates of GFR than prediction equations. 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. The guidelines recommend obtaining 24-hour urine collections only for special clinical circumstances. Some key points to consider when assessing kidney function include:
- Clinical laboratories should report an estimate of GFR using a prediction equation in addition to reporting the serum creatinine measurement 1.
- Use of GFR (rather than serum creatinine measurement) to characterize kidney function is a critical element in improving care of patients with chronic kidney disease 1.
- Autoanalyzer manufacturers and clinical laboratories should calibrate serum creatinine assays using an international standard 1.
- Under most circumstances, untimed urine samples should be used to detect and monitor proteinuria, and it is usually not necessary to obtain a timed urine collection to measure proteinuria 1. In clinical practice, the use of prediction equations, such as the MDRD study equation, is recommended for estimating GFR, rather than relying on 24-hour urine collections for creatinine clearance 1.
From the Research
24-Hour Urine Collection for Creatinine Clearance
- The traditional 24-hour urine collection method for determining creatinine clearance is often considered unreliable and inconvenient, particularly in ambulatory patients and outpatients, due to errors in collection, timing, and measurement of urine volume 2.
- Studies have explored alternative approaches to the traditional 24-hour urine collection, such as using shorter collection periods, to determine creatinine clearance. For example, one study found that creatinine clearance determined from 4-hour and 20-hour urine collections showed no significant difference from that obtained from the traditional 24-hour urine collection 2.
- In critically ill pediatric patients, a 12-hour creatinine clearance was found to be comparable to the traditional 24-hour creatinine clearance, suggesting that a shorter collection period may be sufficient in certain populations 3.
- The accuracy of creatinine clearance measurements can be affected by various factors, including renal function and the presence of certain medical conditions. For instance, metformin treatment has been shown to have an adverse effect on renal function in patients with type 2 diabetes mellitus and moderate chronic kidney disease 4.
- Creatinine clearance is an important measure of renal function, and accurate estimation is crucial for optimizing drug dosing and preventing toxicity. Alternative methods, such as the Schwartz equation, have been proposed, but their reliability can vary depending on the population and clinical context 3, 5.
- Understanding the physiology and clinical application of creatinine is essential for accurate interpretation of creatinine values and translation into changes in kidney function. Limitations and pitfalls of using creatinine as a marker of kidney function have been discussed, and alternative biomarkers are being explored 6.