From the Guidelines
Renal function can be most accurately measured through estimated glomerular filtration rate (eGFR) calculations, such as the CKD-EPI or MDRD equations, which provide a more accurate assessment by incorporating factors like age, sex, race, and body size along with serum creatinine 1.
Methods for Measuring Renal Function
The following methods are used to measure renal function:
- Serum creatinine and blood urea nitrogen (BUN) are the most commonly used laboratory tests.
- eGFR calculations, such as the CKD-EPI or MDRD equations, provide a more accurate assessment by incorporating factors like age, sex, race, and body size along with serum creatinine.
- 24-hour urine collection for creatinine clearance directly quantifies how much creatinine is cleared from the blood over time.
- Cystatin C, a protein filtered by the kidneys, offers an alternative marker less affected by muscle mass than creatinine.
- Urinalysis can detect protein or blood, indicating kidney damage.
- Imaging studies like ultrasound or CT scans evaluate kidney structure.
- Nuclear medicine scans using radioactive tracers can assess functional kidney tissue.
- Inulin clearance remains the gold standard for the most accurate measurement in research or specific clinical scenarios, as inulin is freely filtered at the glomerulus without being reabsorbed or secreted by the tubules.
Clinical Considerations
The choice of measurement depends on clinical context, with eGFR calculations being sufficient for routine monitoring while more specialized tests may be needed for complex cases or research purposes 1. In clinical settings, it is essential to consider the patient's age, sex, race, and body size when estimating renal function, as these factors can significantly impact the accuracy of eGFR calculations 1. Additionally, the use of serum creatinine alone is not sufficient to assess renal function, as it can be affected by factors such as muscle mass and diet 1.
Recommendations
The most recent and highest quality study recommends using eGFR calculations, such as the CKD-EPI or MDRD equations, to measure renal function 1. This approach provides a more accurate assessment of renal function and can help identify patients with chronic kidney disease or acute kidney injury. In specific clinical scenarios, such as in patients with severe renal impairment or those requiring precise measurement of renal function, more specialized tests like inulin clearance may be necessary 1.
From the Research
Options to Measure Renal Function
The following are options to measure renal function:
- 24 hour/timed urine collection for creatinine clearance evaluation 2
- Estimation of eGFR by creatinine based formulae, such as Modification of Diet in Renal Disease Study Equation (MDRD 2006), Chronic Kidney Disease Epidemiology Collaboration (CKD-EPI 2009) and Cockcroft-Gault Formula 2
- Measurement of serum creatinine and cystatin C levels to estimate GFR 3, 4
- Calculation of kinetic GFR when creatinine is changing rapidly 3
- Use of combined creatinine-cystatin C equation to estimate GFR, which may be useful as a confirmatory test for chronic kidney disease 4
- Estimation of GFR using CKD-EPI and MDRD equations with both serum creatinine and cystatin C (eGFRcr-cys) or single analyte (eGFRcr or eGFRcys) 5
Advantages and Limitations of Each Option
- The 24 hour urine collection for creatinine clearance is considered the most accurate method, but it is impractical for routine use 2
- The estimation of eGFR by creatinine based formulae is rapid and reliable, but may have limitations in certain clinical circumstances 2
- The measurement of serum creatinine and cystatin C levels is widely available, but may not accurately reflect GFR in all patients 3, 4
- The combined creatinine-cystatin C equation may provide a more accurate estimate of GFR, but requires further validation 4
- The use of CKD-EPI and MDRD equations with both serum creatinine and cystatin C may provide a more accurate estimate of GFR, but requires further evaluation 5