Understand in one article | How to measure glomerular filtration rate?

Glomerular filtration rate (GFR) refers to the ability of the kidney to produce filtrate per unit time. It is an important indicator for evaluating renal function and is also the basis for staging chronic kidney disease (CKD). Accurate determination of GFR is crucial for drug dose adjustment and kidney disease management. GFR cannot be measured directly and needs to be expressed indirectly by measuring the renal clearance or plasma clearance of a certain marker. Markers can be divided into two categories: one is exogenous markers, including radionuclide markers such as inulin and non-radioactive contrast agents such as iohexol; the other is endogenous markers, including blood urea nitrogen, cystatin C (CysC), serum creatinine (SCr), urine protein, etc. The clearance rate of exogenous markers is recognized as the "gold standard" for determining GFR at home and abroad, but the detection steps are cumbersome and expensive, and it is difficult to popularize clinically, and is mostly used in medical experimental research. Among the endogenous markers, SCr and CysC are often used in clinical tests.

SCrSCr is one of the traditional indicators for assessing renal function. There is an inverse relationship between SCr and GFR, but at high GFR levels, a small change in creatinine may correspond to a huge change in GFR, while at low GFR levels, a large change in creatinine may only correspond to a relatively small change in GFR. This method is widely used, but is affected by many factors, such as muscle mass, age, and gender. SCr is affected by factors such as age, race, diet, muscle content, drugs, inflammation, lifestyle habits, and the measurement method, and the level changes are not very stable.

Although SCr is affected by many factors, using SCr levels to calculate GFR is still the most commonly used method, namely estimated GFR (eGFR). These include MDRD, CKD-EPI, C-G, Cockcroft-Gault, BIS, FAS and other formulas. The applicability and accuracy of each formula vary depending on the situation, so it is necessary to carefully select the appropriate formula. The commonly used formulas in my country are C-G, modified MDRD and CKD-EPI.

● C - G formula

eGFR = endogenous creatinine clearance × 0.84 × 1.73/BSA

Endogenous creatinine clearance = [(140-age) × weight × 0.85 (female)] / (72 × SCr)

BSA (m2) = 0.007184 × weight (kg) 0.425 × height (cm) 0.725

● MDRD formula

eGFR = 170 × (SCr) - 0.999 × (age) - 0.176 × (BUN) - 0.170 × (ALB) 0.318 × 0.742 (female)

Simplified formula: eGFR = 186 × (SCr) - 1.154 × (age) - 0.203 × (0.742 female)

Improved formula: eGFR = 170 × (SCr) - 1.234 × (age) - 0.179 × (0.79 female)

● CKD - EPI formula

male

SCr≤0.9 ml/dl: eGFR=144×(SCr/0.9)-0.411×(0.993)age

SCr＞0.9 ml/dl: eGFR=144×(SCr/0.9)－1.209×(0.993)age

female

SCr≤0.7 ml/dl: eGFR=144×(SCr/0.7)-0.329×(0.993)age

SCr＞0.7 ml/dl: eGFR=144×(SCr/0.7)-1.209×(0.993)age

The MDRD formula was once the preferred GFR assessment formula and is recommended for use in the clinical practice guidelines for kidney disease. The modified MDRD equation was designed by Chinese scholars based on the simplified MDRD equation and according to the characteristics of the Chinese people. It significantly improves the precision and accuracy of the equation when used for Chinese people and reduces deviations.

CysC is relatively more stable and has high sensitivity and specificity in reflecting renal injury, which is superior to other markers. It has more accurate performance in the elderly and patients with cirrhosis, but is also affected by other factors, such as thyroid status, inflammation, diabetes, etc.

Inulin clearance method Inulin is an ideal substance for measuring GFR, which can be completely filtered by the glomerulus and is not secreted or reabsorbed by the renal tubules. However, this method is cumbersome to operate and is mainly used in scientific research.

ECT method This method measures renal function through radionuclides. The results are not affected by age, muscle content, metabolic status, diet, disease status, etc. However, the equipment is expensive and the cost is high. It is affected by factors such as the depth of the kidney and subjective interpretation of the image.

Summary: Accurate determination of GFR is crucial for drug treatment and disease management. At present, there is no ideal formula that is perfectly applicable to all people. Different methods have their own advantages and disadvantages. In clinical practice, it is necessary to select an appropriate method based on the patient's condition and comprehensively consider various factors to ensure accuracy. In the future, it is necessary to refine specific populations through large sample analysis and meta-studies, so as to select appropriate markers and appropriate formulas to reduce errors and obtain a more suitable method for assessing renal function for Chinese people. In the future, in the evolving medical field, it is necessary to continue to explore more accurate methods for measuring GFR.