Learn about glomerular filtration rate test

The article was professionally consulted by a doctor of Biochemistry - Laboratory Department - Vinmec Ha Long International Hospital

Glomerular filtration rate is a test that evaluates the ability of the kidneys to function based on the measurement of substances excreted by the kidneys such as urea, creatinine, ... in the blood.
The test only assesses the relative level of kidney function because it depends on many factors such as food intake, age, gender, mental state.

Glomerular Filtration Rate (GFR) is the amount of blood filtered through the glomerulus per unit time. Since the nephrons function independently of each other, the LVEF of the two kidneys is equal to the LC of each nephron multiplied by the total number of nephrons. The formula for calculating the glomerular filtration rate is as follows:
BP = N x LC of 1 nephron CF = N x K x S x (PGC-PBC)-(GC- BC) With:
N: Number of nephrons in both kidneys K : Ultrafiltration coefficient S: Filtered area PGC: Hydrostatic pressure of the glomerular capillary PBC: Hydrostatic pressure of Bowman's cavity GC: Colloidal pressure of glomerular capillary BC: Colloidal pressure of Bowman's cavity In humans , glomerular filtration rate is only indirectly assessed through the clearance of a substance (clearance). The clearance of a substance (X) is the efficiency of a substance in removing a substance from plasma, indicating how many mL of plasma through the stem is cleared in 1 minute, calculated by the formula:
Cx: = ( Ux x V)/ Px (mL/min)
Where:
Cx is the Clearance of a substance (mL/min)
Ux is the concentration of that substance in the urine (mg/ml)
Px is the concentration of that substance in the blood (mg/ml)
V is the amount of urine in 1 minute (ml/min)

The glomerular filtration rate test is used to evaluate the functioning of the kidneys by measuring the amount of blood that is filtered by the kidneys over a certain period of time (measured indirectly through the assessment of the glomerular filtration rate). filtration of certain substances in the blood such as creatinine, urea). The glomerulus is the organ responsible for filtering the blood. When the glomerulus does not perform its full filtering function, it will lead to impaired kidney function.

3.1 Inuline clearance
Inuline is considered the “gold standard” in the assessment of LC - GFR in terms of accuracy and accuracy. Homer Smith's Inuline clearance measurement technique is performed as follows:
After a night of fasting, the patient is asked to drink about 2 liters of water in the morning. Continuous intravenous infusion of Inuline during clearance measurements. After 1 hour of intravenous infusion, inuline concentrations measured in blood and urine were used to calculate clearance. The final measurement will be the average of 4 measurements. The normal index of inuline clearance is inuline DCL = 130mL/min/1.73 m2 skin (men) and 120mL/min/1.73 m2 skin (women). Although it is a highly accurate test, inuline clearance has many disadvantages: inuline is difficult to find, and the technique for measuring inuline concentration is complicated. Therefore, the clearance of inuline is only used in research, few applications in clinical practice.

3.2 Serum creatinine clearance The glomerular filtration rate test is usually done by measuring the amount of creatinine in the blood. Creatinine is a product made during muscle movement that the kidneys need to filter out of the blood. However, if the kidneys are not working properly, it will lead to high levels of residual creatinine in the blood. Because kidney function is also affected by many other factors, to ensure accuracy, in addition to the measurement of creatinine in the blood, it must also be considered along with factors such as age and sex of the person. is tested through 2 calculation formulas:
Formula to calculate creatinine clearance from blood creatinine concentration and 24-hour urine creatinine concentration. The formula for estimating glomerular clearance is based on serum creatinine levels (proposed by Cockcroft and Gault for adults and non-obese adults.

3.3 Blood urea clearance Urea is freely filtered by the glomerulus and is reabsorbed in the proximal and distal renal tubules. The amount of urea in the blood depends not only on the filtering function of the glomeruli, but also on the blood flow to the kidneys and the production of urea in the liver. Under conditions of dehydration and hypovolemia, urea is reabsorbed, causing hyperuremia. Blood urea is also increased in the case of eating a lot of protein, increasing protein metabolism such as fever, taking steroids, increasing blood reabsorption in the gastrointestinal tract in gastrointestinal bleeding. Urea reduction is seen with reduced protein intake and severe liver disease. The normal value of blood urea nitrogen BUN is 10-15 mg/dl, serum urea is 20-30 mg/dl.
Urea in plasma increases as glomerular filtration rate decreases. Only when the glomerular filtration rate falls below 60ml/min does blood urea begin to rise. Therefore, blood urea is an insensitive test for the diagnosis of decreased glomerular filtration rate. On the other hand, because urea is reabsorbed in the renal tubules, the urea clearance will be less than the true glomerular filtration rate.
Mean index of urea clearance and creatinine clearance: Because creatinine has the property of being frequently excreted additionally, the creatinine clearance is greater than the true glomerular filtration rate. In contrast, urea is reabsorbed so the urea clearance is less than the true glomerular filtration rate. Therefore, taking the arithmetic mean of urea clearance and creatinine clearance will further increase the accuracy of the clearance.

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