The kidneys are an extremely important organ for the human body. To assess their condition and performance, there are many methods and tests. One such indicator is glomerular filtration rate.
What it is
This indicator is the main quantitative characteristic of the function of the kidneys. It reflects how much primary urine is formed in the kidneys over a given period of time.
Glomerular filtration rate may vary under the influence of various factors affecting the body.
This indicator plays a significant role in the diagnosis of renal failure and some other diseases. To determine it, you need to know some of the constants reflected in the calculation formulas, of which there are several variations and varieties.
Normally, glomerular filtration rate is regulated by several body systems (such as kallikrein-kinin, renin-angiotensin-aldosterone, endocrine, etc.). With pathology, most often a lesion of the kidney itself or a malfunction of one of these systems is detected.
What does this indicator depend on and how can it be determined?
Factors Affecting GFR Change
As mentioned above, the glomerular filtration rate depends on several indicators or conditions.
These include:
- Renal plasma flow rate. It is due to the amount of blood flowing through the bringing arteriole to the renal glomeruli. Normally, this indicator in a healthy person is about 600 ml per minute (the calculation was carried out for an average person weighing about 70 kg).
- Pressure in the vessels. Normally, the pressure in the bringing vessel should be significantly greater than in the carrying one. Only then can the process underlying the work of the kidneys be realized - filtration.
- The number of functioning nephrons. As a result of some diseases, it is possible to reduce the number of working kidney cells, which will result in a decrease in the so-called filtration surface, and, accordingly, a low glomerular filtration rate will be detected.
Indications for determining GFR
In what cases is the definition of this indicator necessary?
Most often, the glomerular filtration rate (the norm of this indicator is 100-120 ml per minute) is determined for various diseases of the kidney. The main pathologies in which its determination is necessary are:
- Glomerulonephritis . It leads to a decrease in the number of functioning nephrons.
- Amyloidosis Due to the formation of an insoluble protein compound - amyloid - the filtration capacity of the kidney decreases, which leads to the accumulation of endogenous toxins and poisoning of the body.
- Nephrotoxic poisons and compounds. Against the background of their intake, renal parenchyma may be affected with a decrease in all its functions. Such compounds may be mercuric chloride, some antibiotics.
- Renal failure as a complication of many diseases.
These conditions are basic in which glomerular filtration rate below normal can be observed.
Methods for determining glomerular filtration
Currently, quite a few methods and tests have been created to determine the level of glomerular filtration. All of them have a name (in honor of the scientist who discovered this or that test).
The main methods for studying glomerular function are the Reberg-Tareev test, the determination of glomerular filtration rate according to the Cockcroft-Gold formula. These methods are based on changing the level of endogenous creatinine and calculating its clearance. Based on its changes in blood plasma and urine, a definite conclusion is made regarding renal function.
All people can carry out these tests, since these studies have no contraindications.
The two above samples are the benchmark in the study of renal filtration. Other methods are used less often and are carried out mainly on specific indications.
How is the determination of creatinine level carried out and what are these procedures?
Sample Reberg-Tareev
It is used in clinical practice somewhat more often than the Cockcroft-Gold test.
For research, use blood serum and urine. Be sure to take into account the time of collection of analyzes, since the accuracy of the study depends on this.
There are several options for this sample. The most common technique is as follows: urine is collected over several hours (usually two-hour servings). In each of them, creatinine clearance and minute diuresis (the amount of urine formed per minute) is determined. The calculation of glomerular filtration rate is based on these two indicators.
A little less often is the determination of creatinine clearance in a daily portion of urine or the study of two 6-hour samples.
In parallel, no matter what method the test is performed, blood is taken from a vein on an empty stomach in the morning to assess creatinine concentration.
Cockcroft Gold Test
This technique is somewhat similar in performance to the Tareev test. In the morning, on an empty stomach, the patient is given a drink of a certain amount of liquid (1.5-2 glasses of liquid - tea or water) to stimulate minute diuresis. After 15 minutes, the patient urinates in the toilet (to remove residual urine formed during the night from the bladder). Then the patient is shown peace.
After an hour, the first portion of urine is collected and the time of urination is accurately noted. During the second hour, the second portion is collected. Between urination, 6-8 ml of blood is taken from a patient's vein to determine serum creatinine level.
After minute diuresis and creatinine concentration are determined, determine its clearance. How to determine the glomerular filtration rate?
The calculation formula for its determination is as follows:
- F = (u: p) Λ v , where
u is the concentration of creatinine in the urine,
p - creatinine in blood plasma,
V - minute diuresis,
F - clearance.
Based on the indicator F, a conclusion is made about the filtration capacity of the kidneys.
Determination of filtration rate by the MDRD formula
In contrast to the main methods that allow us to determine the glomerular filtration rate, our MDRD formula is somewhat less widely used. It is widely used by nephrologists in most European countries. In their opinion, the Reberg-Tareev test is low informative.
The essence of this technique is to determine GFR based on gender, age, and serum creatinine level. Often used in determining kidney function in pregnant women.
It looks like this:
- GFR = 11.33 x Crk - 1.154 x age - 0.203 x K, where
Crk - creatinine concentration in the blood (in mmol / l),
K is the sex coefficient (for example, for women it is 0.742).
This formula has proven itself in reducing the level of filtration rate, but its main drawback is incorrect results if the glomerular filtration rate increases. The calculation formula (due to this minus) has been modernized and supplemented (CKD-EPI).
The advantage of the formula is that it is possible to determine age-related changes in kidney function and observe them in dynamics.
Indicator decline
After all the tests and studies carried out, an interpretation of the results is carried out.
A decrease in glomerular filtration rate is observed in the following cases:
- The defeat of the glomerular apparatus of the kidney. The decrease in GFR is almost the main indicator indicating the defeat of this area. However, with a decrease in GFR, a decrease in the concentration ability of the kidneys (in the early stages) may not be observed.
- Renal failure. The main reason for the decrease in GFR and a decrease in filtration capacity. Throughout its stages, a progressive decrease in the clearance of endogenous creatinine, a decrease in the filtration rate to critical numbers, and the development of acute intoxication of the body with endogenous metabolic products are observed.
- A decrease in glomerular filtration can also be observed while taking certain nephrotoxic antibiotics, which leads to the development of acute renal failure. These include some fluoroquinolones and cephalosporins.
Stress test
To determine the filtration ability, you can use the so-called load tests.
For the load, they usually use a single use of animal protein or amino acids (in the absence of contraindications), or resort to the intravenous administration of dopamine.
When loaded with protein, about 100 grams of protein enters the patient (the amount depends on the weight of the patient).
Over the next half hour, in healthy people there is an increase in GFR by 30-50%.
This phenomenon is called the reserve of renal filtration, or PFR (renal functional reserve).
If the increase in GFR did not occur, a violation of the permeability of the renal filter or the development of some vascular pathologies (such as, for example, with diabetic nephropathy) and chronic renal failure should be suspected.
The dopamine test shows similar results and is interpreted in the same way as a protein loading test.
The importance of research data
Why are so many methods of assessing filtration ability created and why is it necessary to determine the glomerular filtration rate?
The norm of this indicator, as you know, changes in various conditions. That is why many methods and studies are currently being created that allow us to evaluate the state of our natural filter and prevent the development of many diseases.
In addition, these diseases provoke most kidney transplant surgeries, which is a rather laborious and complex process, often leading to the need for repeated interventions or more complex interventions.
That is why the diagnosis of the pathology of this organ is so important for both patients and physicians. A timely detected disease is much easier to treat and prevent than its neglected form.