Investigation of the glomerular filtration rate
Last reviewed: 23.04.2024
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To measure the glomerular filtration rate (GFR), the clearance of substances is used, which in the course of transport through the kidneys are only filtered without undergoing reabsorption or secretion in the tubules, dissolve well in water, freely pass through the pores of the basal glomerulus membrane and do not bind to plasma proteins. Such substances include inulin, endogenous and exogenous creatinine, urea. In recent years, ethylenediaminetetraacetic acid and glomerulotropic radiopharmacological preparations, such as diethylenetriamine pentaacetate or iotalamate labeled with radioisotopes, have become widely used as marker substances. Also, unlabeled contrast agents (unlabeled iotalamate and yogexol) were used.
The rate of glomerular filtration is the main indicator of kidney function in healthy and sick people. Its definition is used to evaluate the effectiveness of therapy aimed at preventing the progression of chronic diffuse kidney diseases.
An ideal marker for determining the glomerular filtration rate is inulin, a polysaccharide with a molecular weight of 5200 daltons. It is freely filtered through the glomerular filter, not secreted, not reabsorbed and not metabolized in the kidneys. In this regard, the clearance of inulin is used today as a "gold standard" for determining the glomerular filtration rate. Unfortunately, there are technical difficulties in determining the clearance of inulin, and this is an expensive study.
The use of radioisotope markers also allows the determination of the glomerular filtration rate. The results of the definitions are closely correlated with the inulin clearance. However, radioisotope methods of research are associated with the introduction of radioactive substances, the availability of expensive equipment, as well as the need to comply with certain standards for storage and administration of these substances. In this regard, studies of the glomerular filtration rate using radioactive isotopes are used in the presence of special radiological laboratories.
In recent years, as a GFR marker, a new method has been proposed using serum cystatin C-one of the protease inhibitors. Currently, due to the incompleteness of population studies in which this method is evaluated, there is no information on its effectiveness.
The clearance of endogenous creatinine to the last years was the most widely used method for determining the glomerular filtration rate in clinical practice. To determine the glomerular filtration rate, a 24-hour urine collection is performed (for 1440 minutes) or urine is received at separate intervals (more often 2 intervals of 2 hours) with a preliminary water load to achieve sufficient diuresis. The clearance of endogenous creatinine is calculated by the clearance formula.
A comparison of the results of GFR obtained in the study of creatinine clearance and inulin clearance in healthy individuals revealed a close correlation of the indices. However, with the development of moderate and, especially, pronounced renal failure, GFR calculated by the clearance of endogenous creatinine significantly exceeded (by more than 25%) the GFR values obtained by inulin clearance. With GFR 20 ml / min, the clearance of creatinine exceeded the clearance of inulin by 1.7 times. The reason for the discrepancy between the results was that, in conditions of renal failure and uremia, the kidney starts secreting creatinine with proximal tubules. The preliminary (2 h before the beginning of the study) administration of the cimetidine, a substance that blocks the secretion of creatinine, at a dose of 1200 mg, helps to level the error. After preliminary administration of cimetidine, the clearance of creatinine in patients with moderate and severe renal insufficiency did not differ from the clearance of inulin.
At present, the calculation methods for the determination of GFR, taking into account the serum creatinine concentration and a number of other parameters (sex, height, body weight, age) are widely introduced into clinical practice. Cockcroft and Gault proposed the following formula for calculating GFR, which is currently used by most practitioners.
The glomerular filtration rate for men is calculated by the formula:
(140 - age) х m: (72 х Р кр ),
Where Р кр - concentration of creatinine in blood plasma, mg%; m is the body weight, kg. GFR for women is calculated by the formula:
(140 - age) х mx 0.85: (72 х Р кр ),
Where Р кр - concentration of creatinine in blood plasma, mg%; m is the body weight, kg.
Comparison GFR calculated by the Cockroft-Gault formula with measures of GFR, certain of the most accurate methods, the clearance (clearance of inulin 1 125 -yotalamata), revealed a high comparability of results. In the overwhelming majority of comparative studies, the estimated GFR differed from the true one by 14% or less, in the larger - by 25% or less; in 75% of cases the differences did not exceed 30%.
In recent years, the MDRD (Modification of Diet in Renal Disease Study) formula has been widely adopted for the definition of GFR:
GFR + 6.09 (serum creatinine, mol / l) -0.999 x (age) -0.176 x (0.7b2 for women (1.18 for African Americans) x (serum urea, mol / l) -0.17 x albumin serum, g / l) 0318.
Comparative studies have shown the high reliability of this formula: in more than 90% of cases, deviations in the results of the MDRD formula did not exceed 30% of the measured GFR. Only in 2% of cases the error exceeded 50%.
Normally, the glomerular filtration rate for men is 97-137 ml / min, for women - 88-128 ml / min.
In physiological conditions, the rate of glomerular filtration increases during pregnancy and when eating foods high in protein and decreases with aging. So, after 40 years, the rate of decline in GFR is 1% per year, or 6.5 ml / min per decade. At the age of 60-80 years, the GFR is halved.
In pathology, the rate of glomerular filtration decreases more often, but it can also increase. In diseases that are not associated with kidney pathology, the decline in GFR is most often due to hemodynamic factors - hypotension, shock, hypovolemia, severe heart failure, dehydration, and NSAID intake.
In renal diseases, a decrease in the filtration function of the kidneys is mainly due to structural disorders that lead to a decrease in the mass of the active nephrons, a decrease in the filtering surface of the glomerulus, a decrease in the coefficient of ultrafiltration, a decrease in renal blood flow, and obstruction of the renal tubules.
These factors cause a decrease in the glomerular filtration rate for all chronic diffuse kidney diseases [chronic glomerulonephritis (CGH), pyelonephritis, polycystic kidney disease, etc.], kidney damage in the context of systemic connective tissue diseases, development of nephrosclerosis in the presence of arterial hypertension, acute renal failure , obstruction of the urinary tract, severe damage to the heart, liver and other organs.
In pathological processes in the kidneys, the increase in GFR is significantly less evident due to an increase in the pressure of ultrafiltration, the coefficient of ultrafiltration or renal blood flow. These factors are important in the development of high GFR in the early stages of diabetes mellitus, hypertension, systemic lupus erythematosus, in the initial period of nephrotic syndrome formation. Currently, long-term hyperfiltration is considered as one of the non-immune mechanisms of progression of renal failure.
[Load tests to determine the reserve of glomerular filtration
The rate of glomerular filtration under physiological conditions during the day varies depending on the physical and psychological state of the individual, the composition of the food consumed, and the administration of medications. The maximum values are revealed after eating a large amount of meat protein. The ability of the kidneys to increase GFR in response to the stimulus was the basis for determining the reserve of glomerular filtration or the renal functional reserve (PFR).
To assess the RPF, as a rule, use a single (acute) load protein or amino acids or the introduction of small doses of dopamine.
Acute protein loading
The sample provides for the consumption of 70-90 g of meat protein (1.5 g of protein per 1 kg of body weight, corresponding to 5 g of boiled beef per 1 kg of body weight), 100 g of vegetable protein or intravenous injection of a set of amino acids.
In healthy individuals, in response to acute protein loading or the introduction of amino acids, GFR increases by 20-65% in the next 1-2.5 h after the load. The average RPF is 20-35 ml / min.
The FIU is calculated by the formula:
PFR = (GFR 2 -GFR 1 ): GFR 1,
Where GFR 1 is GFR in basal conditions (in the morning, on an empty stomach), GFR 2 is GFR after administration of meat or amino acids (stimulated GFR). The result is expressed as a percentage.
In diseases of the kidneys, the PFR can remain at a normal level or decrease. A sharp decline (less than 10%) or lack of reserve (less than 5%) indirectly reflects the state of hyperfiltration in functioning nephrons. Low values of PFR can be observed both in normal GFR (in the early stages of development of diabetic nephropathy, in nephrotic syndrome) and in the suppression of GFR in patients with renal insufficiency.