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Kidney function and methods of assessment
Last reviewed: 06.07.2025
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The kidneys play an important role in the body, performing numerous functions. There are various methods for assessing them.
The kidneys perform several functions: depuration and endocrine, and maintain homeostasis.
Renal homeostasis is maintained through volume regulation (maintenance of blood volume and extracellular fluid), osmoregulation (maintenance of a stable concentration of osmotically active substances in the blood and other body fluids), maintenance of a constant ionic composition of the blood through regulation of the excretion of electrolytes and water and regulation of the acid-base balance (ABB).
The depuration function consists of excretion of the end products of nitrogen metabolism (mainly urea), foreign substances (toxins and drugs) and excess organic substances (amino acids, glucose).
The endocrine function consists of the production and secretion of enzymes and hormones by the kidney:
- renin, which plays an important role in regulating water-salt balance and blood pressure;
- erythropoietin, which stimulates erythropoiesis;
- active form of vitamin D - one of the main regulators of calcium and phosphorus levels in the body.
Evaluation of homeopathic and depuration function of the kidneys
To assess the main functions of the kidneys, various research methods are used:
- biochemical (determine serum levels of creatinine, uric acid, urea, sodium, potassium and other electrolytes);
- urine test;
- special methods, which primarily include cleansing (clearance) methods;
- load tests (urine concentration and dilution test, glucose, protein, ammonium chloride load test, etc.);
- radioisotope studies (radioisotope renography, scintigraphy).
Great importance is attached to determining the size of the kidneys using ultrasound examination, the introduction of contrast and isotopic compounds, which allows determining the tactics of treatment measures.
The most important indicators are the level of serum creatinine, the relative density of urine in a single analysis and/or Zimnitsky test, and the size of the kidney.
Serum creatinine is the end product of protein metabolism. It is synthesized in the body at a relatively constant rate and excreted only by the kidneys (mainly by glomerular filtration; secreted to a very small extent in the proximal tubules). Its level under physiological conditions depends on the degree of muscle mass. Normally, the concentration of creatinine in the blood serum is 0.062-0.123 mmol/l). Creatinine clearance is used to determine the SCF.
A relative density of urine in a single analysis and/or Zimnitsky test of more than 1018 g/l indicates intact renal function.
Normal kidney sizes (length from 10 to 12 cm, width from 5 to 7.5 cm and thickness 2.5-3 cm) indicate the absence of pronounced sclerotic processes.
With the development of renal failure, the level of serum creatinine exceeds 0.123 mmol/l, the relative density of urine decreases (less than 1018 g/l) and the size of the kidneys decreases. In addition to an increase in creatinine in the blood, the development of renal failure may be indicated by a decrease in the SCF, an increase in the concentration of uric acid, urea, residual nitrogen or urea nitrogen in the blood serum. In this situation, a decrease in the excretion of creatinine and urea in the urine also acquires great importance.
Methods for assessing the clearance of individual substances
These methods allow obtaining more accurate information about the state of renal functions. Quantitatively, the clearance of a substance is the volume of blood (in milliliters) that, when passing through the kidneys in a unit of time (1 min), is completely cleared of the substance.
The clearance of a substance (X) is calculated using the formula:
C x =(U x xV): P x,
Where C x is the clearance of substance X, U x is the concentration of substance X in urine, P x is the concentration of substance X in blood, V is the minute diuresis. The clearance of a substance is expressed in ml/min.
The clearance method is used to calculate the SCF, the value of renal plasma flow, and to study the osmoregulatory function of the kidneys. The results obtained should be equated to the standard body surface area of 1.73 m2.
In recent years, a number of formulas have appeared that make it possible to evaluate the SCF, as well as the state of sodium and potassium transport in individual segments of the nephron, which is important both for determining the localization of the pathological process in the kidneys and for determining the site of action of individual pharmacological drugs.
Study of renal autoregulatory function
The osmoregulatory function of the kidneys is assessed by their ability to concentrate and dilute urine. In clinical practice, the following indicators are used to characterize the osmoregulatory function of the kidneys:
- relative density of urine in a single analysis;
- Zimnitsky test (determination of fluctuations in the relative density of urine during the day);
- osmolality of serum and urine with calculation of the concentration coefficient, excreted fraction of osmotically active substances, clearance of osmotically free water and reabsorption of osmotically free water.
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