Kidney function tests

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Renal osmoregulation function study

The main function of kidneys is cleansing, by selective excretion of unnecessary nutrients and delay the necessary blood components, which ensures the maintenance of a constant composition of blood. The most simple functional test based on detection of the impaired ability of the kidneys to concentrate and dilute urine.

The study is based on osmotically ability of the kidneys to concentrate and dilute urine. These processes depend on the efficient operation of the nephrons, with a general hemodynamics, which determines the blood rheology, renal blood flow, neurohumoral regulation and other factors. Violation of any part leads to changes in kidney function.

Zimnitsky test

Zimnitsky test based on the study of relative density in separate portions of urine excreted during voluntary urination during the day in a certain rhythm. The study was conducted in normal nutritional mode without liquid restrictions.

Urine was collected every three hours during the day and examined its amount, relative density, and the amount of sodium chloride and urea. The content of chloride and urea is defined in the daytime and nighttime urine samples. The total amount of urine allocated during the day is 65-75% of the drunk liquid.

About the normal reaction of the kidneys is judged by the following criteria:

  • exceed daytime diuresis over night
  • greatest fluctuation of urine volume and relative density from 1.004 to 1.032 in individual portions
  • difference between the highest and lowest relative density, which should not be less than 0.007
  • dramatic increase in urination after taking liquid
  • renal excretion of at least 80% of the administered fluid

About pathology indicate:

  • monotonic urination
  • excess nocturnal urine over a daytime
  • small amplitude of relative density fluctuations (1,007–1,009–1.010–1,012)
  • polyuria

Methods, based on the study of renal cleansing function (clearance), considered the most reliable.

Under the renal clearance is meant the amount of serum (plasma) blood (in ml) that is cleared completely per unit time from any exogenous or endogenous substances. The following types of clearance:

  1. Filtration clearance - the substance is released as a result of filtration and is not reabsorbed in the tubules. Such clearance has creatinine. It determines the amount of glomerular filtration.
  2. Excretion clearance - the substance is isolated by filtration and tubular excretion without reabsorption. This clearance determines the amount of plasma passed through the kidney. Such a substance is diodrast.
  3. Reabsorbtion clearance - substance is isolated by filtration and then completely reabsorbed in the tubules. Such substances include glucose, protein. Their clearance to be 0. At high concentrations of the substance in the blood clearance determines the maximum capacity of the tubules for reabsorption.
  4. Mixed clearance - is observed in the ability of filterable substances to a partial reabsorption. That clearance has urea.

Clearance of the analyte corresponds to the difference between the content of the substance in the urine and plasma in 1 minute. Clearance (C) calculate by the formula:

С = (U х V): Р

where C is the clearance of the test substance in ml/min
U is the concentration of the test substance in urine in mg/ml
V - diuresis in ml/min
P - concentration of the analyte in the plasma in mg/ml

Clearance depends on age, so it is different in children and adults, and the extent of kidney damage.

Most commonly to identify renal dysfunction (functional capacity of the glomeruli and tubules), as well as in the differential diagnosis of nephropathy used creatinine and urea.

Increasing the concentration of creatinine and urea in the blood, concomitant renal dysfunction — a symptom, pathognomonic of renal failure, however, the concentration of creatinine in the blood increases earlier than urea. Determination for identification of renal dysfunction more significant.