Samples for urine concentration

Samples for urine concentration characterize the ability of the kidneys to release an increased amount of osmotically active substances to maintain homeostasis in conditions of artificially created dehydration. Among this type of samples, 36 hours of dehydration (Folgard's test), 24 hours of dehydration, 18 hours of dehydration (with and without previous administration of diuretics), a sample with pitresidine (vasopressin), a sample with a synthetic analog of vasopressin are used.

At 36-hour dehydration, the relative density of urine should normally reach 1025-1040 g / l, and osmolality - 900-1200 mOsm / l; at 24-hour dehydration, the relative density of urine is 1022-1032 g / l, and the osmolality is 900-1100 mOsm / l, respectively. However, due to the nonphysiological conditions of the samples and the high tolerability of these studies, patients were deprived of the time interval of deprivation and limited to 18 hours (the patient was deprived of fluid for the period from 15 hours of the previous day to 9 hours of the day of the study). Relative density of urine in the morning portion on the day of the study should normally be 1020-1024 g / l, and the osmolality of urine - 800-1000 mOsm / l. To achieve more rapid and complete dehydration, additional administration of loop diuretics can be used (6 hours prior to initiation of deprivation) followed by dehydration for 16-18 hours. Under these conditions, the maximum values of the relative density and osmolality of urine correspond to those at 24 hours deprivation.

A test with subcutaneous or intramuscular injection of 5 units of pittresin is widely used to study the concentrating ability of the kidneys. The drug is administered in the evening on the eve of the day of examination and then during the day the relative density and / or osmolality of the urine is determined. In a healthy person, the relative density increases to 1024, and osmolality to 900-1200 mOsm / l.

At present, the method with the introduction of 1-diamino-8-0-arginine-vasopressin (desmopressin), a synthetic analogue of arginine-vasopressin, is used to determine the ability of the kidneys to maximize urine concentration. It has pronounced antidiuretic properties and is practically devoid of vasoconstrictive effect. The ways of its introduction are different: intranasal, intramuscular, intravenous, subcutaneous. The maximum values of osmolality of urine with the introduction of desmopressin reach 1200 mOsm / l, the relative density of urine is 1028-1032.

The disturbance of the function of osmotic concentration is ascertained when the kidneys in the samples for concentrating fail to increase the relative density of urine more than 1016-1020, the values of the osmolality of urine in the samples for concentration below 800 mOsm / l.

The complete loss of the function of osmotic concentration indicates:

• isostenuria - equal osmolality of blood serum and urine (275-295 mOsm / l);
• the relative density of urine is 1010-1011, which characterizes the complete cessation of the processes of urine concentration and dilution;
• hypostenuria is a condition in which the values of the maximum osmolality of urine are lower than the osmolality of the plasma (200-250 mOsm / L), and the relative density of urine is lower than 1010, which indicates complete cessation of urine concentration processes and the persistence of urine dilution processes.

Both isostenuria and hyposthenuria testify to severe kidney damage. They are detected in chronic renal failure and tubulointerstitial nephropathies.

Disturbance of the function of osmotic concentration is observed in all chronic kidney diseases in the stage of moderately expressed renal failure, malignant arterial hypertension (ZAG), Fanconi syndrome, pessessin-resistant non-natural nephrogenic diabetes insipidus, sickle cell anemia, eating small amounts of animal protein (vegetarians).

[1], [2], [3]