Urine concentration tests

Urine concentration tests characterize the ability of the kidneys to excrete an increased amount of osmotically active substances to maintain homeostasis under conditions of artificially created dehydration. Among this type of tests, 36-hour dehydration (Volhard test), 24-hour dehydration, 18-hour dehydration (with and without preliminary administration of diuretics), a test with pitressin (vasopressin), a test with a synthetic analogue of vasopressin are used.

With 36-hour dehydration, the relative density of urine should normally reach 1025-1040 g/l, and the osmolality - 900-1200 mOsm/l; with 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 non-physiological conditions of the tests and the poor tolerance of these studies by patients, the time interval of deprivation was shortened and limited to 18 hours (the patient is deprived of fluid for the period from 3 p.m. the previous day until 9 a.m. on the day of the study). The relative density of urine in the morning portion on the day of the study should normally be 1020-1024 g/l, and the urine osmolality - 800-1000 mOsm/l. To achieve more rapid and complete dehydration, additional administration of loop diuretics (6 hours before the start of deprivation) can be used, followed by dehydration for 16-18 hours. Under these conditions, the maximum values of the relative density and osmolality of urine correspond to those during 24-hour deprivation.

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

Currently, to determine the kidneys' ability to concentrate urine to the maximum, a method is used with the introduction of 1-diamino-8-0-arginine-vasopressin (desmopressin), a synthetic analogue of arginine-vasopressin. It has pronounced antidiuretic properties and is virtually devoid of a vasoconstrictor effect. The routes of its introduction are different: intranasally, intramuscularly, intravenously, subcutaneously. The maximum values of urine osmolality when introducing desmopressin reach 1200 mOsm/l, the relative density of urine is 1028-1032.

Impaired osmotic concentration function is determined by the inability of the kidneys to increase the relative density of urine in concentration tests to more than 1016-1020, and urine osmolality values in concentration tests are below 800 mOsm/l.

The complete loss of the osmotic concentration function is indicated by:

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

Both isosthenuria and hyposthenuria indicate severe kidney damage. They are detected in chronic renal failure and tubulointerstitial nephropathy.

Impaired osmotic concentration function is observed in all chronic kidney diseases at the stage of moderate renal failure, malignant arterial hypertension (MAH), Fanconi syndrome, pitress-resistant nocturnal nephrogenic diabetes insipidus, sickle cell anemia, and consumption of food with a low amount of animal protein (in vegetarians).

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