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Samples for urine dilution
Last reviewed: 23.04.2024
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Urine dilution tests characterize the ability of the kidneys to maximally dilute the urine in conditions of artificially created hyperhydration. The state of hyperhydration is achieved by a water load, which can be single or prolonged.
With a single water load, the subject undergoes an empty stomach for 30-45 minutes, offering a drink of water (water, weak tea) at a rate of 20-22 mg / kg body weight. Then, urine portions are collected in which the relative density and osmolality are determined. With prolonged water load, subjects drink liquid in an amount equal to 2% of body weight for 30-40 min. For the next 3 hours, urine samples are collected every 30 minutes for research. Subsequently, the water load is maintained, finishing every 30 minutes a liquid whose volume is 50 ml more than the portion of the extracted urine.
In healthy individuals, when carrying out a sample for maximum dilution, the relative density of urine is reduced to 1003, and urine osmolality is reduced to 50 mOsm / l. During the first 2 hours of the sample, more than 50% of the total volume of the liquid discharged is released, within 4 hours more than 80%. The maximum rate of urine output exceeds 2-3 ml / min. Concentration index is always less than 1, more often 0,2-0,3; osmolarity clearance does not change significantly. Clearance of osmotically free water - the value is always positive and, as a rule, is more than 10 ml / min.
Dysfunction of the urine dilution is ascertained with the inability of the kidneys in the dilution samples to reduce the relative density of urine below 1004-1005, the urinary osmolality values exceeding 80 mOsm / l in the dilution samples.
The complete loss of osmotic dilution is indicated by isostenuria and hypersthenuria. In a healthy person, the inability to distinguish diluted urine is observed in the transition from prolonged hypokinesia to ordinary motor activity (space flights). The delay in the release of water is due to compensatory redistribution of liquid spaces.
In clinical practice, a violation of the ability of the kidneys to maximum dilution, not directly related to renal pathology, is detected in syndromes with excessive production of antidiuretic hormone. This is possible with ectopic secretion of antidiuretic hormone (lung, pancreas, leukemia, Hodgkin's lymphoma, thymoma); diseases of the central nervous system (CNS) - brain tumors, encephalitis, meningitis; Some diseases of the lungs (pneumonia, tuberculosis, abscess). In addition, the violation of the ability to breed is noted with adrenal insufficiency, hypothyroidism, heart failure, liver cirrhosis, obesity, malabsorption syndrome. These disorders can occur with the use of a number of drugs (antidiuretic hormone analogues, drugs that stimulate the production of antidiuretic hormone, amitriptyline, barbiturates, nicotine, morphine, sulfonylurea derivatives, etc.).
In nephrological practice, impaired ability to dilute urine is observed in chronic diffuse kidney disease and in patients with a transplanted kidney.
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