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Urine dilution tests
Last reviewed: 07.07.2025
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Urine dilution tests characterize the kidneys' ability to maximally dilute urine under conditions of artificially created hyperhydration. The state of hyperhydration is achieved by a water load, which can be single or long-term.
In a single water load, the subject is asked to drink liquid (water, weak tea) on an empty stomach for 30-45 minutes at a rate of 20-22 mg/kg of body weight. Then, urine portions are collected, in which the relative density and osmolality are determined. In a long-term water load, subjects drink liquid in an amount equal to 2% of body weight for 30-40 minutes. In the following 3 hours, urine portions are collected for testing every 30 minutes. Subsequently, the water load is maintained by drinking liquid every 30 minutes, the volume of which is 50 ml more than the portion of excreted urine.
In healthy individuals, when performing a maximum dilution test, the relative density of urine decreases to 1003, urine osmolality - to 50 mOsm/l. During the first 2 hours of the test, more than 50% of the total volume of liquid drunk is excreted, over 4 hours - more than 80%. The maximum rate of urine excretion exceeds 2-3 ml/min. The concentration index is always less than 1, more often 0.2-0.3; osmolar clearance does not change significantly. Clearance of osmotically free water is always a positive value and, as a rule, is more than 10 ml/min.
Impaired urine dilution function is determined by the inability of the kidneys to reduce the relative density of urine below 1004-1005 in dilution tests, and urine osmolality values exceeding 80 mOsm/l in dilution tests.
Isothenuria and hypersthenuria indicate a complete loss of the osmotic dilution function. In a healthy person, the inability to excrete diluted urine is observed during the transition from prolonged hypokinesia to normal motor activity (space flights). The delay in water excretion is due to compensatory redistribution of fluid spaces.
In clinical practice, impaired renal ability to maximally dilute, 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 cancer, pancreas, leukemia, Hodgkin's lymphoma, thymoma); diseases of the central nervous system (CNS) - brain tumors, encephalitis, meningitis; some lung diseases (pneumonia, tuberculosis, abscess). In addition, impaired dilution ability is noted in adrenal insufficiency, hypothyroidism, heart failure, liver cirrhosis, obesity, malabsorption syndrome. These disorders can occur when taking a number of medications (analogues of antidiuretic hormone, 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 diseases and in patients with a transplanted kidney.
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