Method

KRYO (cryoscopic).

Description

Urine osmolality is the degree of concentration of osmotically active particles per 1 kilogram of solvent. The main dissolved molecules in urine are sodium, urea, potassium, chloride, calcium, ammonia, phosphates, and sulfates. Glucose contributes significantly to osmolality only when present in large amounts in the urine. As a measure of urine concentration, it is more accurate than specific gravity. The kidneys' ability to maintain tone and hydroelectrolytic balance in the extracellular compartment can be assessed by measuring urine osmolality. Additional information about kidney function can be obtained by comparing urine osmolality with serum osmolality; the normal ratio ranges between 1-3. In diseases characterized by low concentration, the ratio is low but still ≥ 1.

In a patient with normal fluid and food intake, urine osmolality will be approximately 500-850 mOsm/kg H2O. A healthy kidney can concentrate urine at levels of 800-1400 mOsm/kg, and in cases of excessive fluid intake, it determines osmolality of 40-80 mOsm/kg. Under dehydration conditions, urine osmolality is 3-4 times higher than plasma.

Determination of urine osmolality is an important test in monitoring increased urine volume. Defects in urine concentration (urine volume <2 L/24 hr) should be differentiated from polyuria caused by osmotic or water diuresis with urine volume > 2.5-3 L/24 hr.

Indications

Diagnosis of diabetes insipidus.
Assessment of the kidney's concentrating ability in acute and chronic renal failure.
Evaluation of dehydration.
Monitoring patients after administration of radiocontrast agents.
Assessment of the effectiveness of infusion therapy.
Diagnosis of hyperosmolar comatose states.

Preparation

Blood sampling in the morning on an empty stomach or 4 hours after eating. Drinking plain still water is allowed. Do not smoke 30 minutes before the test. For children under 1 year – no food 30-40 minutes before the test; for children 1-5 years – 2-3 hours.

Limit fluid intake for 12 hours before urine collection if possible. Before collecting urine, perform thorough genital hygiene. Inform the doctor about all medications taken, as well as about intravenous contrast administration for radiological examination if it was performed the day before.

Clinical Significance

Urine osmolality reflects the kidney's concentrating ability and is an important indicator of the body's hydroelectrolytic balance. Osmolality values <150 mOsm/kg H2O indicate water diuresis, characterized by increased urine volume with low dilution. This may be caused by the absence of antidiuretic hormone (ADH) secretion (central diabetes insipidus) or the kidney's inability to respond to ADH stimulation (nephrogenic diabetes insipidus). Osmotic diuresis with osmolality levels >400 mOsm/kg H2O occurs in diabetes, blood nitrogen overload, administration of glucose, urea, or mannitol. The osmotic diuresis test allows monitoring of water excretion under the influence of osmotic diuretics (furosemide).

Urine osmolality1651

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