Pay with Crypto and get 10% off your order!
Evaluate concentrating ability of the kidneys (eg, in acute and chronic renal failure); evaluate electrolyte and water balance; used in work-up for renal disease, syndrome of inappropriate antidiuretic hormone secretion (SIADH), and diabetes insipidus; may be used with urinalysis when patient has had radiopaque substances, has glycosuria or proteinuria;1 evaluate dehydration, amyloidosis. Osmolality is desirable in examination of neonatal urine when protein or glucose are present.2
Osmolality is a better measurement than specific gravity. Osmolality is a measure of renal tubular concentration, depending on the state of hydration.
Simultaneous determination of urine and serum osmolalities facilitates interpretation of results.
High urinary:plasma ratio is seen in concentrated urine. Normal ranges for the U:P ratio are given by Weisberg as approximately 0.2−4.7, and >3.0 with overnight dehydration.1 With poor concentrating ability the ratio is low but still ≥1.0. In SIADH urine sodium and urine osmolality are high for plasma osmolality.3
Low birthweight infants have been reported to have increased serum osmolality with normal urine osmolality.4
The urine osmolar gap is described as the sum of urinary concentrations of sodium, potassium, bicarbonate, chloride, glucose, and urea compared to measured urine osmolality. The gap is normally 80−100 mOsm/kg (SI: 80−100 mmol/kg) H2O. Determination of the urine osmolal gap is used to characterize metabolic acidosis.
1. Weisberg HF. Unraveling the laboratory model of a syndrome: The osmolality model. In: Young DS, Hicks J, Nipper H, et al, eds. Clinician and Chemist. The Relationship of the Laboratory to the Physician. Washington, DC: American Association of Clinical Chemistry;1979:200-243.
2. Leech S, Penney MD. Correlation of specific gravity and osmolality of urine in neonates and adults. Arch Dis Child. 1987 Jul; 62(7):671-673. PubMed 3632012
3. Goldstein CS, Braunstein S, Goldfarb S. Idiopathic syndrome of inappropriate antidiuretic hormone secretion possibly related to advanced age. Ann Intern Med. 1983 Aug; 99(2):185-188. PubMed 6881773
4. Giacoia GP, Miranda R, West KI. Measured vs calculated plasma osmolality in infants with very low birth weights. Am J Dis Child. 1992 Jun; 146(6):712-717. PubMed 1595626
