Urine osmolality is a measure of the concentration of osmotically active particles; it can range from 50 (maximally dilute urine) to 1,200 (maximally concentrated urine) mOsm/kg. Primary particles include:
Sodium
Chloride
Potassium
Urea
This measured value is used to assess:
Plasma electrolyte and fluid balance
Concentrating and diluting ability of the kidney in response to plasma osmolality
In normal physiologic conditions, urine specific gravity corresponds to urine osmolality.
Physiology Principles
Terminology.
One osmole: The amount of particles equal to Avogadro's number (6.02 × 1023).
Osmolarity: The amount of osmotically active particles present per liter of solution (mmol/L).
Osmolality: The amount of osmotically active particles present per kilogram of solvent (mmol/kg). for dilute solutions, osmolarity and osmolality can be used interchangeably.
Tonicity: The osmotic pressure of two solutions separated by a semipermeable membrane. Solutes that can cross will reach equilibrium. Thus tonicity results from solutes that cannot cross the membrane. In the body, tonicity refers to the state of the extracellular fluid (ECF compared to that of the intracellular fluid).
The kidney regulates urine concentration (and osmolality) by 3 physiologic processes:
Maintenance of a hypertonic medullary interstitium by countercurrent exchangers and urea cycling. This is an active transport and energy consuming process.
Different water permeabilities of the ascending and descending loops of Henle.
Effect of arginine vasopressin (AVP; formerly ADH) on distal tubules and collecting ducts. It increases water permeability via effects on V2 receptors. Water flows down an osmotic gradient into the hyperosmotic renal medulla. AVP is normally released in response to increased serum osmolarity; nonosmotic stimuli include pain, nausea, emotional stress, and decreased ECF volume.
Serum osmolality: The osmotic threshold ranges from 270 to 290 mOsm/kg. Sodium is the primary plasma electrolyte and determines plasma osmolality; thus, changes in sodium ion concentration reflect/accompany disturbances in water balance.
Plasma hypoosmolality: Decreased AVP release causing the excretion of a dilute urine
Low urine osmolality (<100 mOsm/kg)
Low specific gravity (<1.003)
Kidneys retain sodium
Plasma hyperosmolality: Increased AVP release causing the excretion of a concentrated urine
High urine osmolality (>200–300 mOs/kg)
High specific gravity (>1.02)
Anatomy
Osmoreceptors are located in the paired supraoptic and paraventricular nuclei of the anterior hypothalamus, where AVP is also synthesized. When stimulated (e.g., hyperosmolality, decreased ECF volume), AVP is axonally transported to the posterior pituitary for secretion.
G-protein–coupled V2 receptors, located on the basolateral cell membrane of the collecting duct, respond to AVP. AVP-induced activation of V2 receptors causes insertion of aquaporin-2 water channels in the apical membrane of principal cells in the collecting duct (1).
Physiology/Pathophysiology
Urine osmolality is utilized in correlation with clinical history and examination, plasma osmolality, serum sodium, and volume status to differentiate between causes of hypernatremia and hyponatremia, as well as polyuria, SIADH, and cerebral salt wasting (CSW).
Urine osmolality is elevated (concentrated urine) in the following conditions:
Dehydration/hypovolemia may present with hypotension, tachycardia, and decreased skin turgor. Lab findings:
Urine osmolality: Increased
Urine volume: Decreased
Serum osmolality: Typically increased
Serum sodium: Both hypernatremia and hyponatremia can occur
Total body sodium: Decreased
Postoperative state (high AVP state)
Urine osmolality: Increased
Urine volume: Variable
Serum osmolality: Decreased
Serum sodium: Decreased
Total body sodium: Unchanged or decreased
Solute diuresis
Urine osmolality: Increased
Urine volume: Increased
Serum osmolality: Typically increased
Serum sodium: Depends on the type of solute
Total body sodium: Decreased
Mineralocorticoid and glucocorticoid deficiency
Urine osmolality: Elevated
Urine volume: Decreased or normal
Serum osmolality: Variable
Serum sodium: Decreased
Total body sodium: Typically decreased with mineralocorticoid deficiency, and normal with glucocorticoid deficiency
Congestive heart failure and cirrhosis
Urine osmolality: Decreased
Urine volume: Variable
Serum osmolality: Decreased
Serum sodium: Decreased
Total body sodium: Increased
SIADH and CSW may present after a neurosurgical procedure or CNS insult with hypoosmolar hyponatremia. Lab findings:
Urine osmolality: Increased
Urine volume: Decreased
Serum osmolarity: Decreased
Serum sodium: Decreased
Total body sodium: Normal
Central diabetes insipidus can develop after a hypophysectomy, injury to the pituitary stalk, head trauma, supra- or interstellar tumor/cyst. Lab findings:
Urine osmolality: Decreased (<250 mOsm/L)
Urine volume: Increased
Serum osmolarity: Increased
Serum sodium: Increased
Total body sodium: Normal
Perioperative Relevance
General anesthesia may interfere with urinary concentrating capacity despite an increase in plasma AVP.
In the postoperative period, nonosmotic release of AVP, along with hypotonic IV fluid administration can lead to hyponatremia (with elevated urine osmolality).
Inert fluids containing glycine, sorbitol, or mannitol are used for irrigation during transurethral resection of the prostate (TURP). They can be rapidly absorbed via open venous sinuses in the prostate gland causing hyponatremia, volume overload, and hypoosmolality.
Methoxyflurane has been associated with polyuric nephrotoxicity especially at doses >1 MAC for 2 hours (2 MAC hours). This volatile agent is no longer used in developed countries.
Equations
Free water clearance (CH2O) = V - {Uosm/Posm} × V, where Uosm (mOsm/kg) = Urine osmolality, Posm (mOsm/kg) = Plasma osmolality, and V = urine flow rate. CH2O is a measure of water regulation by renal tubules. CH2O >0 implies that the kidney is producing dilute urine, whereas CH2O <0 means that the kidney is conserving water, likely under the influence of AVP
Urine-to-plasma osmolality ratio (Uosm:Posm) of >1.5 indicates an intact urine concentrating ability, whereas isosthenuria (Uosm:Posm = 1) in the setting of acute renal failure indicates ATN (in the absence of diuretics).
Estimated total body water (TBW) = Weight (kg) × fraction of body weight (%). Fraction is 0.6 in children; 0.6 and 0.5 in nonelderly men and women, respectively; and 0.5 and 0.45 in elderly men and women, respectively (2).
LinM, LiuSJ, LimIT.Disorders of water imbalance. Emerg Med Clin North Am. 2005;23(3):749–770.
AdroguéHJ.Hyponatremia. N Engl J Med. 2000;342(21):1581–1589.
AdlerSM, VerbalisJG.Disorders of body water homeostasis in critical illness. Endocrinol Metab Clin North Am. 2006;35(4):873–894.
EssonML, SchrierRW.Diagnosis and treatment of acute tubular necrosis. Ann Intern Med. 2002;137(9):744–752.
DecauxG, MuschW.Clinical laboratory evaluation of the syndrome of inappropriate secretion of antidiuretic hormone. Clin J Am Soc Nephrol. 2008;3(4):1175–1184.
Additional Reading⬆⬇
See Also (Topic, Algorithm, Electronic Media Element)
Urine osmolality, in addition to history, symptoms, signs, physical examination, serum sodium and osmolality, and urine volume, can aid in the diagnostic workup of hyponatremia, hypernatremia, polyuria, CSW, and diabetes insipidus.