Osmolar

Urine osmolality

What a urine osmolality result means and how it pairs with serum values. Explanation only.

Urine osmolality is the number of dissolved particles per kilogram of water in a urine sample, in mOsm/kg (equal to mmol/kg in SI units) [1]. It shows how far the kidneys are concentrating or diluting urine, so it is read together with a blood (serum) osmolality and sodium taken at about the same time, not on its own [2, 3].

Explanation only. A urine osmolality result is interpreted by a clinician alongside the history, examination and other tests. This page does not say what is wrong or what to do.

What the test measures

The lab measures urine osmolality with an osmometer, usually by freezing-point depression. It is a measured value, not a calculated one [4]. See how osmometers work. Most urinary osmoles are urea and salts (sodium, potassium and their anions), and the kidneys need these solutes to carry water out of the body [3, 5]. Urine osmolality therefore reflects two things: vasopressin (ADH) activity, which concentrates urine, and how much solute is being excreted [3, 5].

Reference ranges (labelled by source)

MeasurementRangeSource
Random urine sample50–1,200 mOsm/kg waterMedlinePlus [1]; StatPearls Urinalysis gives the same span, “usually 275–900” [2]
After 12–14 h fluid restriction> 850 mOsm/kg waterMedlinePlus [1]
Physiological limitsminimum 50–100, maximum 1,200 mOsm/kgStatPearls Hyponatremia [3]
Variant300–750 mOsm/kgStatPearls Osmometer [4]
Serum, for comparison275–295 mOsm/kgMedlinePlus [6]

Ranges vary by laboratory. MedlinePlus notes that “normal value ranges may vary slightly among different laboratories” [1]. A healthy person’s random value can fall anywhere in this span, depending on fluid intake.

Specific gravity: an approximate stand-in only

Urine specific gravity (USG) is the density of urine relative to pure water. A dipstick, refractometer or hydrometer measures it [2]. USG and osmolality “directly correlate”. StatPearls gives the example that a USG of 1.010 corresponds to roughly 300 mOsm/kg [2]. Its listed normal USG range is 1.002–1.035 (usually 1.016–1.022), and this is also lab-dependent [2].

The two are not interchangeable. USG depends on the mass of the dissolved particles, not their number, so heavy molecules raise it out of proportion. StatPearls lists dextran, IV radiocontrast and proteinuria as causes of falsely elevated USG [2]. Osmolality is “more reliable and accurate than USG” [2]. Treat any USG-to-osmolality conversion as a rough screen, not a substitute for a measured value [4].

The urine:serum osmolality ratio

U:S ratio = urine osmolality ÷ serum osmolality, using measured values from samples collected at about the same time [5].

  • Ratio above 1: the urine is more concentrated than plasma. In hyponatremia the European guideline notes that “in case of non-suppressed vasopressin activity, urine osmolality usually exceeds serum osmolality” [5].
  • Urine osmolality ≤ 100 mOsm/kg: maximally dilute urine, consistent with suppressed vasopressin [5].
  • Between 100 mOsm/kg and the serum value: the guideline calls this a “grey area”. In this range, “one cannot be clear about the presence or absence of vasopressin activity” [5].
  • Ratio close to 1 in advanced kidney disease: “urine osmolality is usually close to serum osmolality (isosthenuria)” [5].

Urine osmolality in the hyponatremia work-up (explanation only)

The 2014 European clinical practice guideline (ESE, ESICM, ERBP; Spasovski et al.) puts urine osmolality early in its diagnostic pathway for hypotonic hyponatremia [5]. Its recommendations include:

  1. “We recommend interpreting urine osmolality of a spot urine sample as a first step” (1D).
  2. “If urine osmolality is ≤100 mOsm/kg, we recommend accepting relative excess water intake as a cause of the hypotonic hyponatraemia” (1D).
  3. “If urine osmolality is >100 mOsm/kg, we recommend interpreting the urine sodium concentration on a spot urine sample taken simultaneously with a blood sample” (1D).
  4. Urine sodium ≤ 30 mmol/L suggests low effective arterial volume. A value > 30 mmol/L prompts assessment of extracellular fluid status and diuretic use (both 2D) [5].

StatPearls describes the same steps. A urine osmolality < 100 mOsm/kg usually indicates primary polydipsia, reset osmostat or potomania, and > 100 mOsm/kg usually indicates a high-vasopressin state such as low effective arterial volume, hypovolemia or SIADH [3].

SIAD/SIADH. The guideline’s essential criteria include all of the following [5]:

  • effective serum osmolality < 275 mOsm/kg
  • urine osmolality > 100 mOsm/kg at some level of decreased effective osmolality
  • clinical euvolemia
  • urine sodium > 30 mmol/L with normal salt and water intake
  • no adrenal, thyroid, pituitary or renal insufficiency
  • no recent diuretic use

Variant: the Schwartz–Bartter criteria as listed in StatPearls use urine sodium > 40 mEq/L [7]. The urine-sodium threshold therefore depends on the source.

Low solute intake and polydipsia. The water the kidneys can excrete depends on the solute they excrete. Per the guideline, “50–100 mmol of solutes, such as urea and salts, are required to remove 1 l of fluid”, depending on how dilute the urine can get [5]. StatPearls gives 600–900 mOsm/day for a regular diet and uses 200 mOsm/day as an example of a low-solute diet (“beer potomania”, “tea and toast”) [3]. The arithmetic shows why low solute intake limits water excretion:

  • 900 mOsm/day ÷ 50 mOsm/kg (most dilute urine) ≈ 18 L of urine per day at most
  • 200 mOsm/day ÷ 50 mOsm/kg ≈ 4 L per day at most

Both figures assume urine at the 50 mOsm/kg floor and 1 kg of water ≈ 1 L. The 18 L figure is consistent with StatPearls’ statement that people with normal renal function can excrete “up to 20 liters of water per day” [3].

This is physiology arithmetic, not a fluid target. StatPearls notes that people with beer potomania are at high risk of overly rapid sodium correction, which is a clinical management issue [3].

Diabetes insipidus and water deprivation (explanation only)

A 2022 international working group proposed renaming central and nephrogenic diabetes insipidus as arginine vasopressin (AVP) deficiency and AVP resistance [8].

The water deprivation test is performed under clinical supervision. When fluids are withheld, plasma osmolality rises and normally triggers AVP release, so urine concentrates. In AVP deficiency or resistance, “the urine fails to concentrate optimally with water deprivation” [9]. In the StatPearls approach to polyuria [9]:

  • hypotonic urine is typically < 300 mOsm/kg
  • a urine osmolality > 800 mOsm/kg indicates adequate AVP action
  • desmopressin given after deprivation reportedly raises urine osmolality by > 100% in complete central DI and up to 50% in partial central DI, with little or no rise in AVP resistance

The test has stopping criteria, including plasma osmolality above 295–300 mOsm/kg or sodium above 145 mmol/L, which is one reason it is never a home experiment [9].

Newer approaches exist. In a 2018 multicentre study of 141 patients with hypotonic polyuria, a hypertonic-saline–stimulated copeptin test was correct in 96.5% of patients, compared with 76.6% for the indirect water-deprivation test [10].

The urine osmolal gap

The urine osmolal gap (UOG) compares measured urine osmolality with a value calculated from measured urine solutes [11, 12]:

UOG = measured U-osm − [2 × (U-Na + U-K) + U-urea + U-glucose]

All terms are in mmol/L (urea nitrogen mg/dL ÷ 2.8; glucose mg/dL ÷ 18) [12]. The leftover osmoles are mostly ammonium salts, and U-NH₄⁺ ≈ UOG ÷ 2 “has traditionally been used” as an estimate [11]. It can help assess acid excretion in metabolic acidosis when urine ammonium is not measured directly.

Caveats:

  • Formulas differ. Halperin’s original 1988 definition subtracts the sum of Na, K, Cl, HCO₃, urea and glucose (the anions counted directly instead of 2 × cations) and reported a normal gap of 80–100 mOsm/kg H₂O [13]. Gap values are only comparable within the same formula.
  • Accuracy is inconsistent. In chronic kidney disease, UOG ÷ 2 correlated closely with measured ammonium (r = 0.925) but overestimated it by 4.7 mmol/L on average [11]. In kidney-transplant recipients, 40% of samples had a negative UOG, and there was no correlation with ammonium [12]. The authors attribute this to “an incorrect assumption that Na+ and K+ completely dissociate from their accompanying anions in urine” [12].
  • Low gap in impaired acidification. In patients with distal renal tubular acidosis the UOG was lower than in acid-loaded controls (about 130 vs 320 mmol/L) [14].
  • Use a measured osmolality (freezing point) and urine chemistry from the same sample.

Worked examples

Example 1: Urine:serum ratio (arithmetic only)

Measured serum osmolality 270 mOsm/kg, and urine osmolality 540 mOsm/kg from a sample taken at the same time.

  1. U:S = 540 ÷ 270 = 2.0
  2. The urine osmolality is above 100 mOsm/kg and above the serum value. The guideline describes this pattern as usual when vasopressin is not suppressed, and its next step is a spot urine sodium [5].

If the urine had instead been 80 mOsm/kg, it would be ≤ 100, the guideline’s “relative excess water intake” category [5]. At 200 mOsm/kg, it would fall in the grey area between 100 and 270 [5].

Example 2: Urine osmolal gap (US units)

Measured U-osm 600 mOsm/kg. U-Na 50 and U-K 40 mmol/L. Urine urea nitrogen 420 mg/dL. U-glucose 0.

  1. Urea: 420 ÷ 2.8 = 150 mmol/L
  2. Calculated: 2 × (50 + 40) + 150 + 0 = 180 + 150 = 330
  3. UOG = 600 − 330 = 270
  4. Traditional estimate: U-NH₄⁺ ≈ 270 ÷ 2 = 135 mmol/L [11]. This is an estimate, and the caveats above apply.

Practice questions

Q1. A random urine osmolality is 1,050 mOsm/kg. Is it outside the MedlinePlus reference span?

Answer

No. The random-sample span is 50–1,200 mOsm/kg water [1]. A concentrated urine can be normal after limited fluid intake. Interpretation needs the serum value and the clinical context.

Q2. A urine has a specific gravity of 1.030 after an IV contrast scan. Can you convert this to an osmolality?

Answer

Not reliably. Radiocontrast falsely elevates specific gravity [2]. Specific gravity reflects particle mass, not particle number, so only a measured osmolality gives the true value.

Q3. Serum osmolality 260 mOsm/kg (measured). Urine osmolality 90 mOsm/kg. Where does this fall in the European pathway?

Answer

Urine osmolality ≤ 100 mOsm/kg falls in the guideline’s “relative excess water intake” category [5]. U:S = 90 ÷ 260 = 0.35. This describes the pattern only. Causes and management are clinical decisions.

Q4. U-osm 450, U-Na 60, U-K 30 mmol/L, urine urea nitrogen 280 mg/dL, U-glucose 0. Calculate the UOG.

Answer

Urea 280 ÷ 2.8 = 100 mmol/L. Calculated = 2 × (60 + 30) + 100 = 280. UOG = 450 − 280 = 170, and UOG ÷ 2 ≈ 85 mmol/L ammonium (traditional estimate) [11]. Agreement with measured ammonium varies [11, 12].

Q5. Using the guideline figure of 50–100 mmol of solute per litre of urine, about how much urine can 300 mOsm/day of solute carry at the most dilute end?

Answer

300 ÷ 50 = 6 L per day at most (300 ÷ 100 = 3 L at the less dilute end) [5]. This is arithmetic illustrating solute dependence, not a fluid target.

If you are looking at your own urine test

This page explains the measurement. It cannot interpret your result. Discuss it with the clinician who ordered it, together with your blood results. Your lab’s printed range is the one that applies [1].

Do not change how much you drink, restrict fluids, or take salt tablets, urea or other supplements to “fix” a number. A water-deprivation test is done only under medical supervision.

Get urgent medical help. Call your local emergency number if someone with signs of dehydration is confused, sleepier than normal or hard to wake, has difficulty breathing or is taking lots of quick breaths, or has blue, grey, pale or blotchy skin, lips or tongue, or skin that feels cold [15]. Also call your local emergency number if someone has a seizure for the first time, a seizure that lasts more than 5 minutes (or longer than is usual for them), does not fully wake up afterwards, has several seizures without waking up in between, or has difficulty breathing after a seizure [16]. For anything less urgent but worrying, such as feeling unusually tired, dizziness on standing that does not go away, dark yellow urine or passing less urine than usual, or a fast heart rate, contact your doctor or local urgent-care service [15].

Educational content, not medical advice. Reference ranges vary by laboratory and method, and a value outside a range is not a diagnosis. Verify results against the reporting laboratory’s reference interval and discuss them with a clinician. See our disclaimer and how we check content.

Related: Serum osmolality calculator · Osmolar gap · Osmolality vs osmolarity · How osmometers work · Corrected sodium for hyperglycemia · Free water deficit calculator

References

  1. MedlinePlus Medical Encyclopedia. Osmolality urine test. US National Library of Medicine. Review date 2025-07-01. https://medlineplus.gov/ency/article/003609.htm
  2. Queremel Milani DA, Jialal I. Urinalysis. In: StatPearls [Internet]. Treasure Island (FL): StatPearls Publishing. https://www.ncbi.nlm.nih.gov/books/NBK557685/
  3. Rout P, Afzal M. Hyponatremia. In: StatPearls [Internet]. Treasure Island (FL): StatPearls Publishing. https://www.ncbi.nlm.nih.gov/books/NBK470386/ (PMID 29262111)
  4. Larkins MC, Zubair M, Thombare A. Osmometer. In: StatPearls [Internet]. Treasure Island (FL): StatPearls Publishing. https://www.ncbi.nlm.nih.gov/books/NBK589659/
  5. Spasovski G, Vanholder R, Allolio B, et al.; Hyponatraemia Guideline Development Group. Clinical practice guideline on diagnosis and treatment of hyponatraemia. Nephrol Dial Transplant. 2014;29 Suppl 2:i1–i39. doi:10.1093/ndt/gfu040. PMID 24569496. https://pubmed.ncbi.nlm.nih.gov/24569496/ (co-published in Eur J Endocrinol. 2014;170(3):G1–G47, doi:10.1530/EJE-13-1020, PMID 24569125)
  6. MedlinePlus Medical Encyclopedia. Osmolality blood test. US National Library of Medicine. https://medlineplus.gov/ency/article/003463.htm
  7. Yasir M, Mechanic OJ. Syndrome of Inappropriate Antidiuretic Hormone Secretion. In: StatPearls [Internet]. Treasure Island (FL): StatPearls Publishing. https://www.ncbi.nlm.nih.gov/books/NBK507777/
  8. Arima H, Cheetham T, Christ-Crain M, et al.; Working Group for Renaming Diabetes Insipidus. Changing the Name of Diabetes Insipidus: A Position Statement of the Working Group for Renaming Diabetes Insipidus. J Clin Endocrinol Metab. 2022;108(1):1–3. doi:10.1210/clinem/dgac547. PMID 36355385. https://pubmed.ncbi.nlm.nih.gov/36355385/
  9. Hui C, Khan M, Khan Suheb MZ, Radbel JM. Arginine Vasopressin Disorder (Diabetes Insipidus). In: StatPearls [Internet]. Treasure Island (FL): StatPearls Publishing. https://www.ncbi.nlm.nih.gov/books/NBK470458/
  10. Fenske W, Refardt J, Chifu I, et al. A copeptin-based approach in the diagnosis of diabetes insipidus. N Engl J Med. 2018;379(5):428–439. doi:10.1056/NEJMoa1803760. PMID 30067922. https://pubmed.ncbi.nlm.nih.gov/30067922/
  11. Fujimaru T, Shuo T, Nagahama M, et al. Assessing urine ammonium concentration by urine osmolal gap in chronic kidney disease. Nephrology (Carlton). 2021;26(10):809–813. doi:10.1111/nep.13937. PMID 34288275. https://pubmed.ncbi.nlm.nih.gov/34288275/
  12. Raphael KL, Ix JH. Correlation of urine ammonium and urine osmolal gap in kidney transplant recipients. Clin J Am Soc Nephrol. 2018;13(4):638–640. doi:10.2215/CJN.13311117. PMID 29519951. https://pmc.ncbi.nlm.nih.gov/articles/PMC5969467/
  13. Halperin ML, Margolis BL, Robinson LA, et al. The urine osmolal gap: a clue to estimate urine ammonium in “hybrid” types of metabolic acidosis. Clin Invest Med. 1988;11(3):198–202. PMID 3135965. https://pubmed.ncbi.nlm.nih.gov/3135965/
  14. Kim GH, Han JS, Kim YS, et al. Evaluation of urine acidification by urine anion gap and urine osmolal gap in chronic metabolic acidosis. Am J Kidney Dis. 1996;27(1):42–47. doi:10.1016/s0272-6386(96)90029-3. PMID 8546137. https://pubmed.ncbi.nlm.nih.gov/8546137/
  15. NHS. Dehydration (page last reviewed 01 May 2026). Accessed 2026-10-18. https://www.nhs.uk/conditions/dehydration/
  16. NHS. What to do if someone has a seizure (fit) (page last reviewed 19 December 2023). Accessed 2026-10-18. https://www.nhs.uk/conditions/what-to-do-if-someone-has-a-seizure-fit/

Sources

Numbered references are listed in the article above.