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Formal name: Osmolality (plasma, urine, stool)

At a Glance

Why Get Tested?

To help evaluate the body's water and electrolyte balance; to investigate low sodium levels in the blood (hyponatremia) and increased or decreased urine production; to detect alcohol poisoning due to ingestion of toxins such as methanol or ethylene glycol; to monitor the effectiveness of treatment for conditions affecting osmolality; to help determine the cause of chronic diarrhea

When to Get Tested?

When someone has a low blood sodium level, is taking mannitol (an osmotic diuretic), may have ingested methanol, ethylene glycol, or isopropyl alcohol, is producing significantly increased or decreased amounts of urine, or has chronic diarrhea

Sample Required?

A blood sample drawn from a vein in your arm and/or a random urine sample; sometimes a fresh, liquid stool sample

Test Preparation Needed?

None may be required or you may be instructed to fast for 6 hours before the test; follow any instructions provided. Inform your health care provider of all medications you are taking, especially mannitol.

The Test Sample

What is being tested?

Osmolality is a measure of the number of dissolved particles in a fluid. The osmolality test reflects the concentration of substances such as sodium, potassium, chloride, glucose, and urea in a sample of blood, urine, or sometimes stool. It is used to evaluate the balance between water and dissolved particles in the blood and urine, to detect the presence of substances that may affect this balance, and to assess the kidneys' ability to concentrate urine.

Water balance in the body is a dynamic process that is regulated by controlling the amount of water excreted in the urine and by increasing or decreasing water drinking by regulating "thirst." Osmotic sensors in the body perceive and react to increases and decreases in the amount of water and particles in the bloodstream. When blood osmolality increases, indicating either a decrease in the amount of water in the blood or an increase in the number of particles, the hypothalamus secretes antidiuretic hormone (ADH), which tells the kidneys to conserve water. This results in more concentrated urine with higher urine osmolality and more dilute blood with lower osmolality. As blood osmolality decreases, ADH secretion is suppressed, the kidneys excrete increased amounts of dilute urine, the amount of water in the body decreases, and blood osmolality returns to normal.

Blood osmolality is primarily a measure of sodium dissolved in the serum. Sodium is the major electrolyte in the blood, urine, and stool. It works with potassium, chloride, and CO2 (in the form of bicarbonate) to maintain electrical neutrality in the body and acid-base balance. Sodium comes into the body in the diet and is normally conserved or excreted in the urine by the kidneys to maintain its concentration in the blood within a healthy range.

Urine osmolality primarily measures the waste products urea and creatinine. Urea and creatinine are produced and removed by the body at a relatively constant rate.

Glucose and urea are not electrolytes but as particles (molecules), they do contribute to osmolality. Normally their contributions are small, but when someone has high blood glucose (hyperglycemia, as found in diabetes) or high blood urea (seen in diseases such as kidney failure), their influence can be significant.

Glucose is osmotically active. This means it can draw water out of the body's cells, increasing the amount of fluid in circulation, which in turn increases the amount of dilute urine produced. Mannitol, a drug used to treat cerebral edema, also has this property. Toxins such as methanol, isopropyl alcohol, ethylene glycol, propylene glycol, and acetone, and drugs such as acetylsalicylic acid (aspirin) can also affect osmolality when ingested in sufficiently large amounts.

Osmolality can be measured or it can be estimated from the major solutes expected to be in the blood. The difference between measured and calculated (estimated) results is called the "osmotic gap" or "osmolal gap." An increase in the osmotic gap (greater than 10) indicates the presence of other substances such as toxins, aspirin, or mannitol. The osmotic gap is sometimes used for evaluation when a person is being treated with mannitol or when someone is suspected of having ingested a toxin such as methanol or too much of a drug such as aspirin. Other common causes of an elevated osmolal gap are alcoholic ketoacidosis, kidney failure, diabetic ketoacidosis, and shock.

How is the sample collected for testing?

A blood sample is obtained by inserting a needle into a vein in the arm. A random urine sample is collected using a clean catch method (see description under "Urinalysis: How is the sample collected for testing?"). A fresh (refrigerated or frozen within about 30 minutes of collection), liquid stool that is not contaminated by urine is collected in a clean container. Bacteria in the stool can change the results of the test within a short period of time.

NOTE: If undergoing medical tests makes you or someone you care for anxious, embarrassed, or even difficult to manage, you might consider reading one or more of the following articles: Coping with Test Pain, Discomfort, and Anxiety, Tips on Blood Testing, Tips to Help Children through Their Medical Tests, and Tips to Help the Elderly through Their Medical Tests.

Another article, Follow That Sample, provides a glimpse at the collection and processing of a blood sample and throat culture.

Is any test preparation needed to ensure the quality of the sample?

No test preparation may be needed or you may be instructed to fast (nothing to eat or drink except water) for 6 hours before the test. Follow any instructions you are given. Because some medications can interfere with this testing, inform your health care provider of all of the medications you are taking, especially if you are taking mannitol.

The Test

Common Questions

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Article Sources

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NOTE: This article is based on research that utilizes the sources cited here as well as the collective experience of the Lab Tests Online Editorial Review Board. This article is periodically reviewed by the Editorial Board and may be updated as a result of the review. Any new sources cited will be added to the list and distinguished from the original sources used.

Sources Used in Current Review

MedlinePlus Medical Encyclopedia. Osmolality - blood. Available online at http://www.nlm.nih.gov/medlineplus/ency/article/003463.htm. Accessed October 2013.

MedlinePlus Medical Encyclopedia. Osmolality - urine. Available online at http://www.nlm.nih.gov/medlineplus/ency/article/003609.htm. Accessed October 2013.

Healthline. Blood Osmolality Test. Available online at http://www.healthline.com/health/osmolality-blood through http://www.healthline.com. Accessed October 2013. Accessed October 2013.

Tuazon SA et al. Serum Osmolality. Medscape. Available online at http://emedicine.medscape.com/article/2099042-overview through http://emedicine.medscape.com. Accessed October 2013.

Medscape Editorial Staff. Urine Osmolality. Medscape. Available online at http://emedicine.medscape.com/article/2088250-overview through http://emedicine.medscape.com. Accessed October 2013.

University of Rochester Online Medical Encyclopedia. Osmolality (Stool). Available online at http://www.urmc.rochester.edu/encyclopedia/content.aspx?ContentTypeID=167&ContentID=osmolality_stool through http://www.urmc.rochester.edu. Accessed October 2013.

Drugs.com. Mannitol. Available online at http://www.drugs.com/cdi/mannitol.html through http://www.drugs.com. Accessed October 2013.

Advanced Instruments, Inc. Osmolarity vs. Osmolality. Available online at http://www.aicompanies.com/index.cfm/AIUniversity/OsmolalityExplained/Osmolarity_vs._Osmolality through http://www.aicompanies.com. Accessed October 2013.

Christine L. Snozek, PhD. Lab Tests Online adjunct board member.

Sources Used in Previous Reviews

Thomas, Clayton L., Editor (1997). Taber's Cyclopedic Medical Dictionary. F.A. Davis Company, Philadelphia, PA [18th Edition]. Pp 1361

Pagana, Kathleen D. & Pagana, Timothy J. (2001). Mosby's Diagnostic and Laboratory Test Reference 5th Edition: Mosby, Inc., Saint Louis, MO. Pp 613-616

Peng, K. (2004 May 15). Management of Hyponatremia. American Family Physician [On-line journal]. Available online at http://www.aafp.org/afp/20040515/2387.html through http://www.aafp.org.

Agha, I. (2004 February 23, Updated). Osmolality. MedlinePlus Medical Encyclopedia [On-line information]. Available online at http://www.nlm.nih.gov/medlineplus/ency/article/003463.htm.

Agha, I. (2004 February 11, Updated). Osmolality-Urine. MedlinePlus Medical Encyclopedia [On-line information]. Available online at http://www.nlm.nih.gov/medlineplus/ency/article/003609.htm.

(© 2005). Electrolyte & Osmolality Profile, Fecal. ARUP's User's Guide [On-line information]. Available online at http://www.aruplab.com/guides/ug/tests/0020699.jsp through http://www.aruplab.com.

Reynolds, R. and Seckl, J. (2005 October 10) Hyponatraemia for the Clinical Endocrinologist. Medscape, From Clinical Endocrinology 2005;63(4):366-374 [On-line journal article]. Available online at http://www.medscape.com/viewarticle/514125?src=search through http://www.medscape.com.

Dufour, D. R. (1993 July 13). Osmometry, The Rational Basis for Use of an Underappreciated Diagnostic Tool. Industry Workshop presentation for AACC Meeting New York, New York [On-line information]. PDF available for download at http://www.osmolality.com/pdf/Osmometry.pdf through http://www.osmolality.com.

Dugdale, D. (Updated 2009 August 10). Osmolality - blood. MedlinePlus Medical Encyclopedia [On-line information]. Available online at http://www.nlm.nih.gov/medlineplus/ency/article/003463.htm. Accessed January 2010.

Dugdale, D. (Updated 2009 August 7). Osmolality – urine. MedlinePlus Medical Encyclopedia [On-line information]. Available online at http://www.nlm.nih.gov/medlineplus/ency/article/003609.htm. Accessed January 2010.

Simon, E. and Hamrahian, S. (Updated 2009 May 29). Hyponatremia. eMedicine [On-line information]. Available online at http://emedicine.medscape.com/article/242166-overview through http://emedicine.medscape.com. Accessed January 2010.

Lewis, J. (Revised 2009 May). Water and Sodium Balance. Merck Manual for Healthcare Professionals [On-line information]. Available online at http://www.merck.com/mmpe/sec12/ch156/ch156b.html?qt=osmolality&alt=sh#sec12-ch156-ch156b-657a through http://www.merck.com. Accessed January 2010.

Pagana, K. D. & Pagana, T. J. (© 2007). Mosby's Diagnostic and Laboratory Test Reference 8th Edition: Mosby, Inc., Saint Louis, MO. Pp 684-687.

Clarke, W. and Dufour, D. R., Editors (© 2006). Contemporary Practice in Clinical Chemistry: AACC Press, Washington, DC. Pp 469, 341.