Osmolality and Osmolal Gap
- Also Known As:
- Serum Osmolality
- Plasma Osmolality
- Urine Osmolality
- Osmotic Gap
- Formal Name:
- Osmolality (serum
- plasma
- urine
- stool)

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At a Glance
Why Get Tested?
Primarily, to detect poisoning due to ingestion of toxins such as methanol or ethylene glycol; to help evaluate the body’s water and electrolyte balance; to investigate low sodium levels in the blood (hyponatremia)
When To Get Tested?
When you may have ingested methanol, ethylene glycol, or isopropyl alcohol; when you have a low blood sodium level
Sample Required?
A blood sample drawn from a vein; sometimes a random urine sample is collected
Test Preparation Needed?
No test preparation may be needed; follow any instructions you are given. Some health care practitioners may instruct you to fast (nothing to eat or drink except water) for 6 hours before the test, and/or limit fluids for 12-14 hours before the test.
You may be able to find your test results on your laboratory’s website or patient portal. However, you are currently at Testing.com. You may have been directed here by your lab’s website in order to provide you with background information about the test(s) you had performed. You will need to return to your lab’s website or portal, or contact your healthcare practitioner in order to obtain your test results.
Testing.com is an award-winning patient education website offering information on laboratory tests. The content on the site, which has been reviewed by laboratory scientists and other medical professionals, provides general explanations of what results might mean for each test listed on the site, such as what a high or low value might suggest to your healthcare practitioner about your health or medical condition.
The reference ranges for your tests can be found on your laboratory report. They are typically found to the right of your results.
If you do not have your lab report, consult your healthcare provider or the laboratory that performed the test(s) to obtain the reference range.
Laboratory test results are not meaningful by themselves. Their meaning comes from comparison to reference ranges. Reference ranges are the values expected for a healthy person. They are sometimes called “normal” values. By comparing your test results with reference values, you and your healthcare provider can see if any of your test results fall outside the range of expected values. Values that are outside expected ranges can provide clues to help identify possible conditions or diseases.
While accuracy of laboratory testing has significantly evolved over the past few decades, some lab-to-lab variability can occur due to differences in testing equipment, chemical reagents, and techniques. This is a reason why so few reference ranges are provided on this site. It is important to know that you must use the range supplied by the laboratory that performed your test to evaluate whether your results are “within normal limits.”
For more information, please read the article Reference Ranges and What They Mean.
What is being tested?
Osmolality is a measure of the number of dissolved particles in a fluid. A test for osmolality measures the amount of dissolved substances such as sodium, potassium, chloride, glucose, and urea in a sample of blood and sometimes in urine. Alternatively, it can be estimated from the major solutes expected to be in the blood or urine.
Water balance in the body is a dynamic process that is regulated by controlling the amount of water eliminated in the urine by the kidneys and by increasing or decreasing water drinking by regulating “thirst.” In a healthy person, the body perceives and reacts to changes in the amount of water and particles in the blood.
- When blood osmolality increases with a decrease in the amount of water in the blood or an increase in the number of particles such as sodium, chloride, and glucose, a gland called the hypothalamus releases antidiuretic hormone (ADH). The kidneys respond to ADH by conserving water and producing urine that is more concentrated. The retained water dilutes the blood and lowers blood osmolality back to normal. This also increases blood volume and blood pressure. If this is not sufficient to restore the water balance, then thirst is also stimulated so that the affected person will drink more water.
- When blood osmolality decreases, the release of ADH is suppressed, the kidneys release more dilute urine, the amount of water in the body decreases, thirst is diminished, and blood osmolality increases back toward normal.
A blood (serum) osmolality test is primarily a measure of sodium dissolved in the serum (the liquid portion of blood). Sodium is the major electrolyte in the blood and urine. 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 eliminated in the urine by the kidneys to maintain its concentration in the blood within a healthy range.
In addition to electrolytes, glucose and urea contribute to osmolality. Normally their contributions are small, but when someone has high blood glucose (hyperglycemia, as found in untreated diabetes) or high blood urea (seen in diseases such as kidney failure), their influence can be significant.
Serum osmolality is often used in cases of suspected poisoning or overdose. Toxins such as methanol, isopropyl alcohol, ethylene glycol, propylene glycol, and acetone, and drugs such as salicylates (aspirin) can also affect osmolality when ingested in sufficiently large amounts.
A urine osmolality test primarily measures the waste products urea and creatinine. Urea and creatinine are produced and removed by the body at a relatively constant rate.
A serum osmolal gap (osmotic gap) may also be calculated. It is the difference between measured and calculated (estimated) osmolality results. In order to calculate the osmolal gap, tests for blood sodium, blood urea nitrogen (BUN), and glucose must be performed to calculate the expected osmolality. Some versions of the expected osmolality calculation also include the measurement of ethanol. An increase in the osmolal gap (greater than 10) indicates the presence of substances such as toxins, aspirin (salicylates), or mannitol.
Common Questions
View Sources
Sources Used in Current Review
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Sources Used in Previous Reviews
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