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Osmolality Blood Test

Last updated May 9, 2019

Approved by: Maulik P. Purohit MD, MPH

The Osmolality Blood Test is often performed to determine the cause of diarrhea.

QWhat are the other Names for this Test? (Equivalent Terms)

  • Osmo Blood Test

What is Osmolality Blood Test? (Background Information)

  • Osmolality refers to the number of dissolved solute particles in a solvent, in this case blood. Osmolality is reported in osmoles (Osm) per kilogram of blood
  • Osmolality is similar to osmolarity, which measures the concentration of a compound in osmoles (Osm) per liter of total solution
  • An osmole is a quantity of dissolved particles that contributes to the osmotic pressure of a solution. The greater the difference in osmolarity between two sides of a semipermeable membrane, the greater the osmotic pressure
  • Osmotic pressure causes a cell placed in salt water to shrink. This is because the higher osmolarity outside the cell relative to the inside, creates osmotic pressure that is directed towards the cell outside
  • Importantly, the cell’s membrane is semi-permeable. It allows water to pass, but does not allow other molecules that are charged, polar, or too large, to pass
  • Solutes in the body include electrolytes and non-electrolytes, such as glucose
  • Electrolytes are chemicals that dissociate in solution and conduct an electrical current. They are important to the body’s acid-base, electrical, and hydrostatic stability
  • Electrolytes are mainly acquired through the diet. Major electrolytes include sodium (Na+), potassium (K+), chloride (Cl-), bicarbonate (HCO3-), and phosphate (HPO42-)
  • Electrolytes are necessary because they allow cells to regulate the flow of water across their membranes. This is called osmotic regulation. Osmotic regulation protects cells from bursting due to an influx of water if their surroundings lack salt
  • When osmolality is measured, it is compared to an expected result that has been calculated. The difference is the osmotic gap. If the osmotic gap is too great, toxins or diseases may be present
  • The Osmolality Blood Test measures the osmolality of blood to determine abnormalities. It is often performed to detect alcohol poisoning and kidney disease and to determine the cause of diarrhea

What are the Clinical Indications for performing the Osmolality Blood Test?

Following are the clinical indications for performing an Osmolality Blood Test:

  • Excessive thirst
  • Altered mental status
  • Headache
  • Vision problems
  • Fatigue

How is the Specimen Collected for Osmolality Blood Test?

Following is the specimen collection process for Osmolality Blood Test:

Sample required: Blood

Process of obtaining blood sample in adults:

  • A band is wrapped around the arm, 3-4 inches above the collection site (superficial vein that lies within the elbow pit)
  • The site is cleaned with 70% alcohol in an outward spiral, away from the zone of needle insertion
  • The needle cap is removed and is held in line with the vein, pulling the skin tight
  • With a small and quick thrust, the vein is penetrated using the needle
  • The required amount of blood sample is collected by pulling the plunger of the syringe out slowly
  • The wrap band is removed, gauze is placed on the collection site, and the needle is removed
  • The blood is immediately transferred into the blood container, which has the appropriate preservative/clot activator/anti-coagulant
  • The syringe and the needle are disposed into the appropriate “sharp container” for safe and hygienic disposal

Preparation required: No special preparation is needed prior to the test. 

What is the Significance of the Osmolality Blood Test Result?

The significance of Osmolality Blood Test result is explained:

  • Increased osmolality levels in blood may indicate:
    • Azotemia
    • Diabetes insipidus
    • Diabetic ketoacidosis
    • Hypercalcemia
    • Hypernatremia
  • Decreased osmolality levels in blood may indicate:
    • Adrenocorticoid insufficiency
    • Hyponatremia
    • Syndrome of inappropriate antidiuretic hormone production (SIADH)

The laboratory test results are NOT to be interpreted as results of a "stand-alone" test. The test results have to be interpreted after correlating with suitable clinical findings and additional supplemental tests/information. Your healthcare providers will explain the meaning of your tests results, based on the overall clinical scenario. 

Additional and Relevant Useful Information:

  • In general, the Osmolality Test measures the osmolality of blood, urine, or stool, to determine abnormalities
  • Certain factors interfere with the results of the Osmolality Test. These include hydration status, pregnancy, and consumption of electrolyte-rich foods

Certain medications that you may be currently taking may influence the outcome of the test. Hence, it is important to inform your healthcare provider of the complete list of medications (including any herbal supplements) you are currently taking. This will help the healthcare provider interpret your test results more accurately and avoid unnecessary chances of a misdiagnosis.

Please visit our Laboratory Procedures Center for more physician-approved health information:


References and Information Sources used for the Article:

Reviewed and Approved by a member of the DoveMed Editorial Board
First uploaded: June 2, 2015
Last updated: May 9, 2019