Osmolality of Serum (or Plasma)
This test measures the total concentration of dissolved particles (solutes) in your blood, helping assess hydration status and detect conditions like dehydration or overhydration.
["Osmolality helps diagnose fluid balance issues and supports diagnosis of metabolic disorders."]
Key facts at a glance
- LOINC
- 36011-5
- Short code
- XR tomo Heel
- Synonyms
- XR tomo Heel
- Category
- chemistry
- Sample
- serum
- Container
- Red-top tube (SST)
- Units
- N/A
- Fasting
- false
- Population
- adult
- Sex
- all
- Age range
- 18–65 years
- Method
- xr-tomography
- Specimen volume
- 5 mL
- Stability
- Room temperature: 4h, Refrigerated: 24h, Frozen: 7d
- Hemolysis max
- 500
- Lipemia max
- 1000
- Icterus max
- 500
- TAT
- 4–24 hours
- Order
- routine
Why Is This Test Done?
Your healthcare provider may order this test to evaluate if you’re dehydrated, overhydrated, or have an imbalance in electrolytes or other solutes in your blood. It’s also used in monitoring conditions like kidney disease, diabetes, or severe illness where fluid balance is critical.
What to Expect Before the Test
No special preparation is needed. You’ll have a small needle inserted into a vein (usually in your arm) to draw a blood sample. The process is quick and typically painless. If you’re on dialysis or have a central line, your sample may be drawn from that site instead.
What Happens During the Test?
The blood sample is sent to a lab where it’s analyzed to determine the total solute concentration. Results are usually available within 1–2 days, depending on lab processing times.
What Do the Results Mean?
Osmolality results are compared to a reference range that varies by lab. High osmolarity may indicate dehydration, diabetes insipidus, or kidney problems, while low osmolarity could signal overhydration or liver/kidney dysfunction. Your provider will interpret results in the context of your overall health.
Clinical interpretation
How abnormal results are interpreted and what conditions they suggest.
Osmolality is a key marker for detecting and managing fluid-electrolyte imbalances. Abnormal levels can signal dehydration (e.g., from vomiting, diarrhea, or insufficient fluid intake), overhydration (e.g., from excessive water intake or kidney issues), or metabolic disorders like diabetes. It’s also used to monitor patients on dialysis or those with severe infections or burns, where fluid balance is critical.
Short name
Serum Osmolality
Frequently asked questions
What causes high osmolarity in blood?
What causes low osmolarity in blood?
How often should I get this test?
Sources
- 1
- 2Lab Tests OnlineSource
- 3
Click any link to access the full source. PubMed Central (PMC) links provide free full-text access.
This information is for educational purposes only and is not a substitute for professional medical advice, diagnosis, or treatment.
This content is for general informational and educational purposes only and is not a substitute for professional medical advice, diagnosis, or treatment. Always seek the guidance of a qualified healthcare provider with any questions you have about a medical condition, and never disregard professional medical advice or delay seeking it because of something you read here.
If you think you may have a medical emergency, call your local emergency number or go to the nearest emergency department immediately.
Medically reviewed by WellAlly Editorial Team Last reviewed August 16, 2026. · Read our editorial policy