Free Insulin, Serum/Plasma — Concentration
A blood test that measures unbound insulin, usually interpreted with glucose and other tests to assess insulin production and blood sugar regulation.
["Free insulin helps evaluate how much active insulin is circulating and why blood glucose may be high or low."]
Key facts at a glance
- LOINC
- 6901-3
- Short code
- Insulin Free SerPl-aCnc
- Synonyms
- Insulin Free SerPl-aCnc
- Category
- hormone
- Sample
- serum
- Container
- Red-top tube (SST)
- Units
- N/A
- Ref low
- 2.6
- Ref high
- 24.9
- Fasting
- false
- Population
- adult
- Sex
- all
- Age range
- 18–65 years
- CPT
- 83525
- SNOMED
- 413618004
- 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
What is this test?
This test measures free insulin, the unbound form of insulin that can act on the body's cells. It is performed on a blood sample and is usually reviewed alongside glucose, C-peptide, or other diabetes tests.
Why might I need this test?
Free insulin may be used to investigate low blood sugar, especially when symptoms occur unexpectedly. It can also help clinicians assess insulin resistance, diabetes, or the body's response to meals and insulin therapy.
How is the test performed?
A blood sample is collected, and the amount of free insulin in the serum or plasma is measured using a laboratory method. The results are reported as a concentration and interpreted with the other parts of your medical evaluation.
How do I prepare for the test?
Fasting for the period specified by your clinician may be required, particularly when the test is being used to evaluate low blood sugar. Continue taking medicines as prescribed unless your clinician instructs you otherwise, and tell the laboratory about insulin injections, supplements, and recent meals.
Reference range
Fasting: 2.6-24.9 μIU/mLN/A
No special preparation usually required. Stay well hydrated. Inform your healthcare provider of all medications and supplements you are taking.
Clinical interpretation
How abnormal results are interpreted and what conditions they suggest.
A high insulin level together with low blood glucose may indicate that the body is producing too much insulin, which can occur with an insulin-secreting tumor in rare cases. Elevated insulin with high glucose may suggest insulin resistance, while lower levels may be related to diabetes, fasting, or the timing and type of insulin treatment. Results must be interpreted with glucose levels, symptoms, medicines, and other laboratory findings.
Short name
Free insulin
Frequently asked questions
What is the difference between free and total insulin?
Is a fasting blood sample required?
Do insulin medicines affect the result?
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