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Fasting Serum Insulin

Published: August 9, 2026Updated: August 16, 2026

Measures the amount of insulin in a blood sample taken after fasting to help assess how the body regulates blood glucose.

Quick summary

["Fasting insulin helps evaluate pancreatic insulin production and the body's response to glucose."]

Medically reviewed by WellAlly Editorial TeamLast reviewed August 16, 2026Editorial policy

Key facts at a glance

LOINC
62805-7
Short code
Fasting serum insulin proto
Synonyms
Fasting serum insulin proto
Category
liver
Sample
serum
Container
Red-top tube (SST)
Units
N/A
Ref low
10
Ref high
40
Critical range
50 N/A – 400 N/A
Fasting
true
Population
adult
Sex
all
Age range
18–65 years
Method
phenx
CPT
84484
SNOMED
302774002
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?

A fasting serum insulin test measures insulin, a hormone that helps glucose enter cells for energy. The result is often interpreted along with blood glucose and other diabetes tests.

Why might I need this test?

This test may be used to investigate unusually high or low blood glucose, evaluate insulin production, or assess insulin resistance. It is sometimes part of the evaluation for diabetes or hypoglycemia.

How is the test performed?

A blood sample is drawn from a vein and tested in a laboratory to measure the insulin in the serum. You generally do not need to do anything special immediately before the blood draw.

How do I prepare for the test?

You will usually be asked to fast overnight, typically for at least 8 hours, and to drink only water during that time. Follow the specific instructions from your healthcare professional, because fasting and eating can affect insulin levels.

Reference range

Adult: 10-40 U/LN/A

Specimen: serum

Fasting for 8-12 hours required. Water is permitted. Avoid chewing gum and smoking before the test.

Clinical interpretation

How abnormal results are interpreted and what conditions they suggest.

A high fasting insulin level may occur with insulin resistance or type 2 diabetes, particularly when blood glucose is also elevated. A low level may occur with impaired insulin production or conditions that cause hypoglycemia. Fasting insulin should be interpreted with other laboratory findings and clinical information rather than used alone to make a diagnosis.

Short name

Fasting insulin

Frequently asked questions

Does fasting insulin diagnose diabetes?
No. Fasting insulin is usually interpreted together with fasting glucose, hemoglobin A1c, or other glucose tests.
What happens if I eat before the test?
Eating can temporarily change insulin levels and may make the result harder to interpret. Tell the laboratory if you were unable to fast as instructed.
Is a blood draw the only way to measure insulin?
Serum insulin is commonly measured from a venous blood sample, although other insulin tests may use different specimens or stimulation procedures.
  1. 1
    MedlinePlus — Laboratory TestsSource
  2. 2
    Lab Tests OnlineSource
  3. 3
    Mayo Clinic — Tests & ProceduresSource

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.

Medical Disclaimer

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