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Glucose in Serum (Fasting)

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

This test measures your blood sugar level after you’ve fasted for a set period. It helps doctors assess your blood sugar control, especially if you have or are at risk for diabetes.

Quick summary

["A fasting serum glucose test checks your blood sugar levels to identify diabetes or prediabetes risk."]

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

Key facts at a glance

LOINC
40297-4
Short code
GH 3.5h p chal SerPl-mCnc
Synonyms
GH 3.5h p chal SerPl-mCnc
Category
chemistry
Sample
serum
Container
Red-top tube (SST)
Units
N/A
Fasting
false
Population
adult
Sex
all
Age range
18–65 years
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 This Test Matters

Your doctor may order this test to diagnose diabetes, monitor existing diabetes, or check for conditions like prediabetes. High or low blood sugar levels can cause serious health problems if left unmanaged.

What to Expect Before the Test

You’ll need to fast (not eat or drink anything except water) for 8–12 hours before the test. Bring your fasting instructions with you. Avoid alcohol and strenuous activity on the day of the test.

During the Test

A healthcare provider will draw blood from a vein in your arm. You may feel a slight pinch, but it usually takes only a few minutes. You can resume eating and drinking normally afterward.

After the Test

You can return to your usual activities immediately. If you had symptoms like dizziness or nausea before the test, contact your doctor. Store any test results securely and share them with your healthcare team if needed.

Clinical interpretation

How abnormal results are interpreted and what conditions they suggest.

High fasting glucose levels (typically above 126 mg/dL) often indicate diabetes or impaired glucose tolerance. Low levels (below 70 mg/dL) may signal hypoglycemia, especially in people with diabetes or other conditions. Regular monitoring helps prevent long-term complications like nerve damage, kidney disease, or heart disease.

Short name

Fasting Serum Glucose

Frequently asked questions

How long does fasting last for this test?
Fasting typically requires 8–12 hours, meaning you shouldn’t eat or drink anything (except water) after midnight the night before the test.
Can I take my usual medications before the test?
Yes, but check with your doctor first. Some medications (like insulin or diabetes medications) may need adjustment. Never skip doses without approval.
What if I feel lightheaded or shaky before the test?
If you experience symptoms, inform the phlebotomist immediately. They may provide a quick glucose check or adjust your position. Severe symptoms could mean you need medical evaluation, especially if you have diabetes.
  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