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Glucose in Serum (Fasting)

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

This test measures the amount of glucose (sugar) in your blood after you haven’t eaten for at least 8 hours. It helps check how well your body uses sugar for energy.

Quick summary

["A fasting glucose test helps identify diabetes or prediabetes risk."]

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

Key facts at a glance

LOINC
1632-9
Short code
GH 3h p 1 g/kg Glc PO SerPl-mCnc
Synonyms
GH 3h p 1 g/kg Glc PO SerPl-mCnc
Category
glucose
Sample
serum
Container
Red-top tube (SST)
Units
N/A
Fasting
true
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 Is This Test Done?

Your doctor may order this test to diagnose diabetes, check for high blood sugar, or monitor existing diabetes management. It’s also used in screening for conditions like prediabetes or gestational diabetes.

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. Avoid smoking or chewing gum during this time. Bring your fasting blood sample to the lab on the day of the test.

What Happens During the Test

A healthcare provider will draw blood from a vein in your arm. You may feel a small pinch, but it usually doesn’t cause discomfort. The blood sample will be sent to a lab for glucose measurement.

What the Results Mean

Results are compared to standard reference ranges to determine if your blood sugar is within normal limits, borderline, or elevated. Your doctor will discuss these findings with you.

Clinical interpretation

How abnormal results are interpreted and what conditions they suggest.

High fasting glucose levels may indicate diabetes or insulin resistance, increasing risks for heart disease, nerve damage, or kidney problems. Low levels (rarely tested alone) can cause fatigue, dizziness, or confusion. Regular monitoring helps manage these conditions effectively.

Short name

Fasting Serum Glucose

Frequently asked questions

What causes fasting glucose levels to be abnormal?
Factors like genetics, obesity, poor diet, lack of physical activity, stress, or certain medications can affect fasting glucose levels. Illness, infections, or hormonal imbalances may also play a role.
How often should I get this test?
Your doctor will recommend a testing schedule based on your health status. For people without diabetes, tests may be done every 1–3 years. Those with diabetes or risk factors may need more frequent checks.
Can I eat or drink anything before the test?
No, you must fast for 8–12 hours before the test. Only water is allowed. Avoid sugary drinks or snacks, as they can affect results.
  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