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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 gone without food for at least 8 hours. It helps check for diabetes or other blood sugar issues.

Quick summary

["A fasting blood sugar test helps assess your risk for diabetes or other metabolic conditions."]

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

Key facts at a glance

LOINC
33136-3
Short code
GA from TTD.Hansmann85
Synonyms
GA from TTD.Hansmann85
Category
chemistry
Sample
serum
Container
Red-top tube (SST)
Units
N/A
Fasting
false
Population
adult
Sex
all
Age range
18–65 years
Method
us-estimated-from-ttd-hansmann
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 Is Done

Your doctor may order this test if you have symptoms like excessive thirst, frequent urination, unexplained weight changes, or fatigue. It’s also used to diagnose diabetes, prediabetes, or to monitor existing diabetes management.

What to Expect Before the Test

You’ll need to fast (not eat or drink anything except water) for 8 to 12 hours before the test. Avoid smoking or taking certain medications that can affect blood sugar levels. Bring a list of your current medications to your appointment.

How the Test Is Performed

A healthcare provider will draw blood from a vein in your arm using a small needle. You may feel a slight sting, but it usually doesn’t cause much discomfort. The blood sample is sent to a lab for analysis.

What the Results Mean

Results are typically reported in milligrams per deciliter (mg/dL). A normal fasting glucose level is generally below 100 mg/dL. Levels between 100 and 125 mg/dL may indicate prediabetes, while 126 mg/dL or higher suggests diabetes.

Clinical interpretation

How abnormal results are interpreted and what conditions they suggest.

◆Associated findings1

Short name

Fasting Serum Glucose

Frequently asked questions

What causes fasting blood sugar to be higher than normal?
Several factors can raise fasting blood sugar, including poor diet, lack of physical activity, certain medications (like steroids), infections, or underlying conditions like diabetes. Stress or illness can also temporarily increase levels.
What should I do if my fasting blood sugar is high?
If your test results show high fasting glucose, follow your doctor’s advice for lifestyle changes, such as eating balanced meals, staying active, and managing stress. They may also recommend further testing or adjustments to your treatment plan.
Can I eat or drink anything before my fasting blood sugar test?
No, you must fast for 8 to 12 hours before the test. Only water is allowed. Avoid sugary drinks or snacks, as they can affect your 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