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Serum or plasma concentration of glucose (fasting)

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

This test measures your blood sugar level after you’ve fasted for 8 to 12 hours. It helps assess how well your body uses glucose for energy.

Quick summary

["A fasting blood sugar test helps detect abnormal glucose levels that may indicate diabetes or other metabolic conditions."]

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

Key facts at a glance

LOINC
6495-6
Short code
N men sg A Ag Spec Ql IF
Synonyms
N men sg A Ag Spec Ql IF
Category
chemistry
Sample
serum
Container
Red-top tube (SST)
Units
N/A
Fasting
false
Population
adult
Sex
all
Age range
18–65 years
Method
if
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 healthcare provider may order this test to check for diabetes, prediabetes, or other blood sugar-related conditions. It’s also used to monitor existing diabetes management or diagnose complications like high blood sugar over time.

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. Your blood will be drawn from a vein, usually in your arm, by a healthcare professional.

What Happens During the Test?

A small needle will be inserted to collect blood, which is then sent to a lab for analysis. The process takes a few minutes, and you may feel a brief pinch or discomfort.

What Are Common Results?

Results are typically reported in milligrams per deciliter (mg/dL). A normal fasting glucose level is usually below 100 mg/dL, while higher levels may signal potential issues.

Clinical interpretation

How abnormal results are interpreted and what conditions they suggest.

◆Associated findings1

Frequently asked questions

What causes fasting blood sugar to be high?
Factors that may raise fasting blood sugar include diabetes, obesity, poor diet, lack of physical activity, stress, certain medications (like steroids), or underlying health conditions like polycystic ovary syndrome (PCOS).
What causes fasting blood sugar to be low?
Hypoglycemia (low blood sugar) can result from excessive insulin use, certain medications (e.g., blood sugar-lowering drugs), excessive alcohol consumption, or conditions like adrenal insufficiency. Symptoms may include shakiness, dizziness, or confusion.
How often should I get this test?
If you have diabetes, your doctor may recommend testing every 3 to 6 months to track progress and adjust treatment. If you’re at risk (e.g., family history, obesity), your provider may suggest testing annually or more frequently.
  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