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Serum or plasma mass concentration of glucose (fasting)

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

This test measures the amount of glucose (sugar) in your blood after you’ve fasted for a set period. It helps doctors check how well your body uses sugar for energy.

Quick summary

["A fasting blood sugar test helps assess your blood sugar control and identify potential diabetes or metabolic issues."]

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

Key facts at a glance

LOINC
46408-1
Short code
GH 15M pre arg SerPl-mCnc
Synonyms
GH 15M pre arg 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 Is This Test Done?

Your doctor may order this test to check for diabetes, prediabetes, or other conditions that affect blood sugar levels. It’s often done as part of routine health checks or when symptoms like excessive thirst, fatigue, or unexplained weight changes appear.

How Is the Test Performed?

You’ll need to fast (not eat or drink anything except water) for 8–12 hours before the test. A small blood sample is taken from a vein, usually in your arm, and sent to a lab for analysis.

What Are Common Results?

Results are typically reported in milligrams per deciliter (mg/dL). While specific ranges vary by lab, higher than expected levels may signal diabetes or insulin resistance, while lower levels could indicate hypoglycemia or other metabolic issues.

What Does This Test Tell Your Doctor?

High fasting blood sugar levels may indicate diabetes, where your body struggles to regulate blood sugar properly. Low levels might suggest conditions like hypoglycemia or hormonal imbalances. Your doctor will use these results alongside other factors to guide treatment or further testing.

Clinical interpretation

How abnormal results are interpreted and what conditions they suggest.

Clinical Significance: Fasting Blood Sugar Tests Play a Key Role in Diabetes Diagnosis and Monitoring

Short name

Fasting Blood Sugar

Frequently asked questions

How long should I fast before this test?
You’ll need to fast for 8–12 hours before the test. Avoid eating or drinking anything except water during this time.
What if I feel dizzy or weak during the blood draw?
It’s normal to feel a slight pinch or discomfort during the draw. If you feel dizzy, tell the phlebotomist immediately—they can help by applying pressure or slowing the process.
Can I take my usual medications before the test?
Check with your doctor or lab before taking any medications, including over-the-counter supplements. Some medications (like insulin or certain diabetes drugs) may 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