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Serum or plasma 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 8 to 12 hours. It helps doctors check how well your body uses sugar for energy.

Quick summary

["A fasting blood glucose test helps diagnose diabetes or prediabetes and assesses blood sugar control."]

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

Key facts at a glance

LOINC
31908-7
Short code
N men sg A Ag Spec Ql
Synonyms
N men sg A Ag Spec Ql
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

Fasting blood glucose is critical for diagnosing conditions like diabetes, where your body struggles to regulate blood sugar properly. It also helps monitor existing diabetes to prevent complications like nerve damage or heart disease.

When You Might Need It

Your doctor may order this test if you have symptoms like excessive thirst, frequent urination, unexplained weight loss, or fatigue. It’s also part of routine check-ups for people at higher risk of diabetes.

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. Bring a list of medications you’re taking, as some may affect results.

What Happens During the Test

A healthcare provider will draw blood from a vein in your arm. The process is quick and usually painless. You may feel a slight pinch, but it doesn’t last long.

Clinical interpretation

How abnormal results are interpreted and what conditions they suggest.

Clinical Significance

Short name

Fasting blood glucose

Frequently asked questions

A high fasting blood glucose level (above normal ranges) may indicate diabetes or insulin resistance. Low levels (rarely seen in fasting tests) could signal hypoglycemia or other metabolic issues. Regular monitoring helps prevent long-term health problems.
This test is typically done once, but your doctor may recommend follow-up tests (like hemoglobin A1c) to track blood sugar trends over time.
What does a ‘normal’ result mean?
A ‘normal’ fasting blood glucose result doesn’t mean you’re completely healthy—it just means your blood sugar isn’t dangerously high. Some people with prediabetes may have borderline results, so follow-up testing is often needed.
Can I eat or drink anything before the test?
No, you must fast for 8 to 12 hours. Only water is allowed. Skipping this step can lead to inaccurate 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