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Glucose in Serum (Fasting)

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

This test measures your blood sugar level after you have not eaten for at least 8 hours. It helps check for diabetes or other blood sugar disorders.

Quick summary

["A fasting blood sugar test helps diagnose diabetes or monitor blood sugar levels over time."]

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

Key facts at a glance

LOINC
33116-5
Short code
GA from HL.ASUM00
Synonyms
GA from HL.ASUM00
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-hl-asum
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 type 1 or type 2 diabetes, prediabetes, or gestational diabetes. It can also help check for other conditions that affect blood sugar, such as hypoglycemia (low blood sugar) or hormonal imbalances.

What to Expect During the Test

You’ll need to fast (not eat or drink anything except water) for 8–12 hours before the test. A healthcare provider will draw blood from a vein, usually in your arm. The process takes a few minutes and may cause a slight pinch or discomfort.

How the Results Are Interpreted

Results are typically reported as a numerical value (mg/dL or mmol/L), but your doctor will discuss what the number means for your health. High levels may indicate diabetes or prediabetes, while very low levels could signal hypoglycemia or other issues.

Common Causes of Abnormal Results

Factors like stress, illness, certain medications, or recent food intake can affect blood sugar levels. If your results are outside expected ranges, your doctor may recommend further testing or lifestyle adjustments.

Clinical interpretation

How abnormal results are interpreted and what conditions they suggest.

This test is critical for early diabetes detection and management. High fasting glucose levels often appear before symptoms like frequent urination or extreme thirst develop, making it a key screening tool.

Short name

Fasting Serum Glucose

Frequently asked questions

What does a ‘fasting’ requirement mean?
‘Fasting’ means you shouldn’t eat or drink anything except water for at least 8 hours before the test. Skipping this step can give inaccurate results.
How often should I get this test?
Your doctor will determine how often you need this test based on your health history, symptoms, and risk factors. Some people with diabetes check it monthly, while others may need it less frequently.
Can I take my usual medications before the test?
Yes, but check with your doctor first. Some medications (like insulin or diabetes drugs) may affect your blood sugar, so they may recommend timing or adjustments.
  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