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Glucose, Serum (Fasting)

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

This test measures the amount of glucose (sugar) in your blood after you’ve fasted for 8–12 hours. It helps assess your blood sugar levels and overall metabolic health.

Quick summary

["A fasting glucose test checks if your blood sugar is within a healthy range after not eating."]

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

Key facts at a glance

LOINC
14763-7
Short code
Glucose 30M p Glc SerPl-sCnc
Synonyms
Glucose 30M p Glc SerPl-sCnc
Category
glucose
Sample
serum
Container
Red-top tube (SST)
Units
N/A
Ref low
70
Ref high
100
Critical range
50 N/A – 400 N/A
Fasting
true
Population
adult
Sex
all
Age range
18–65 years
CPT
82947
SNOMED
260395008
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 to diagnose or monitor conditions like diabetes, prediabetes, or to check for high blood sugar levels. It’s also used during pregnancy (gestational diabetes screening) or if you have symptoms like excessive thirst, fatigue, or frequent urination.

What to Expect Before the Test

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

What the Results Mean

While specific ranges vary by lab, higher-than-normal results may indicate diabetes or insulin resistance, while lower-than-normal results could signal hypoglycemia (low blood sugar). Your doctor will interpret results in the context of your overall health.

Common Conditions Linked to This Test

Abnormal results may signal diabetes, prediabetes, or metabolic disorders. It’s also used to evaluate how well medications (like insulin) are working or to screen for conditions like liver or kidney disease.

Reference range

Fasting: 70-100 mg/dLN/A

Specimen: serum

Fasting for 8-12 hours required. Water is permitted. Avoid chewing gum and smoking before the test.

Clinical interpretation

How abnormal results are interpreted and what conditions they suggest.

Clinical Significance

Short name

Fasting Serum Glucose

Frequently asked questions

How long does fasting last before the test?
Typically, you should fast for 8–12 hours, meaning no food or liquids (except water) after midnight the night before the test.
Can I take my usual medications on the fasting day?
Check with your doctor first, as some medications (like insulin or diabetes meds) may need adjustment or timing changes.
What if my blood sugar is too low before the test?
If you experience symptoms like shakiness, dizziness, or sweating, inform the phlebotomist immediately. They may adjust the procedure or provide a quick sugar source (like juice) to stabilize you before testing.
  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