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Glucose, Serum (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 check how well your body uses sugar for energy.

Quick summary

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

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

Key facts at a glance

LOINC
1499-3
Short code
Glucose 1h p .5 g/kg Glc IV SerPl-mCnc
Synonyms
Glucose 1h p .5 g/kg Glc IV SerPl-mCnc
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 Is This Test Done?

Your doctor may order this test to check for diabetes, high blood sugar, or other conditions affecting how your body processes sugar. It’s also used to monitor treatment for conditions like 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. Your doctor will provide specific instructions. Bring any medications you’re currently taking to your appointment.

How the Test Works

A healthcare professional will draw blood from a vein in your arm. The blood sample is sent to a lab to measure glucose levels using an enzymatic method.

What the Results Mean

Results are compared to general reference ranges, but your doctor will interpret them in the context of your overall health. High or low levels may signal diabetes, hypoglycemia, or other health concerns.

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.

High fasting blood sugar levels (above normal ranges) often indicate diabetes or insulin resistance. Low levels may suggest hypoglycemia or other metabolic issues. Your doctor will discuss results with you and recommend next steps if needed.

Short name

Fasting Blood Sugar

Frequently asked questions

What causes fasting blood sugar levels to be high?
High fasting blood sugar can be caused by diabetes, poor insulin function, certain medications, stress, or infections. Lifestyle factors like diet, physical activity, and obesity also play a role.
How can I prepare for a fasting blood sugar test?
Follow your doctor’s instructions for fasting. Avoid eating or drinking anything except water for 8 to 12 hours before the test. Wear comfortable clothing and bring your medications if prescribed.
Can I take my diabetes medication before the test?
If you take diabetes medication, ask your doctor whether to take it before the test. Some medications may need to be adjusted or skipped to get an accurate result.
  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