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Serum 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 assess your blood sugar levels and check for conditions like diabetes or prediabetes.

Quick summary

["A fasting blood glucose test helps monitor your risk for diabetes and understand how your body processes sugar."]

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

Key facts at a glance

LOINC
1533-9
Short code
Glucose 3h p 75 g Glc PO SerPl-mCnc
Synonyms
Glucose 3h p 75 g Glc PO 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 This Test Matters

Your doctor may order this test to diagnose or manage diabetes, check for high blood sugar over time, or evaluate how medications affect your blood sugar. It’s also used in screening for conditions like prediabetes or gestational diabetes.

How the Test Works

A small blood sample is taken from your arm, usually while you’re fasting for 8–12 hours. The sample is sent to a lab, where glucose levels are measured using an enzymatic method. Results are compared to general reference ranges to identify any abnormalities.

What to Expect Before the Test

You’ll need to avoid eating or drinking anything except water for the specified fasting period. Wear comfortable clothing and bring any medications you’re currently taking, as your doctor may ask for them.

What to Expect During the Test

The test involves a quick, painless blood draw from your arm. You may feel a slight pinch, but it usually takes only a few minutes. You can resume normal activities immediately afterward.

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 Blood Glucose

Frequently asked questions

What does a high or low result mean?
High fasting blood glucose levels may indicate diabetes or insulin resistance. Repeatedly elevated results could signal prediabetes, which is a warning sign for future diabetes risk. Low levels might suggest hypoglycemia (low blood sugar), especially if you’re taking diabetes medications or insulin.
How often should I get this test?
Your doctor will determine the frequency based on your health history, symptoms, and treatment plan. For example, people with diabetes may need tests every few months to monitor progress. Regular check-ups help adjust treatments and prevent complications.
Can I eat or drink anything before the test?
No, you must fast for 8–12 hours before the test, meaning only water is allowed. Avoid sugary drinks or snacks to ensure accurate 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