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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 8–12 hours. It helps doctors check how well your body uses glucose for energy.

Quick summary

["A fasting blood sugar test helps diagnose diabetes or other blood sugar issues."]

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

Key facts at a glance

LOINC
54268-8
Short code
Glucose pre OKG SerPl-sCnc
Synonyms
Glucose pre OKG 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 Is This Test Done?

Your doctor may order this test to check for diabetes, prediabetes, or other conditions that affect blood sugar. It’s also used to monitor treatment progress if you have a known condition.

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.

How the Test Works

A small amount of your blood is taken and sent to a lab. The lab measures the glucose concentration in your serum using an enzymatic method to determine your fasting blood sugar level.

What the Results Mean

Results are typically reported in milligrams per deciliter (mg/dL). Your doctor will interpret the results based on your medical history and symptoms.

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.

This test is critical for diagnosing diabetes (fasting glucose ≥126 mg/dL) or prediabetes (fasting glucose 100–125 mg/dL). Abnormally high or low levels can indicate metabolic disorders or other health risks.

Short name

Fasting Blood Sugar

Frequently asked questions

What causes fasting blood sugar to be high?
High fasting blood sugar can be caused by diabetes, insulin resistance, certain medications, stress, or poor diet. If you suspect this, discuss symptoms with your doctor.
What if I have a low blood sugar (hypoglycemia) before the test?
If you experience symptoms like shakiness or dizziness before testing, inform your doctor or lab staff immediately. They may adjust your fasting window or retest later.
How often should I get this test?
Your doctor will determine how often you need this test based on your health status. Regular check-ups may include fasting blood sugar tests every 1–3 years for those without diabetes.
  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