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

Quick summary

["A fasting blood sugar test checks your blood glucose levels when you haven’t eaten, helping identify risks for diabetes or other metabolic conditions."]

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

Key facts at a glance

LOINC
14768-6
Short code
Glucose BS SerPl-sCnc
Synonyms
Glucose BS 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 It’s Done

Your doctor may order this test if you have symptoms like extreme thirst, frequent urination, fatigue, or unexplained weight changes. It’s also used to diagnose or monitor conditions like diabetes, prediabetes, or hormonal imbalances.

What to Expect

You’ll need to fast for 8–12 hours before the test. A healthcare provider will draw blood from a vein in your arm. The process takes a few minutes, and you may feel slight discomfort.

How Results Are Used

Results help determine if your blood sugar is within a healthy range. Abnormally high or low levels may signal underlying health concerns that need further evaluation.

Common Causes of Abnormal Results

High fasting glucose levels can indicate diabetes or insulin resistance, while very low levels may suggest hypoglycemia (low blood sugar), especially in people with certain medical conditions or on medications.

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.

◆Associated findings1

Short name

Fasting Blood Sugar

Frequently asked questions

What does a ‘fasting’ requirement mean?
You must avoid eating or drinking anything—except water—8 to 12 hours before the test. This helps ensure the test reflects your body’s natural glucose levels when you haven’t recently eaten.
How long should I fast before the test?
Typically, you should fast for at least 8 hours, but your doctor may specify a longer duration (up to 12 hours). Check with your provider for exact instructions.
Can I take my usual medications before the test?
Yes, but avoid taking medications that contain sugar (e.g., some cough syrups) or insulin unless instructed otherwise. Always confirm with your doctor or lab technician.
  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