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Glucose, Serum or Plasma 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 at least 8 hours. It helps assess how well your body uses glucose for energy.

Quick summary

["A fasting blood glucose test checks your blood sugar levels when you’re not eating, which is important for detecting diabetes or other metabolic conditions."]

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

Key facts at a glance

LOINC
59814-4
Short code
Glucose 7 AM SerPl-sCnc
Synonyms
Glucose 7 AM 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 healthcare provider may order this test to diagnose or monitor conditions like diabetes, prediabetes, or to check for signs of high blood sugar, especially if you have symptoms like excessive thirst, frequent urination, or unexplained weight changes.

What to Expect

You’ll need to fast (not eat or drink anything except water) for 8–12 hours before the test. A small blood sample will be taken from a vein, usually in your arm, by a phlebotomist.

Sample Collection

The blood is collected in a tube and sent to a lab where it’s analyzed to measure glucose concentration. The specimen is either serum (blood without clotting factors) or plasma (blood with clotting factors).

Testing Process

The test uses an enzymatic method to detect glucose, which is a common and reliable way to measure blood sugar levels accurately.

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 Glucose

Frequently asked questions

What does a fasting blood glucose test measure?
It measures the concentration of glucose in your blood after fasting, which helps determine if your blood sugar is within a healthy range or if you have diabetes or prediabetes.
How long should I fast before this test?
You should fast for at least 8 hours, meaning no food or liquids (except water) except for water before the test.
Can I take my usual medications before the test?
Check with your healthcare provider first, as some medications (like insulin or certain diabetes drugs) may need to be adjusted or temporarily stopped 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