Skip to content

Fasting Serum Glucose (Mass Concentration)

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 doctors assess your blood sugar levels and identify conditions like diabetes.

Quick summary

["A fasting blood sugar test checks if your body processes sugar properly and helps prevent long-term health risks."]

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

Key facts at a glance

LOINC
14771-0
Short code
Glucose p fast SerPl-sCnc
Synonyms
Glucose p fast SerPl-sCnc
Category
glucose
Sample
serum
Container
Red-top tube (SST)
Units
N/A
Ref low
10
Ref high
40
Critical range
50 N/A – 400 N/A
Fasting
true
Population
adult
Sex
all
Age range
18–65 years
CPT
84484
SNOMED
302774002
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 check for diabetes or prediabetes, monitor existing diabetes, or evaluate symptoms like excessive thirst, fatigue, or unexplained weight changes. It’s also used before certain treatments or surgeries.

How the Test Works

You’ll fast (no food or drink except water) for 8–12 hours before the test. A small blood sample is taken from your arm, usually by a phlebotomist. The sample is sent to a lab where glucose levels are measured using an enzymatic method.

What to Expect Before the Test

No special preparation is needed beyond fasting. Wear comfortable clothing and bring any medications you’re currently taking. If you have a history of blood clots or bleeding disorders, let your doctor know.

What to Expect During the Test

The blood draw is quick and painless. You may feel a slight pinch, but most people tolerate it well. If you’re nervous, ask your doctor about numbing cream or other options.

Reference range

Adult: 10-40 U/LN/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

How long does fasting last for this test?
You’ll need to fast for at least 8 hours before the test, typically overnight. Avoid eating or drinking anything except water during this time.
Can I take my usual medications before the test?
Yes, but always check with your doctor first. Some medications (like insulin or certain blood pressure drugs) may affect your results.
Will I feel different after the blood draw?
You may feel a slight bruise or tenderness at the site, but most people don’t experience lasting effects. If you have a bleeding disorder, inform your doctor beforehand.
  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