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Fasting Serum Glucose 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 assess how well your body uses sugar for energy.

Quick summary

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

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

Key facts at a glance

LOINC
77145-1
Short code
Glucose p fast SerPlBld-sCnc
Synonyms
Glucose p fast SerPlBld-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 diagnose or monitor conditions like diabetes, prediabetes, or metabolic disorders. It also helps track how well medications are working for managing blood sugar levels.

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. The process takes a few minutes and is generally painless.

What Happens During the Test

A healthcare provider will clean the area with an antiseptic, insert a small needle, and collect a blood sample into a tube. The needle is removed quickly, and you may feel a brief pinch. The sample is sent to a lab for analysis.

What the Results Mean

Results are compared to standard reference ranges, but your doctor will interpret them in the context of your health history and symptoms. High or low readings may signal potential issues that need further evaluation.

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 it take to get my results?
Results are typically available within 1–3 business days, depending on your lab’s processing time. Your doctor will contact you with the final report.
Can I eat or drink anything before the test?
No, you must fast for 8–12 hours before the test. Only water is allowed. Avoid alcohol, caffeine, or sugary drinks/snacks during this time.
Will I feel any pain or discomfort?
You may feel a slight pinch when the needle is inserted, but most people describe it as mild. The area may sting briefly afterward. Most people don’t experience lasting discomfort.
  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