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Serum/plasma 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 8–12 hours. It helps assess how well your body regulates blood sugar levels.

Quick summary

["A fasting blood sugar test checks for healthy glucose control and can help detect diabetes or other metabolic conditions."]

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

Key facts at a glance

LOINC
25669-3
Short code
Glucose 3.5h p Glc SerPl-sCnc
Synonyms
Glucose 3.5h p Glc 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 This Test Matters

Your doctor may order this test if you have symptoms like excessive thirst, frequent urination, unexplained weight changes, fatigue, or if you have a family history of diabetes. It’s also used during pregnancy, before surgeries, or to monitor chronic conditions like diabetes or prediabetes.

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 doctor or lab technician will take a blood sample from a vein, usually in your arm. The process is quick and typically painless.

What the Results Mean

Results are not provided as a single numeric range here, but they’re compared to your doctor’s reference values. High or low readings may signal potential health concerns that need further evaluation.

Common Reasons for Testing

Doctors may check fasting blood sugar levels to diagnose diabetes, check for prediabetes, evaluate symptoms of metabolic disorders, or monitor treatment progress for conditions like diabetes or kidney disease.

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

How long does it take to get my results?
Results are usually available within 1–3 business days, depending on your lab. 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 during this time.
What if my blood sugar is too low or too high?
If your results are outside expected ranges, your doctor will discuss next steps, which may include retesting, lifestyle changes, or additional tests to understand the cause.
  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