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Serum or plasma concentration of glucose (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 sugar for energy and can help detect diabetes or other metabolic conditions.

Quick summary

["A fasting blood sugar test helps identify potential diabetes or insulin resistance."]

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

Key facts at a glance

LOINC
49950-9
Short code
GH 2 AM SerPl-aCnc
Synonyms
GH 2 AM SerPl-aCnc
Category
chemistry
Sample
serum
Container
Red-top tube (SST)
Units
N/A
Fasting
false
Population
adult
Sex
all
Age range
18–65 years
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 Is This Test Done?

Your healthcare provider may order this test if you have symptoms like excessive thirst, frequent urination, unexplained weight changes, or fatigue. It’s also used to monitor diabetes management or screen for 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. Avoid sugary drinks or snacks. Bring your fasting instructions with you.

How the Test Is Performed

A blood sample is taken from a vein, usually in your arm. You may feel a small pinch, but it won’t cause significant discomfort. The test is quick and done in a lab.

What the Results Mean

Results are compared to a reference range, but exact values depend on your lab. High levels may indicate diabetes or insulin resistance, while low levels could signal hypoglycemia or other conditions.

Clinical interpretation

How abnormal results are interpreted and what conditions they suggest.

◆Associated findings1

Short name

Fasting Blood Sugar (Glucose)

Frequently asked questions

What causes fasting blood sugar to be high?
High fasting blood sugar can result from diabetes, insulin resistance, stress, certain medications (e.g., steroids), or hormonal imbalances. Lifestyle factors like poor diet or lack of physical activity may also play a role.
What causes fasting blood sugar to be low?
Low fasting blood sugar (hypoglycemia) can occur from excessive insulin, certain medications (e.g., diabetes drugs), alcohol consumption, or liver/kidney issues. Symptoms include shakiness, dizziness, or confusion.
Do I need to fast for this test?
Yes, fasting is required unless your provider specifies otherwise. Follow your lab’s instructions precisely to ensure accurate results.
  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