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Glucose in Serum (Fasting)

Published: August 9, 2026Updated: August 16, 2026

This test measures your blood sugar level after you’ve fasted for a set period. It helps doctors check for diabetes, prediabetes, or other blood sugar-related conditions.

Quick summary

["A fasting blood sugar test helps assess your body’s ability to manage glucose, often used to diagnose diabetes or monitor metabolic health."]

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

Key facts at a glance

LOINC
47925-3
Short code
GH 5M p Arg+Ins SerPl-aCnc
Synonyms
GH 5M p Arg+Ins SerPl-aCnc
Category
hormone
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 This Test Matters

Your fasting blood sugar level reflects how well your body processes glucose, the main energy source for your cells. Consistently high or low levels can signal health risks, including diabetes, heart disease, or kidney problems.

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 in your arm. The process takes a few minutes and is usually painless.

How the Test Works

A small amount of blood is drawn from your arm and sent to a lab. The lab uses an automated method to measure glucose concentration in your serum (liquid part of blood). This test is quick and doesn’t require special preparation beyond fasting.

Possible Results and Next Steps

Results may be compared to general reference ranges (e.g., normal: typically below 100 mg/dL; prediabetes: 100–125 mg/dL; diabetes: 126 mg/dL or higher). If your result is outside these ranges, your doctor may recommend follow-up tests or lifestyle changes.

Clinical interpretation

How abnormal results are interpreted and what conditions they suggest.

◆Associated findings1

Short name

Fasting Serum Glucose

Frequently asked questions

What does a high fasting blood sugar mean?
A high fasting blood sugar (above reference ranges) may indicate diabetes, insulin resistance, or other metabolic disorders. It can also be caused by stress, illness, or certain medications. Your doctor will discuss next steps, such as additional tests or treatment plans.
What does a low fasting blood sugar mean?
Very low fasting blood sugar (hypoglycemia) can cause symptoms like shakiness, dizziness, or confusion. While rare in fasting tests, it may signal conditions like adrenal insufficiency or certain medications. Report symptoms to your doctor immediately.
How often should I get this test?
Your doctor will determine how often to repeat this test based on your health history, symptoms, and other test results. Typically, it’s done once or twice a year for people with diabetes or prediabetes, or as recommended for other conditions.
  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