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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 at least 8 hours. It helps doctors check for diabetes or other blood sugar issues.

Quick summary

["A fasting blood sugar test helps assess your risk of diabetes and manage blood glucose levels."]

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

Key facts at a glance

LOINC
55512-8
Short code
GH 30M p Gc SerPl-mCnc
Synonyms
GH 30M p Gc SerPl-mCnc
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 This Test Matters

Your doctor may order this test if you have symptoms like excessive thirst, frequent urination, unexplained weight changes, or a family history of diabetes. It’s also used to monitor blood sugar levels in people with known diabetes or prediabetes.

What to Expect Before the Test

You’ll need to fast (not eat or drink anything except water) for 8 to 12 hours before the test. Bring your fasting schedule to your appointment to confirm the timing. Wear comfortable clothing and plan to arrive early if you need help with fasting.

During the Test

A healthcare provider will clean and insert a small needle into a vein in your arm. You may feel a slight pinch, but it’s usually over quickly. Blood will be drawn into a tube, and the site will be cleaned and bandaged afterward.

After the Test

You can eat and drink normally right after the test. If you had a needle inserted, apply gentle pressure to the site to stop any bleeding. Avoid heavy lifting or strenuous activity for a few hours.

Clinical interpretation

How abnormal results are interpreted and what conditions they suggest.

◆Associated findings1

Short name

Fasting Serum Glucose

Frequently asked questions

How long does fasting last for this test?
You’ll need to fast for at least 8 hours before the test, but some labs may require up to 12 hours. Follow your doctor’s or lab’s specific instructions to ensure accurate results.
Can I take my usual medications before fasting?
If your medications are essential (like insulin or diabetes medications), ask your doctor whether to take them on the fasting day. Otherwise, avoid them unless instructed otherwise.
What if I feel dizzy or weak during the blood draw?
Tell the healthcare provider right away. They may give you a quick sugar drink or adjust your position to help you feel better until the test is complete.
  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