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Glucose in Serum (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 glucose test checks for healthy sugar control and helps detect diabetes or prediabetes early."]

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

Key facts at a glance

LOINC
56884-0
Short code
GH 160M p Exc SerPl-mCnc
Synonyms
GH 160M p Exc 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 fatigue. It’s also used during pregnancy (gestational diabetes screening) or before certain treatments.

What to Expect Before the Test

You’ll need to fast (no food or drink except water) for 8–12 hours before the test. Bring any medications you take daily with you. Avoid smoking or strenuous activity before the draw.

How the Test Works

A healthcare provider will draw blood from a vein in your arm. The sample is sent to a lab where glucose levels are measured using an enzymatic method.

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

Frequently asked questions

What questions do patients typically ask?
Is fasting strictly necessary? Yes, skipping food ensures the test reflects your natural blood sugar levels. If you miss fasting, your results may be invalid.
What if my results are outside the normal range?
If your fasting glucose is high, your doctor may recommend lifestyle changes (like diet/exercise) or further testing (e.g., HbA1c). If low, they may check for causes like medications or underlying conditions.
Can I eat or drink anything before the test?
No—only water is allowed. Alcohol, caffeine, or sugary drinks can affect results. Follow your doctor’s specific instructions.
  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