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Glucose in Serum (Fasting)

Published: August 9, 2026Updated: August 17, 2026

This test measures your blood sugar level after you’ve fasted for at least 8 hours. It helps your doctor check for diabetes, prediabetes, or other blood sugar issues.

Quick summary

["Fasting blood sugar levels help assess your risk for diabetes and metabolic health."]

Medically reviewed by WellAlly Editorial TeamLast reviewed August 17, 2026Editorial policy

Key facts at a glance

LOINC
40298-2
Short code
GH pre CLN SerPl-mCnc
Synonyms
GH pre CLN SerPl-mCnc
Category
chemistry
Sample
serum
Container
Red-top tube (SST)
Units
N/A
Ref low
0
Ref high
17
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 to monitor blood sugar control in people with diabetes.

What to Expect Before the Test

You’ll need to fast (no food or drink except water) for 8–12 hours before the test. Your doctor may provide specific instructions. Bring any medications you’re currently taking, as some may affect results.

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 automated method, often an enzymatic technique.

What the Results Mean

Results are typically reported in milligrams per deciliter (mg/dL). A fasting glucose level below 100 mg/dL is normal, while levels above 126 mg/dL may indicate diabetes. Values between 100–125 mg/dL suggest prediabetes.

Reference range

NCBI: NSE 0-17 ng/mLN/A

Specimen: serum

No special preparation usually required. Stay well hydrated. Inform your healthcare provider of all medications and supplements you are taking.

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 it take to get my results?
Results are usually available within 1–3 business days, depending on your lab’s processing time. Your doctor will contact you with the final report.
Can I eat or drink anything before the test?
No, you must fast (no food or liquids except water) for 8–12 hours before the test. Check with your doctor if you have questions about medications or special diets.
What if my blood sugar is too low or too high?
If your results are low (hypoglycemia), you may feel shaky, dizzy, or confused. If high (hyperglycemia), you might experience excessive thirst or fatigue. Your doctor will discuss next steps based on your 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 17, 2026. · Read our editorial policy