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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 at least 8 hours. It helps assess your blood sugar levels and identify risks for diabetes or other metabolic conditions.

Quick summary

["A fasting glucose test checks if your blood sugar is within a healthy range after not eating."]

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

Key facts at a glance

LOINC
47758-8
Short code
GH 15M p Gc SerPl-aCnc
Synonyms
GH 15M p Gc 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 This Test Is Done

Your doctor may order this test if you have symptoms like excessive thirst, frequent urination, unexplained weight changes, or fatigue. It’s also used as part of screening for diabetes, prediabetes, or to monitor existing diabetes management.

What to Expect Before the Test

You’ll need to fast (not eat or drink anything except water) for at least 8 hours before the test. Your doctor may provide specific instructions. Bring any medications you’re currently taking, as some may affect results.

During the Test

A healthcare provider will draw blood from a vein in your arm. You may feel a small pinch, but it usually doesn’t cause discomfort. The blood sample is sent to a lab for analysis.

What the Results Mean

Results are typically reported in milligrams per deciliter (mg/dL). While reference ranges vary by lab, higher than normal levels may indicate diabetes or insulin resistance, while lower levels could suggest conditions like hypoglycemia (low blood sugar).

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 glucose level mean?
A high fasting glucose level may signal diabetes or prediabetes, where your body struggles to regulate blood sugar properly. It can increase your risk for long-term health issues like heart disease, nerve damage, or kidney problems. Your doctor may recommend lifestyle changes, medications, or further testing.
What does a low fasting glucose level mean?
Low fasting glucose levels may indicate hypoglycemia, which can cause symptoms like shakiness, dizziness, confusion, or sweating. This can happen with certain medications, liver or kidney disease, or hormonal imbalances. If you experience these symptoms, talk to your doctor.
Can food or drinks before the test affect the results?
Yes, eating or drinking anything other than water before the test can raise your blood sugar and give an inaccurate result. Follow your doctor’s fasting instructions carefully 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