Glucose, Fasting (Serum)
This test measures your blood sugar level after you’ve gone without food for at least 8 hours. It helps your doctor check for diabetes, prediabetes, or other blood sugar issues.
["A fasting glucose test helps assess your risk for diabetes and monitor blood sugar control."]
Key facts at a glance
- LOINC
- 70208-4
- Short code
- Glucose pre 100 g Glc PO SerPl-sCnc
- Synonyms
- Glucose pre 100 g Glc PO SerPl-sCnc
- Category
- glucose
- Sample
- serum
- Container
- Red-top tube (SST)
- Units
- N/A
- Ref low
- 70
- Ref high
- 100
- Critical range
- 50 N/A – 400 N/A
- Fasting
- true
- Population
- adult
- Sex
- all
- Age range
- 18–65 years
- CPT
- 82947
- SNOMED
- 260395008
- 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 is this test done? A fasting blood glucose test measures the amount of sugar in your blood after you have not eaten for a set period of time. Your doctor may order it to screen for diabetes or prediabetes, to check how well your diabetes treatment plan is working, or to help find the cause of symptoms such as increased thirst, frequent urination, blurred vision, or unusual tiredness.
Your doctor may order this test to diagnose or monitor conditions like type 1 or type 2 diabetes, prediabetes, or to check for high blood sugar levels. It’s also used before surgeries or during pregnancy to assess risk factors.
How is the test performed? A member of your healthcare team will draw a small sample of blood, usually from a vein on the inside of your elbow or the back of your hand. The sample is placed into a tube and sent to a laboratory, where your glucose level is measured and the result is reported to your doctor.
You’ll fast (not eat or drink except water) for 8–12 hours before the test. A healthcare provider will draw blood from a vein in your arm. The blood sample is sent to a lab for analysis.
What should you do before the test? You will typically be asked to fast for 8 to 12 hours beforehand, which means no food or beverages other than water. Your doctor will tell you whether to take your regular medications, and you may be advised to avoid alcohol and strenuous exercise before the test. Be sure to follow the specific instructions your care team gives you.
Avoid caffeine, alcohol, or sugary drinks before testing. Wear comfortable clothing if you’ll need to have your arm exposed. If you have diabetes, follow your doctor’s specific fasting instructions.
What happens during the test? An elastic band is wrapped around your upper arm to make the vein easier to see, and the site is cleaned with an antiseptic. A needle is gently inserted, which may cause a brief sting, and the blood is collected into a small tube. The needle is then removed, pressure is applied, and a bandage is placed on the spot. The whole process usually takes only a few minutes.
You’ll sit comfortably while a small needle is inserted into your arm. A small amount of blood is collected into a vial or tube. The process usually takes a few minutes and may cause a brief pinch or discomfort.
Reference range
Fasting: 70-100 mg/dLN/A
Fasting for 8-12 hours required. Water is permitted. Avoid chewing gum and smoking before the test.
Clinical interpretation
How abnormal results are interpreted and what conditions they suggest.
Clinical Significance
Short name
Fasting Serum Glucose
Frequently asked questions
A fasting glucose test helps determine if your blood sugar is within a healthy range. High levels may indicate diabetes or insulin resistance, while very low levels could signal hypoglycemia or other metabolic issues. Your doctor will use the results to guide treatment or further testing.
Results are typically available within 1–3 business days, depending on your lab’s processing time. Your doctor will contact you with the final report.
If your results are outside the normal range, your doctor may recommend additional tests (like an oral glucose tolerance test) or lifestyle changes, such as diet adjustments or exercise. Follow-up visits will help monitor your condition.
Sources
- 1
- 2Lab Tests OnlineSource
- 3
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.
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