Glucose in Serum or Plasma (Fasting)
This test measures the amount of glucose (sugar) in your blood after you’ve fasted for at least 8 hours. It helps assess how well your body uses sugar for energy.
["A fasting blood glucose test checks your blood sugar levels to detect diabetes or prediabetes risk."]
Key facts at a glance
- LOINC
- 55381-8
- Short code
- Glucose 2.5h p 75 g Glc PO SerPl-sCnc
- Synonyms
- Glucose 2.5h p 75 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 This Test Matters
Your doctor may order this test to check for diabetes, high blood sugar, or other metabolic conditions. It’s also used to monitor treatment progress for people already diagnosed with diabetes or related disorders.
What to Expect Before the Test
You’ll need to fast (not eat or drink anything except water) for 8–12 hours before the test. Your blood will be drawn from a vein, usually in your arm. The process is quick and typically painless.
How the Test Works
A small sample of your blood is taken and sent to a lab. The lab uses an automated method (often enzymatic or electrochemical) to measure glucose levels in your serum or plasma.
What the Results Mean
Results are compared to general reference ranges, but your doctor will interpret them based on your medical history and other test results. Abnormally high or low levels may signal health concerns that need further evaluation.
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.
High fasting glucose levels (above normal ranges) may indicate diabetes, insulin resistance, or prediabetes. Consistently low levels could signal hypoglycemia or other metabolic issues. Regular monitoring helps prevent long-term complications like nerve damage, kidney disease, or heart problems.
Short name
Fasting Blood Glucose
Frequently asked questions
What causes fasting blood glucose to be high?
Hypoglycemia (low blood sugar) can result from excessive insulin use (e.g., from diabetes medication), certain medications (like beta-blockers or insulin), alcohol consumption, or prolonged fasting. It may also occur in people with adrenal or pituitary disorders.
Can I eat or drink anything before the test?
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