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Glucose, 25-hour fasting, measured by enzymatic method in serum/plasma

Published: August 9, 2026Updated: August 16, 2026

This test checks your blood sugar level after a long period of fasting, typically 25 hours, to help assess how your body manages glucose.

Quick summary

["A fasting glucose test helps diagnose conditions like diabetes or identify blood sugar imbalances over time."]

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

Key facts at a glance

LOINC
13865-1
Short code
Glucose 2.5h p 50 g Lac PO SerPl-mCnc
Synonyms
Glucose 2.5h p 50 g Lac PO SerPl-mCnc
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

Fasting glucose tests are crucial for diagnosing diabetes or prediabetes, as they reveal how your body processes glucose over an extended period. Unlike standard fasting tests (e.g., 8–12 hours), a 25-hour test may better reflect long-term metabolic patterns, though results should be interpreted with clinical context.

How the Test Works

Your blood is drawn after fasting for 25 hours. The sample is analyzed using an enzymatic method to measure glucose concentration in your serum or plasma.

What to Expect Before the Test

Do not eat or drink anything (except water) for 25 hours before the test. Avoid caffeine, nicotine, or strenuous activity during this period. Your provider may give specific instructions based on your health status.

What the Results Mean

Results are compared to clinical guidelines, but reference ranges may vary by lab. High or low readings could signal diabetes, insulin resistance, or other metabolic disorders. Always discuss results with your healthcare provider.

Reference range

Fasting: 70-100 mg/dLN/A

Specimen: serum

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 Glucose (25-hour)

Frequently asked questions

Why was this test ordered?
Your doctor may recommend this test if you have symptoms of diabetes (e.g., frequent urination, extreme thirst), a family history of diabetes, or if you’re managing chronic conditions like hypertension or heart disease. It may also be used in research or specialized metabolic studies.
What does a normal result indicate?
A normal result suggests your body handles glucose efficiently over a prolonged fast, but it doesn’t rule out undiagnosed conditions. Follow-up tests (e.g., HbA1c) may be needed for further evaluation.
Can I eat or drink anything before the test?
No—strict fasting is required. Ask your provider if special dietary adjustments are needed, especially if you have kidney disease or other health concerns.
  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