Blood sugar level measured after 8-12 hours of fasting. One of the most widely used tests for diabetes screening, prediabetes diagnosis, and follow-up of patients with diabetes.
Indication
- Routine diabetes screening in adults over 45
- Screening in individuals with diabetes risk factors such as obesity, sedentary lifestyle, or family history
- Symptoms such as excessive thirst, frequent urination, and unexplained weight loss
- Monitoring in known diabetes patients
- Initial assessment in pregnancy diabetes screening
- Routine biochemistry before surgery
- Evaluation of polycystic ovary syndrome and metabolic syndrome
Preparation
- No food or drink for 8-12 hours before the test (water is allowed)
- Fasting period should not exceed 14 hours
- No smoking before the test
- Acute illness (fever, flu) may affect results; screening can be postponed during active illness
- All medications used (especially corticosteroids, antipsychotics, diuretics) should be reported to the physician
How it's performed
- In the morning, on an empty stomach, 2-3 mL of blood is drawn from an arm vein
- Blood is placed in a gel or sodium fluoride tube
- Plasma glucose level is measured on an automated biochemistry analyzer
- Result is reported in mg/dL
- If borderline or elevated, an OGTT (oral glucose tolerance test) or HbA1c may be requested
Post-procedure
- Results are interpreted considering age, pregnancy status, and accompanying conditions
- If results are equivocal, the test is repeated on a different day
- When diagnosis is confirmed, HbA1c, lipid profile, and kidney function tests are added
- If diabetes is detected, nutrition, exercise, and medication therapy are planned as needed
- In prediabetes, follow-up every 6-12 months is recommended
Risks
- Temporary bruising at the blood draw site
- Rare hematoma
- Dizziness or hypoglycemia from prolonged fasting (especially in those using insulin/sulfonylureas)
- Very rarely, local infection
FAQ
How are results interpreted?
Generally accepted thresholds: below 100 mg/dL is normal, 100-125 mg/dL indicates impaired fasting glucose (prediabetes), and 126 mg/dL or above on two separate measurements is significant for diabetes diagnosis. A single measurement is not sufficient for diagnosis; the physician evaluates results with the clinical picture.
What is the ideal fasting duration?
8-12 hours is ideal. Fasting longer than 14 hours may falsely lower results; an appointment closest to breakfast time is preferred.
Can I drink water?
Plain water is allowed during fasting. Tea, coffee, sugary or carbonated drinks should not be consumed.
Can a single high result diagnose diabetes?
No. Without typical symptoms (excessive thirst, frequent urination, weight loss), diagnosis requires either two separate days with results of 126 mg/dL or above, or additional tests such as HbA1c.
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