How long to fast before a blood test, and what to avoid

Why some tests need fasting

Food and energy-containing drinks temporarily raise blood glucose and triglycerides. The result then no longer reflects your normal state and may be misinterpreted. Fasting for the recommended time makes results comparable with previous tests.

Tests that usually require fasting include fasting blood sugar, lipid profile and triglycerides. Others, such as a complete blood count, urine analysis or thyroid tests, do not require fasting.

How many hours

Typically 8-12 hours for glucose and lipid tests. Fasting longer than 12-14 hours is not better, because the body starts breaking down fat and results can drift again.

For a morning test, fasting from after dinner and drinking plain water normally works well. The practical target is a blood draw about 8-10 hours after your last meal.

What you may drink and what to avoid

Plain water is fine during the fast, but avoid all energy-containing drinks: coffee and tea with milk or sugar, juices, soft drinks and alcohol.

• Plain water is allowed and makes the blood draw easier
• Avoid coffee, tea, milk, juice and energy drinks
• Avoid alcohol for at least 24 hours before the test
• Avoid smoking on the morning of the test, as it affects blood pressure and some values
• Regular medicines: ask your doctor whether to skip a dose, especially for blood pressure, diabetes and lipid medicines

If you forgot to fast

Tell our staff before the sample is taken. We may reschedule or switch to tests that remain valid in that state, which is better than testing and needing to repeat it.

If a blood test is urgently needed for treatment reasons, a doctor will decide whether the result still provides useful clinical information without fasting.