Many people may wonder “Why must we fast before a blood test?” The answer is that the water we drink and the food we eat affect blood sugar and blood lipid levels.
Why must we avoid drinks and food before a blood test?
Blood tests can diagnose certain diseases that cannot be analyzed from medical history or diseases without symptoms. Therefore, blood test results require the highest clarity and accuracy. Nutrients and substances from various drinks we consume enter the bloodstream and may cause blood test results to vary from the true values. It is essential to fast from food and drinks before certain blood tests to ensure the most accurate results. After the blood draw, you can eat normally while waiting for the results.
How to prepare before a blood draw?
The fasting period varies depending on the purpose of the blood test. Those undergoing blood tests should prepare properly and follow these guidelines:
- Blood sugar test (Fasting Blood Sugar or FBS) requires fasting from food, drinks, snacks, candies, and chewing gum for at least 8 hours before the blood draw. However, plain water is allowed because anything other than water can affect blood glucose levels.
- Lipid profile test requires fasting from food, drinks, snacks, candies, and chewing gum for at least 10-12 hours, but plain water is also allowed. For triglycerides and LDL (bad cholesterol) tests, alcohol consumption should be avoided for 3 days before the blood draw.
- Serum iron test generally does not require fasting, but the doctor may consider individual preparation depending on medications taken, as some drugs can affect serum iron levels, making them higher or lower than actual.
- Vitamin B12 level test requires fasting for 12 hours before the blood draw.
- Kidney function tests (BUN, Creatinine) do not require fasting, but red meat intake should be reduced 2-3 days before the test.
The accuracy of blood test results depends largely on the testing procedure and the patient’s compliance. Therefore, before undergoing a blood test, it is important to understand the preparation clearly and strictly follow the doctor’s instructions.
Behaviors to avoid before a blood test
Besides “fasting from food and drinks”, there are other behaviors to avoid before a blood test, such as:
- Drinking alcoholic beverages
- Smoking
- Chewing gum
- Exercising
If the patient has done any of the above, they must inform the doctor honestly. The doctor may reschedule the blood test for a later date when the patient has properly prepared.
“Blood test results” are an important part of diagnosis to find the cause of disease or treatment options. To obtain the most accurate diagnosis, “before undergoing a blood test”, we must strictly follow the doctor’s instructions for our own benefit.
