A fasting blood test requires you to avoid eating — and sometimes drinking anything other than water — for a specified period before the blood is drawn. The most common fasting window is 8–12 hours. The purpose is to measure a baseline state unaffected by the metabolic response to food.
Why fasting matters for certain tests
When you eat, your blood glucose rises as carbohydrates are absorbed. Triglycerides rise as dietary fats enter the bloodstream. Insulin responds to glucose. These changes persist for hours after eating. Tests that measure these parameters need to capture the stable baseline state, not the post-meal response.
Other tests — full blood count, thyroid function, HbA1c, kidney function, inflammatory markers — are not materially affected by eating and can be done at any time of day.
Practical guidance
- Water is always allowed and encouraged — being well-hydrated makes veins easier to find
- Take your regular medications with water unless your doctor has specifically asked you to omit them
- Avoid strenuous exercise the night before — it can affect glucose and creatinine
- Book a morning slot so the fast coincides with sleep — the hardest part of fasting is feeling hungry while waiting
- If you take insulin or other diabetes medications, get specific advice about timing from your doctor
- If you need to eat for medical reasons (pregnancy, hypoglycaemia risk), tell the phlebotomist — they may still be able to process the test with a note about non-fasting status