Fasting instructions exist because food in the stomach changes the measurement, not because fasting is good for you. Understanding which tests are affected makes the instructions much easier to follow correctly.
What is actually affected
- Blood glucose and lipids. These are the tests that genuinely require fasting — typically 8 to 12 hours.
- Abdominal ultrasound. A full stomach and gas obscure the organs. This is about what is in the stomach, not about metabolism.
- Liver and kidney chemistry. Mildly affected; your centre will say whether it matters for their panel.
- Most other tests. Blood count, thyroid function, tumour markers and urine tests generally do not require fasting.
Water is almost always allowed
Unless you have been told otherwise, drink water. Dehydration makes blood draws harder and can distort several results. Being thirsty is not the point of fasting.
Medications
Do not stop a prescribed medication on your own to prepare for a checkup. Blood pressure medication, cardiac medication, thyroid replacement and anticoagulants are usually continued. If you take diabetes medication or insulin, ask the centre in advance — this is the one category where the instruction genuinely depends on your personal regimen.
Can I drink coffee?
Black coffee without sugar or milk is usually acceptable, but it can affect some results and it does affect blood pressure. Ask, or skip it — one morning without coffee is not the hard part of a checkup.
What if I accidentally ate something?
Tell the centre when you arrive. In most cases they will move the fasting tests to another morning rather than report a misleading number.
How long before the appointment should I arrive?
Fifteen to twenty minutes. You will be registering, and the fasting clock is running.