What fasting actually means, and what a cup of tea does to it
Ten to twelve hours, water only. Why the number moves, what still counts, and what to do if you slipped and had breakfast.
Almost every week somebody arrives at six in the morning, sits down for a fasting sugar, and mentions halfway through that they had a plain tea on the way. They are being honest, and it matters, so here is what we do with that information and why we ask in the first place.
Why the number moves
Glucose is not a fixed property of a person. It is a number that rises after a meal and settles again, and the whole point of a fasting blood sugar is to catch it at its settled baseline. That is the value the reference interval was built against, and it is the value your doctor is comparing against the last one.
Triglycerides move even more. A fatty meal the night before can lift them substantially, and because the LDL cholesterol on most lipid reports is calculated from the triglyceride figure rather than measured directly, one heavy dinner can move two numbers on the same page.
The rule, plainly
Ten to twelve hours, water only.
- Water is fine, and you should drink some. A dehydrated patient is harder to draw from.
- Plain tea is not water. Neither is coffee, and neither is a tea with one spoon of sugar “just to get here”.
- Chewing betel, and smoking, both count as breaking it.
- Keep taking your prescribed medicines unless the doctor who ordered the test has specifically said otherwise. Stopping a blood pressure tablet to get a cleaner lipid result is a bad trade, and it is not your call or ours to make.
Longer is not better. Past about fourteen hours the body starts doing something different again, so an overnight fast beginning after dinner and ending at half past six in the morning is exactly right.
Which tests actually need it
Fasting is required for the fasting blood sugar, the lipid profile, and iron studies. It is preferred but not essential for a liver profile, a bone profile and a uric acid.
A random blood sugar, a full blood count, a thyroid panel and almost everything else do not need it at all. If you have been told to fast for an FBC, somebody has been over-cautious. Ring us and we will tell you exactly what your prescription needs.
If you slipped, say so
This is the part worth remembering. Tell us, and nothing bad happens. We can take the sample and mark it clearly, we can switch you to a post prandial sugar which is a real test in its own right, or we can ask you to come back tomorrow, which takes one morning.
What we cannot do is unknow it later. A fasting sugar that was not fasting is not a slightly wrong result, it is a result labelled as something it is not, sitting in your record next to last year’s, being compared against it by a doctor who has no way to tell. A repeat visit costs you an hour. A wrong baseline can follow you for years.
One practical note
The queue is shortest at half past six, which is also when you most want to be seen, because you have not eaten. Come early, bring the prescription, and there will be a tea shop open by the time we are finished with you.