Do I need to fast? Which blood tests need it and which do not
The question labs hear most is “do I need to fast?”. It depends on the test: not everything needs it, and fasting the wrong way skews results as much as not fasting. Here is what needs it, what does not, and how to do it properly.
Tests that need fasting
Fasting blood sugar (FBS) needs at least 8 hours. A full lipid panel — triglycerides especially — is best after 9–12, because fat from a meal raises the value for hours. A glucose tolerance test (OGTT) needs 8–10 hours and two hours at the lab. Any check-up package that includes the first two will say to fast.
Tests that do not
Complete blood count (CBC), liver and kidney function, thyroid, long-term sugar (HbA1c), vitamin D and most infection and immunity tests can be done at any time. HbA1c is why some labs offer a no-fasting diabetes screen — but if that set also includes lipids, the first rule applies again.
How to fast properly
Fast from food and calorie-containing drinks, not from water — sip plain water, and do drink it, because it makes the draw easier. Skip coffee even without sugar, chewing gum and cigarettes. Take regular medicines as usual unless told otherwise (some diabetes tablets wait until after the draw). Fasting beyond 14 hours, or until you are ravenous, skews values the other way. The easiest method: a normal dinner, sleep, and the first appointment of the morning.
