What « fasting » means
Fasting means eating nothing and drinking only water for a set number of hours before the sample. Coffee, tea, juice and chewing gum all count as food.
How long depends on the test: often 8 to 12 hours for blood glucose and a lipid profile. Some tests need no fasting at all.
Why it matters
A recent meal changes the amount of sugar and fat in the blood for several hours. The reference ranges printed on the report were set on fasting samples, so for the comparison to mean something, yours should be fasting too.
In practice
When you put your request together, the laboratory merges the preparation of all your tests into a single instruction, with the time after which you should stop eating.
Take your medicines as usual unless your doctor says otherwise, and bring something to eat after the sample.