Good preparation makes a real difference to accuracy; a small slip such as a cup of coffee before fasting sugar can push the number up.
Rule one: not all tests need fasting. Only fasting sugar and lipids need 8–12 hours; water is fine, but tea, coffee, juice and smoking are not.
Drink water, wear loose sleeves and never stop a medicine on your own — tell the lab what you take, as some tests are drawn before the dose.
For hormones such as prolactin or testosterone, early morning after rest is best; ovulation hormones on day 2 or 3 of the cycle.
