Prepare for your Weight Loss Blood Test by following these instructions. Please take your sample before 10am. Take this test when any symptoms of short-term illness have settled. Avoid heavy exercise for 48 hours beforehand. Avoid fatty foods for eight hours before your test, you do not need to fast. Take this test two to five days after the start of your period, ideally on day three. It can be taken any time if you do not have periods. Hormonal contraception can affect this test, taking a break from this and using barrier contraception will give more accurate results. Corticosteroid medication can affect this test, ask your doctor whether to stop before testing. Avoid taking your sample from a finger used to apply hormone gels/pessaries in the past 4 weeks. Use gloves to apply these. You should take this test before you take any medication or vitamin/mineral supplements. Do not take biotin supplements for two days before this test, discuss this with your doctor if it is prescribed.