Few questions come up more often before a blood test than this one: do I need to fast? Many people still skip breakfast out of habit, just in case. In practice, fasting before a blood test is needed for a small number of tests and not for most. This guide separates the myths from the facts, using the collection instructions London Medical Laboratory (LML) publishes for each test in the BPR catalogue.
What fasting means
When a test does need fasting, the NHS describes it as not eating or drinking anything other than water for a short time before the sample. The length depends on the test. The NHS also notes that you may sometimes be asked to stop certain medicines before a test, but only a GP, nurse or specialist should tell you to do that. Never stop a prescribed medicine on your own because of a blood test.
Tests in the catalogue that ask for fasting
Three examples show how specific the instructions can be:
- Insulin Resistance (Fasting): measures glucose and insulin and calculates HOMA-IR, a score of how the body responds to insulin. LML asks for an 8-hour fast, because both glucose and insulin change after eating.
- Growth Hormone (Fasting): measured after an overnight fast. LML also asks for 30 minutes of rest before the sample, and the sample needs urgent handling.
- Apolipoprotein E (12 hours fasting): a protein involved in how the body handles fats and cholesterol, measured after a 12-hour fast.
HOMA-IR is a calculated score, and its interpretation varies. A review of studies on insulin resistance found a high degree of variability in the cut-off points used, depending on age, sex and ethnicity (Tahapary et al., 2022). That is one more reason to discuss the result with a professional rather than compare it with a number found online.
Myth: you always need to fast for cholesterol
This is the most persistent one. For decades, lipid profiles were taken fasting. In 2016, a joint consensus statement from the European Atherosclerosis Society and the European Federation of Clinical Chemistry and Laboratory Medicine concluded that fasting is not routinely required for a lipid profile, because the changes after usual meals are small (Nordestgaard et al., 2016). The NHS says the same in plain words: you might be asked to fast for up to 12 hours, but most people do not need to.
The Cholesterol Lipid Profile in the catalogue does not list fasting in its collection notes, and LML runs it on a finger-prick home sample or a phlebotomy sample. If a professional has asked you for a fasting lipid profile for a specific reason, follow that instruction.
Myth: a fasting sample is always more accurate
Not necessarily. Fasting matters for markers that move with food, such as glucose and insulin, and for tests validated under fasting conditions. For many others, as the cholesterol example shows, the change after a usual meal is small. The right sample is the one the test was designed for.
