Which tests need fasting
Fasting matters when eating would change the result. Blood glucose rises after a meal, so a fasting glucose test needs 8 to 12 hours without food. Triglycerides rise sharply after eating fat, so a fasting lipid profile gives a more reliable reading, although many laboratories now accept non-fasting cholesterol tests for routine screening. Iron and some hormone tests are also affected by food and time of day.
Which tests do not
A full blood count, kidney and liver function, thyroid function, vitamin D, B12, folate, inflammatory markers such as CRP, and most hormone tests can be done at any time without fasting. HbA1c, the three-month average of blood sugar used to assess diabetes risk, does not need fasting either, which is one reason it has largely replaced fasting glucose for screening.
How to fast properly
Have your last meal the evening before and book a morning appointment. Water is fine and encouraged, as being well hydrated makes the blood draw easier. Avoid tea, coffee, juice, chewing gum and alcohol. Take regular medication unless you have been told otherwise, and if you are diabetic or take medication that must be taken with food, ask before fasting.
If you forget
Tell the person taking your blood. Some tests can still go ahead, others will be flagged as non-fasting on the report so the result is interpreted correctly, and a few will need rebooking. It is far better to say so than to have a misleading result.
At Omnia
When you book Randox blood testing at our clinic in Chiswick, W4, we tell you whether your panel includes any fasting markers and, if it does, offer an early appointment so you are not going hungry for long. Results are reviewed with you, and any non-fasting sample is noted in the interpretation.