How to fast properly before a blood test

Which tests need fasting, how long for, what you can drink, and what happens if you slip up.

Fasting instructions are given quickly at a counter and forgotten by the time you get home. Here is the whole thing in one place.

Which tests actually need it

Fewer than people assume. Every test page on this site states its own requirement, but broadly:

Fasting required

• Lipid profile: 9 to 12 hours
• Fasting blood glucose: 8 to 10 hours
• Liver function tests: 8 hours preferred
• Ultrasound of the abdomen: 6 hours, plus a full bladder for pelvic views

No fasting needed

• Complete blood count
• HbA1c
• Thyroid profile
• Vitamin D and B12
• Most infection and antibody tests

If you have booked several tests at once, your booking page shows a single combined instruction covering all of them. Follow that rather than working it out test by test.

What "fasting" actually means

No food and no calorie-containing drinks. That includes:

• Tea and coffee with milk or sugar
• Juice, soft drinks and energy drinks
• Milk, lassi and yoghurt drinks
• Chewing gum and mints
• Alcohol, ideally avoided for 24 hours before a lipid or liver test

Water is not only allowed, it is encouraged. Being well hydrated makes the vein easier to find and the sample easier to draw. Dehydration is the single most common reason a collection takes longer than it should.

Black tea and black coffee with nothing added are a grey area. They do not add calories, but caffeine can nudge glucose. If your test is specifically for blood sugar, skip them.

Timing it so it is bearable

The simplest approach is to finish dinner by 9pm, sleep through most of the fast, and book an early-morning slot. That way the majority of the fasting happens while you are asleep.

This is a large part of why our home-collection window opens at 7am. The earlier the slot, the shorter the waking fast.

Medications

Do not stop prescribed medication to fast unless your doctor has specifically told you to. For most medicines, taking them with a sip of water does not affect the result.

Tell us when you book if you:

• Take medication that must be swallowed with food
• Take insulin or other diabetes medication, since fasting affects your dosing and your safety
• Are diabetic and prone to hypoglycaemia

For anyone on insulin, the fasting duration and slot timing should be agreed with your doctor first.

If you slip up

Eat something by mistake? Say so. It is not a problem. We will either reschedule the fasting tests or note it on the sample so the laboratory can interpret the result correctly. A result reported as fasting when it was not is far worse than a rescheduled appointment.

Special cases

• Children. Fasting a young child overnight is not always appropriate. Ask us and we will confirm what is actually needed.
• Pregnancy. Follow your obstetrician's instruction; glucose tolerance testing has its own protocol.
• Elderly or frail patients. Book the earliest possible slot and keep drinking water.

Nothing here overrides an instruction from your own doctor. If what you were told differs from this page, follow your doctor.