What to eat and drink before a blood test

One of the most common questions we hear before an appointment is whether the patient needs to fast. The answer depends entirely on which test is being collected, and getting it wrong in either direction causes problems: eating before a fasting panel can skew results, while unnecessary fasting before a non-fasting test just makes the morning harder than it needs to be.

Which tests require fasting, and for how long ¶

A twelve-hour fast is required for the lipid panel and the cardiac risk extended panel. This means nothing to eat or drink except water from the evening before your appointment. The reason is that dietary fat consumed in the hours before collection temporarily raises triglyceride levels and can affect LDL calculations, producing a result that does not reflect your baseline.

The comprehensive metabolic panel requires a four-hour fast. This is a shorter window because the markers most sensitive to food intake. Primarily blood glucose and certain liver enzymes. Stabilise within a few hours of a light meal. If you have an early morning appointment, skipping breakfast is usually sufficient.

Tests that do not require fasting ¶

The complete blood count, thyroid function panel (TSH, free T3, free T4), vitamin D, and most hormone panels do not require fasting. You can eat and drink normally before these appointments. The STI comprehensive panel and allergy IgE panels also have no fasting requirement.

If you are combining a fasting and a non-fasting panel in a single appointment. Which is common. You will need to follow the fasting requirement for the most restrictive test in your order. Your booking confirmation will specify this clearly.

What you can have during a fast ¶

Water is always permitted and is actually encouraged, as adequate hydration makes venipuncture easier and reduces the likelihood of a difficult draw. Plain black coffee without sugar or milk is generally acceptable for lipid panels, though some clinicians prefer patients to avoid it entirely for glucose-sensitive tests. When in doubt, stick to water.

Medications should generally be taken as normal unless your physician has specifically told you otherwise. If you take medication with food and are fasting, call us before your appointment and we can advise on timing.

If you are unsure about preparation for a specific panel, the safest approach is to call us the day before. It takes two minutes and avoids a wasted trip.