How many fasting hours are needed for an OGTT?

An OGTT compares blood glucose before and after a measured drink. Food, sweetened beverages and some preparation errors can alter the starting measurement, making the test harder to interpret. A fasting window is therefore a controlled part of the appointment rather than a general dieting instruction.

Instructions may differ according to the type of OGTT, whether you are pregnant, and the laboratory's protocol. Do not assume that advice from a previous test applies to this appointment.

Ask the clinic exactly when to finish food and which drinks are allowed. Arrange an appointment that makes the fasting period manageable, keep taking medicines only as instructed, and bring something to eat after the final blood sample. Avoid strenuous activity while waiting because it may affect how your body uses glucose.

If you lose track of the time, eat, smoke, chew gum or drink something other than permitted fluids, tell staff honestly. They can protect the usefulness of the result by advising whether to proceed or rebook.

Call the clinic if fasting causes repeated vomiting or you cannot keep water down. While waiting, report dizziness, shaking, sweating, nausea or marked weakness immediately. Staff can assess you and help you sit safely. Emergency help is needed for fainting, severe confusion, breathing difficulty or chest pain.

Ask for the precise fasting start time, whether medication should be taken, whether brushing your teeth is acceptable, and what to do if you arrive outside the planned window. Ask when results will be available and who will discuss them with you.