Blood Tests Before Treatment: Planning for Fasting, Repeat Blood Draws, and Getting Results

After receiving a blood test order, ask the ordering doctor what question each test is intended to answer and which tests require fasting. After the blood draw, also confirm who will review the results and when you will be told about treatment arrangements.

Key points

• Preparation requirements vary by test. If you have not been told to fast, you do not need to fast on your own.
• Do not stop medicines on your own. Give the ordering doctor a list of your medicines and supplements, and confirm how to take each one before the blood draw.
• If you have eaten or drunk anything, or have not followed the preparation instructions, tell the staff taking your blood. The team will decide whether to proceed.
• An abnormal flag on a report does not mean treatment is canceled. Ask the ordering doctor to explain the results in the context of your symptoms, changes over time, and treatment plan.

Before your appointment: Check the tests and preparation

Tests such as a complete blood count, blood chemistry tests, and clotting tests answer different questions. Doctors may not order the same set for everyone. A complete blood count provides information about blood cells, blood chemistry tests can help assess metabolism and organ function, and clotting tests address specific questions about clotting or bleeding. Ask the doctor how each result may affect your tests or treatment.[3]

Ask the hospital to confirm the tests ordered, the department that ordered them, the hospital site and time for the blood draw, whether the order is in the system, what identification to bring, and how to pay. If you recently had tests abroad, give the care team the complete original reports first. Ask which results they can use, which tests need to be repeated, and why. The doctor may request repeat tests if your condition has changed or treatment requires more recent results.

For each test that needs special preparation, ask exactly when preparation starts and ends, and whether there are restrictions on plain water, other drinks, exercise, or smoking. If you have diabetes, are pregnant, are physically frail, or have felt unwell while fasting before, tell the ordering doctor in advance so they can arrange preparation suited to you. If you are also having anesthesia or other tests that day, follow the relevant team's instructions on food and drink. Do not apply routine blood test instructions to those procedures.

List all prescription medicines, over-the-counter medicines, vitamins, and supplements you use. Include the dose, your usual dosing time, and when you last took each one. Ask the doctor about each item: “Should I take this as usual before this blood draw? If I need to make a change, when should I resume?” Do not stop medicines on your own. If the instructions are unclear, contact the ordering doctor before the blood draw.

On the day of your blood draw: Bring reports and check your identity

Bring the identification, test orders, and previous reports required by the hospital. Wherever possible, reports should retain your name, the sample collection date, results, units, reference ranges, and laboratory details. A translated summary can help with communication, but keep the originals for checking.

Follow the blood collection site's process to check your name and one other identifying detail, and confirm that the tube labels match your identity. Before the draw, tell the staff if you have fainted during a blood draw, have had difficulty with needle insertion, have a bleeding problem or dialysis access, or are receiving an intravenous infusion. If you have not followed the fasting or medicine instructions, explain exactly when you ate, drank, or took medicines. The team will decide whether to collect the sample or reschedule.

After the results are available: Confirm the next steps

Before the blood draw, ask where to get the report, when it is expected to be available, who is responsible for reviewing it, and how you will be notified of abnormal results. Once you receive the report, give the complete report to the ordering or treating doctor. Confirm whether treatment will proceed as planned or whether repeat tests or changes are needed. If you have not received a response, contact the department you were told to follow up with.

High or low flags beside results need to be interpreted alongside that laboratory's reference ranges, your previous results, symptoms, and treatment plan. Laboratories may use different units, methods, and reference ranges, so bring the original reports when comparing results. A result within the reference range does not necessarily rule out a problem.[4] The doctor may decide to proceed as planned, repeat tests, arrange further assessment, or change the plan. If you develop severe, sudden symptoms such as chest pain, difficulty breathing, or fainting, seek local emergency care immediately. Do not wait for an explanation of the report.

Frequently asked questions

Do I need to fast for every blood test?

Not necessarily. Ask the ordering doctor about the specific tests you are having. If fasting is needed, confirm when to start and which liquids you may drink.[1]

Can I drink water while fasting?

Plain water is usually allowed when fasting for a routine blood test. However, if you are also preparing for anesthesia or another procedure, follow the specific instructions from the team responsible.[1][2]

What if I accidentally drank coffee?

Tell the staff taking your blood exactly what you drank and when. The team will decide whether the blood draw can proceed as planned or needs to be rescheduled.[1]

Should I take a missed medicine dose after the blood draw?

Do not take a make-up dose or change the timing on your own. If the original plan is unclear or you missed a dose, ask the prescribing or ordering doctor for instructions specific to that medicine.[1]

Why do I need another tube of blood drawn?

This may be due to sample quality, the need to recheck a result, or a clinical need. For example, the breakdown of red blood cells in a sample, called hemolysis, or clotting in a sample may affect some results, and the laboratory may request another blood draw. Ask which test needs repeating, why another sample is needed, and whether you need to prepare again. Do not assume that results from a faulty sample mean your condition has changed.

Does a result shown in red mean treatment must be canceled?

Not necessarily. Ask your treating doctor to explain the result based on how abnormal it is, previous trends, and the specific treatment. Do not decide on your own to stop medicines or cancel treatment.

What should I do if I feel dizzy after the blood draw?

Tell the staff taking your blood immediately, and rest as directed by the staff on site. If you have other tests or surgery that day, check whether food or drink restrictions still apply before eating or drinking. If symptoms are severe or do not improve, ask the medical staff on site to assess you.

Sources

1. MedlinePlus: Fasting for a Blood Test, checked on 2026-11-06.

2. NHS Kingston and Richmond: Fasting Before Surgery, checked on 2026-11-06; a hospital example. Your individual instructions should come from your care team.

3. MedlinePlus: What You Need to Know About Blood Testing, checked on 2026-11-06.

4. MedlinePlus: How to Understand Your Lab Results, checked on 2026-11-06.