How to prepare for a blood test

Published 3 September 2026

Good preparation makes a blood test reflect your health, not a slip on the day. The main things that matter are fasting when asked, drinking water, timing, and telling the lab about your medicines. The single most important step is simple: follow the specific instructions your doctor or lab gives you.

Do I need to fast, and for how long?

Some tests need fasting and many do not. Fasting glucose and a lipid panel are usually done after 8 to 12 hours without food [ADA, 2024]. A thyroid test or a full blood count usually needs no fasting. If you are unsure, ask the lab which of your tests need an empty stomach.

Can I drink water before a blood test?

Yes, and you should. Plain water is fine during a fast and helps in two ways. It keeps you hydrated, and it makes the vein easier to find. Being low on fluids can also nudge some results, so water helps the sample reflect you [National Kidney Foundation, 2023]. Avoid tea, coffee, juice and milk while fasting, as these count as food.

Does the time of day matter?

For some tests, yes. Fasting samples are usually taken in the morning, after an overnight gap. A few hormones follow a daily rhythm and are read against morning ranges [Endocrine Society, 2023]. Try to book the time your doctor suggests, and keep repeat tests to a similar hour so the trend is fair.

Should I take my usual medicines?

Usually yes, unless your doctor says otherwise. Do not stop a prescribed medicine to prepare for a test on your own. Instead, tell the lab and your doctor what you take, including vitamins and supplements. Some can influence certain results, so the report is read with that in mind [WHO, 2023].

What about exercise, alcohol and food the day before?

A hard workout can raise a few markers for a day or two, so keep to your usual routine before the test. Alcohol the night before can affect sugar and liver results. A very heavy, fatty meal can lift triglycerides. None of this needs a special diet — just an ordinary day and the fast if one is required [ADA, 2024].

A quick checklist

Confirm which tests need fasting. Drink plain water. Take your usual medicines unless told otherwise, and list them for the lab. Book the suggested time. Keep the day before ordinary. Bring your previous reports so results can be compared.

Why does preparation change the numbers?

A blood test is a snapshot of one moment. Food, fluids, exercise and stress all move certain markers up or down for a while [WHO, 2023]. Preparation is simply about removing the temporary changes, so the report reflects your usual state. That makes the result easier to compare with past and future tests.

What should I bring to the lab?

Bring a photo ID and your doctor's request slip. Carry a list of your medicines and supplements. If you have earlier reports, bring them too, as a trend is far more useful than one value [WHO, 2023]. If you tend to feel faint, mention it, and let the staff know before the draw.

Does a repeat test need the same preparation?

Yes, ideally. To compare two results fairly, keep the conditions similar. Fast if the first was fasting. Use the same time of day where you can [ADA, 2024]. Consistent preparation is what lets you and your doctor read a real change, rather than a difference in how the samples were taken.

What if I feel unwell on the day?

A recent fever or infection can shift some results for a short time. If you are acutely unwell, ask whether the test should wait, unless it is urgent [WHO, 2023]. For a routine check, a few days' delay often gives a truer picture of your usual health.

Frequently asked questions

Can I drink water before a fasting blood test?

Yes. Plain water is allowed and encouraged. It keeps you hydrated and makes the blood draw easier. Avoid tea, coffee and juice while fasting [ADA, 2024].

What happens if I accidentally ate before a fasting test?

Tell the person taking the sample. They may reschedule, or note it so the result is read correctly. It is better to repeat than to read a fed sample as fasting.

Do I need to stop my medicines before a blood test?

Not on your own. Keep taking prescribed medicines unless your doctor tells you otherwise, and let the lab know what you take [WHO, 2023].

Does a fasting test have to be in the morning?

Usually, because it follows an overnight gap without food. Morning timing also suits hormones that change through the day. Follow the time your doctor suggests.

Can I brush my teeth or take medicine with water while fasting?

Brushing your teeth is fine. Plain water is allowed, so taking a usual medicine with water is generally okay, unless your doctor says otherwise [WHO, 2023]. Avoid swallowing toothpaste, and skip sugary mouthwash before a fasting sample.