Bloodwork Basics

How to Prepare for a Blood Test: Fasting, Training, Timing, and Supplements

The conditions of a blood draw can change the numbers. Learn how fasting, time of day, recent training, hydration, and supplements such as biotin affect common lab results.

By GetBloodWorx TeamPublished

A blood test measures one moment. What you ate, when you trained, how much you drank, and what time you walked into the lab are all part of that moment, and several of them can move a result far enough to change how it reads.

Good preparation does not make results look better. It makes them describe your usual state, and it makes one test comparable with the next.

Follow the instructions you were given

The specific instructions that come with your order take priority over any general guide, including this one. MedlinePlus is direct on this point: follow all instructions from your provider, and if you did not follow them exactly, say so, because even small changes can affect results.

Fasting

MedlinePlus describes a typical fast as 8 to 12 hours, with the exact duration set by your provider. Tests that commonly call for one include blood glucose, cholesterol and triglycerides, and insulin.

During a fast:

  • Plain water is fine. Flavored or carbonated waters are not.
  • No coffee, juice, soda, or other drinks, including black coffee.
  • No gum, smoking, or exercise, all of which MedlinePlus lists as things to avoid while fasting.

If you eat or drink something by mistake, tell the lab or your provider rather than hoping it will not matter. The test may be rescheduled.

Fasting is not required for everything. A European consensus statement concluded that fasting is not routinely necessary for a lipid profile, since the average changes after a normal meal are small. A panel usually combines many tests on one sample, though, so if any of them needs a fast, the whole draw is done fasted. Following the fasting instruction for the panel as a whole is the simplest way to keep results comparable.

Time of day

Some hormones follow a daily rhythm, so the clock is part of the result.

Testosterone is the clearest example. A study of men sampled across clinic hours found that in men aged 30 to 40, testosterone was 20 to 25 percent lower at 4 p.m. than at 8 a.m. The Endocrine Society guideline recommends measuring fasting morning total testosterone, and MedlinePlus describes the sample as being taken between 7 and 10 in the morning. Cortisol also peaks in the morning, and its reference ranges assume a morning draw.

For a panel that includes hormones, a morning appointment handles both requirements at once: the overnight fast is easier, and the timing matches what the reference ranges expect. Total vs. Free Testosterone goes deeper on why timing changes that result.

Training before the draw

MedlinePlus lists avoiding strenuous exercise among the standard preparations for lab testing, and for people who train hard this is the instruction most often overlooked.

Hard sessions can shift several markers. In one small study of healthy men, an unfamiliar, intensive weightlifting session raised AST and ALT, with the increase lasting at least seven days in some participants; Can Hard Training Raise AST and ALT? covers that in detail. Exercise also produces a short-term inflammatory response, which matters when a panel includes hs-CRP.

There is no single rest period that suits every marker and every athlete. If a result is being used for a specific decision, ask the ordering professional whether training should be modified beforehand and for how long. For routine tracking, the more useful habit is consistency: draw at a similar point in your training week each time.

Hydration

Fasting from food does not mean fasting from water. Arriving dehydrated concentrates the sample, and MedlinePlus names dehydration as the most common cause of a high hematocrit. Drink water as you normally would, and avoid testing straight after a session that left you depleted. Our guide to hematocrit and hemoglobin explains why fluid status moves those numbers.

Supplements and medicines

Tell your provider about everything you take, including vitamins and over-the-counter supplements. MedlinePlus advises that you may be asked to avoid certain medicines or supplements before a test, and that you should never stop a prescribed medicine unless your provider tells you to.

Biotin deserves a specific mention. It is common in hair, skin, and nail products and in many multivitamins, sometimes at high doses. The U.S. Food and Drug Administration has warned that biotin can significantly interfere with certain lab tests and cause incorrect results that may go undetected. If you take biotin, or a product that contains it, tell the lab and your provider before the draw and ask whether it should be paused.

A short checklist

  • Read the instructions that came with your order, and ask if anything is unclear.
  • Confirm whether you need to fast, and for how long.
  • Book a morning appointment if the panel includes hormones.
  • Drink plain water as usual.
  • Avoid hard or unfamiliar training beforehand, and keep the timing consistent between tests.
  • List your medicines and supplements, biotin included.
  • Note anything unusual: recent illness, poor sleep, travel, or, for women, where you are in your cycle.

That last point is easy to skip and often the most useful. When a result looks unexpected, the notes from that week are what let a provider tell a real change from a bad morning. If you are deciding when to test again, see How Often Should Athletes Get Bloodwork?.

Sources & further reading

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