Tests and results
How to Prepare for a Blood Test
What fasting actually means, which everyday things can shift a result, and the questions to ask when the test is ordered rather than on the morning.
A blood test measures what is in your blood at one moment. Quite a lot of ordinary things change what is in your blood at that moment, and a result taken under the wrong conditions can send an investigation down a path that was never needed.
The specific instructions for your test come from whoever ordered it. This page is about knowing what to ask.
Ask when the test is ordered, not on the day
Four questions, at the appointment where the test is ordered:
- Do I need to fast, and for how many hours?
- Should I take my usual medications that morning, or hold any?
- Is there a best time of day for this test?
- Is there anything I should stop beforehand, including supplements?
The second one matters most and is asked least. Some medications are deliberately held before a draw, and some are deliberately taken. Guessing either way can make the result uninterpretable.
What fasting usually means
Where fasting is required it typically means nothing but water for a set number of hours beforehand - often eight to twelve, but follow the number you were given rather than one you read.
Water is normally allowed and is worth drinking. Being well hydrated makes the draw easier and dehydration genuinely shifts several values, including hemoglobin, creatinine, and sodium.
Coffee usually breaks a fast even without milk or sugar. So does chewing gum and, in most instructions, smoking.
If you accidentally ate, say so rather than staying quiet. A rescheduled draw costs an hour. A misinterpreted result can cost considerably more.
Everyday things that move results
None of these are disease, and all of them are common reasons a result comes back unexpected:
- Strenuous exercise in the day or two before can raise liver enzymes, creatinine, and muscle-related values considerably
- Dehydration concentrates the blood and raises several values at once
- Biotin supplements, common in hair and nail products, interfere with many thyroid and hormone assays and can produce dramatically wrong results
- Alcohol in the preceding days affects triglycerides and liver values
- Recent illness, even a mild cold, raises inflammatory markers and can shift thyroid results
- Time of day matters for TSH, cortisol, and iron
- A difficult draw - repeated fist clenching, a long tourniquet, a struggle to find a vein - can produce a falsely high potassium
- Pregnancy shifts the expected ranges for a long list of tests
If any of these applied on the day, mention it. It is often the whole explanation.
On the day
Bring your ID and insurance card, and confirm the lab is in-network for your plan - hospital-based labs and independent labs frequently have different network status, and this is a routine source of surprise bills. Ask the ordering office which lab to use rather than assuming.
Drink water unless you were told otherwise. Wear a sleeve that rolls up easily. If you have a side that works better for draws, or you have fainted before, say so before they start rather than after.
Afterward
Results often appear in your portal before anyone has reviewed them. That is normal - our page on results arriving before your doctor calls covers how to read the layout and what a flag actually means.
To understand what an individual test measures while you wait, the lab test reference explains each one in plain English. To turn your results into a list of questions for your appointment, there is a sheet builder that produces a printable Word document.
Ask when and how you will be told the results, and what to do if you hear nothing. Results that are never followed up are a real failure in the system, and the patient chasing them is often what closes the loop.
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