Preventive care
What to do in the week before a blood test
A blood test measures you on one particular morning. Several ordinary things in the days before it can nudge the numbers, which is worth knowing before you read too much into a single report.
Fasting, when it is asked for
If the lab asked you to fast, it usually means no food and no drinks other than water for a set number of hours beforehand. Tea with sugar counts as breaking it. If nobody told you to fast, you do not need to.
Water helps more than people expect
Being poorly hydrated concentrates the blood, which can nudge several values and also makes the sample harder to draw. Plain water is fine right up to the test, even when you are fasting.
Leave hard exercise for after
A heavy session in the day or two before a test can raise muscle-related enzymes, which then looks like a finding that needs explaining. Ordinary walking is not a problem.
Timing and the day itself
- Morning appointments make fasting easier, and some values naturally vary through the day.
- Alcohol in the previous day or two affects liver values and triglycerides.
- Mention any medicines and supplements you take, including ones you buy without a prescription.
- Where you can, use the same lab you used last time, so the numbers stay comparable.
What not to change
Do not stop a prescribed medicine to make a result look better, and do not eat unusually in the days beforehand. A report that reflects an ordinary week is far more useful to a doctor than a flattering one.
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