What to do before a blood test
Updated 7/6/2026 · 4 min read
A poorly prepared blood test sometimes means doing it again. A few simple rules are enough to make your results reliable. Here is what to know before heading to the lab — and the points to confirm with your doctor or the lab, since the instructions depend on the test ordered.
"Fasting": what it really means
Fasting means eating and drinking nothing (except water) for a set time before the draw. The usual rule is 8 to 12 hours, but not every test requires it.
Generally concerned: fasting glucose and the lipid panel (cholesterol, triglycerides). Many other tests (blood count, TSH, serologies) don't require fasting. When in doubt, the instruction is on the prescription or can be confirmed with the lab.
- Fasting glucose: strict fasting, often 8 to 12 h.
- Lipid panel: 12 h fasting recommended.
- Blood count, electrolytes, TSH: usually no fasting.
Water, yes. Coffee and tobacco, no
Drinking still water is not only allowed but recommended: well hydrated, your veins are easier to find and the draw is more comfortable.
On the other hand, coffee, tea, juice, cigarettes and sugary gum break the fast or skew certain measurements. Avoid them the morning of the test.
Medications, exercise and stress
Never stop a treatment on your own for a blood test: always ask your doctor whether to take it or delay it. Some measurements (thyroid, iron) are sensitive to the timing of the dose.
Avoid intense exercise the day before and that morning (it changes certain muscle markers) and try to arrive relaxed: stress and effort can influence a few values.
On the day: what to bring
To avoid going back, prepare your file:
- The prescription and your insurance card or proof of coverage.
- The list of your current medications.
- Your previous results if the doctor wants to compare.
- Something to pass the time and a snack for afterward if you were fasting.
Keep everything in one place
Between the prescription, the lab's instructions and your old results, it's easy to lose track. Centralizing your documents and reminders prevents forgetting and a wasted trip.
Parato keeps the history of your tests, translates results into plain language and helps you prepare questions for your doctor — without ever replacing their interpretation.
Parato helps you prepare for your appointments. It does not replace medical advice and is not a medical device. In an emergency, call your local emergency number.