"Fasting" before bloodwork causes a lot of confusion — and getting it wrong can mean a wasted trip or inaccurate results. Here's the practical version.
Which tests require fasting?
| Test | Fasting needed? | Typical hours |
|---|---|---|
| Fasting glucose | Yes | 8–12 |
| Lipid panel | Often | 9–12 |
| Comprehensive metabolic panel | Usually | 8–12 |
| HbA1c | No | — |
| CBC | No | — |
| Thyroid (TSH) | No | — |
Can I drink water?
Yes — and you should. Plain water is fine and actually helps. Being well hydrated makes your veins easier to find and your blood easier to draw. Fasting means no food and no caloric drinks, not no water.
Can I drink coffee?
Generally no if you're fasting. Black coffee has almost no calories, but caffeine can affect blood sugar and other results, and most labs ask you to skip it. If you've already had some, tell the phlebotomist so it can be noted.
How long should I fast?
Most fasting tests want 8–12 hours. The easiest approach is to schedule a morning draw and simply not eat after dinner the night before. Fasting much longer than 12 hours isn't better and can actually skew some results.
What about the newer "non-fasting" lipid panels?
Guidelines increasingly accept non-fasting lipid panels for routine cholesterol screening, since LDL and HDL change little with food. If your doctor is specifically tracking triglycerides, fasting still gives the most reliable number. When in doubt, fast — it's valid either way.
Other prep tips
- Avoid alcohol for 24 hours before a lipid panel or liver tests.
- Skip intense exercise right before a draw — it can temporarily shift some values.
- Biotin supplements can interfere with thyroid and other tests; many providers suggest pausing them 2–3 days prior.
Once you have your results, our Lab Report Decoder can explain each value in plain English.