Whether you need to fast before blood work depends on which test you ordered. Some panels require an 8-to-12-hour fast to produce accurate values; many others do not.
Tests that benefit from fasting: the lipid panel (cholesterol, HDL, LDL, triglycerides), fasting glucose, and the Comprehensive Metabolic Panel (CMP). For these, do not eat or drink anything except plain water for 8 to 12 hours before the draw. The Comprehensive Wellness Panel — because it includes the lipid panel and CMP — also benefits from fasting.
Tests that do NOT require fasting: A1C, thyroid panels (TSH, Free T3, Free T4), vitamin D, iron, B12, testosterone, estradiol, cortisol, and most hormone panels. These can be drawn any time of day. Cortisol is the main exception — it is best collected in the morning when levels naturally peak.
During the fast, plain water is fine and actually helps — well-hydrated veins are easier to draw from. Coffee, even black coffee, can affect some metabolic values; coffee with cream or sugar definitely breaks the fast. Morning medications that include sugar coatings or are sweetened can affect glucose readings — check with your prescriber whether to take them before the draw or hold until after.
If you forgot to fast and your test requires it, you have two options: reschedule for tomorrow morning, or come in anyway and discuss with our staff whether a non-fasting result is still useful for your purpose. Many physicians now accept non-fasting lipid panels because total cholesterol and HDL are minimally affected.
Key takeaways
- Fasting: lipid, glucose, CMP, comprehensive wellness panel
- No fasting: A1C, thyroid, vitamin D, hormones, iron, B12
- Plain water is OK and recommended
- Black coffee can affect some values; coffee with cream definitely breaks the fast