The short answer
Sometimes. A hemoglobin A1c, a TSH and a complete blood count do not need fasting. A fasting glucose does, by definition. Lipid panels are frequently run non-fasting now, but fasting is still requested in some circumstances. Metabolic panels include glucose, so fasting is sometimes asked for depending on the question. The instruction attached to your specific order is the one that counts.
Common tests at a glance
| Test | Fasting usually needed? |
|---|---|
| Hemoglobin A1c | No |
| TSH and thyroid tests | No |
| Complete blood count | No |
| Fasting glucose | Yes |
| Lipid panel | Sometimes — follow your order |
| Basic or comprehensive metabolic panel | Sometimes, because glucose is included |
| Iron studies and vitamin levels | Sometimes — these have their own timing rules |
What fasting actually means
- Nothing to eat or drink other than water for the period specified, commonly overnight.
- Water is fine and helpful — dehydration makes the draw harder and can skew some results.
- Black coffee and tea are not automatically fine; they affect some tests, so ask rather than assume.
- No chewing gum, sweets or sugary drinks.
- Alcohol the evening before can affect triglycerides and liver tests.
Medications and supplements
Do not skip a prescribed medication to fast unless you have been told to. Ask when the order is placed — some medicines are deliberately taken after the draw, some are timed to it, and some supplements interfere with assays. Biotin in particular can distort thyroid and other tests, so mention any supplement you take.
Practicalities that make it easier
Book early in the day
An overnight fast is far easier than a daytime one, and most collection sites open early.
Drink water
Two glasses before you leave makes the draw quicker and more comfortable.
Bring the order and photo identification
Along with a list of your medications and supplements.
Bring something to eat for afterwards
Particularly if you take diabetes medication, in which case ask about timing beforehand.
Say if you ate
The site can often still run most of the panel, or reschedule only the affected test.
General guidance. Your own order's instructions override this page, and anyone taking insulin or other glucose-lowering medication should ask about fasting safely before doing it.