Quick answerFast 8 to 12 hours for tests that include glucose, such as a CMP, and when your order says so. A1C, TSH, hs-CRP, and Lp(a) usually do not need fasting. Drink water, and follow the instructions on your lab order.
Common tests and fasting
| Test | Fasting? |
|---|---|
| Lipid panel | Often optional. Some labs and some situations ask for 9 to 12 hours. |
| CMP | Often yes, 8 to 12 hours. |
| A1C | No. |
| TSH | No. |
| ApoB | Usually no. |
| Ferritin | Often suggested overnight. |
| Testosterone | Morning draw, usually fasting. |
Practical tips
- Book a morning slot so the fast overlaps with sleep.
- Drink water. Dehydration makes the draw harder.
- Skip alcohol the day before.
- Bring your lab order and a photo ID if the seller requires one.
Questions people ask
Can I drink coffee before a fasting blood test?
Plain black coffee is often discouraged. Water is the safe choice. Follow the instructions on your lab order.
Can I take my medicines while fasting?
Usually yes, with water. Ask your clinician if you are unsure, especially about diabetes medicines.
What if I forgot to fast?
Tell the lab. Some tests are fine, and some need a repeat. Cholesterol often does not require fasting.
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