How to prepare for a blood test
The short answer
Check whether your test needs fasting (only some do; water is fine), and take your usual medicines unless your prescriber says otherwise. Tell the lab if you take biotin, time thyroid and cycle-dependent hormone tests correctly, and bring a photo ID and your lab order.
The essentials
- Check your test's instructions first. Only some tests need fasting, usually for 8 to 12 hours; our lipid panel, for example, asks for 9 to 12.
- Water is fine while you fast.
- Take your usual medicines unless your prescriber tells you otherwise. On levothyroxine, have your blood drawn first and take your dose after.
- Take biotin? Read the biotin question below before you book.
- Skip alcohol for 24 hours before a lipid panel.
- Some hormone tests need to be timed to your menstrual cycle. Check our cycle guide.
- Bring a photo ID and your lab order.
Which tests need fasting?
Of our tests, 12 need fasting. The other 71 don't: eat and drink as usual.
| Test | Fast for |
|---|---|
| Basic Metabolic Panel | 8 hours |
| C-Peptide | 8 hours |
| Comprehensive Metabolic Panel | 8 hours |
| Glucose, Fasting | 8 hours |
| Homocysteine | 8 hours |
| Insulin, Fasting | 10 hours |
| Iron and Total Iron Binding Capacity | 8 hours |
| Leptin | 8 hours |
| Lipid Panel (Standard) | 9 hours |
| Lipid Panel, Cardio IQ | 9 hours |
| Lipoprotein Fractionation (NMR) | 9 hours |
| Prolactin | Yes |
If any test in your order needs fasting, fast for the whole order: it is one draw.
What can skew your result
Fasting: only for some tests
Most blood tests do not need fasting. In our catalog, the ones that do include fasting glucose, fasting insulin (10 to 12 hours), lipid panels (9 to 12 hours), the comprehensive and basic metabolic panels, iron and TIBC (8 to 12 hours), homocysteine, C-peptide and leptin. Tests such as TSH, HbA1c, vitamin D and a CBC do not need fasting. Each test page lists its own requirement.
Forgot to fast? Tell the lab. For a lipid panel the test can still be done, but not fasting may affect triglycerides and the calculated LDL. Our fasting guide has more detail.
Water, coffee and alcohol
Water is allowed while you fast. Coffee is less simple: most of our fasting instructions say water only, while our Cardio IQ lipid panel allows black coffee. If you are unsure, stick to water. See the coffee guide for more.
For lipid panels, avoid alcohol for 24 hours before the draw. For the lipoprotein fractionation test, our instructions say 48 hours.
Exercise
A hard workout can change some results. Our instructions ask you to avoid strenuous exercise the night before a fasting insulin test and the day before a lipoprotein fractionation or Cardio IQ lipid panel. See the exercise guide for other tests.
Medicines and supplements
Keep taking prescribed medicines as usual unless your prescriber tells you otherwise. Never stop a prescribed medicine just for a test.
A few test instructions are specific. On thyroid medicine such as levothyroxine, have your blood drawn before your daily dose. For an iron and TIBC test, skip iron supplements the day before. For vitamin D and omega-3 tests, keep taking your supplements as usual, so the result reflects your normal routine. If you use a topical estrogen, do not apply it near the arm where blood will be drawn. Before a celiac (tTG) test, keep eating gluten for at least 2 to 4 weeks.
Biotin is the supplement most likely to cause a misleading result. See the biotin question below.
Timing
Some tests depend on when you are drawn. Book a morning appointment for an iron test. Hormone tests such as FSH, estradiol and progesterone depend on your cycle day; see the menstrual cycle guide. If you take thyroid medicine, see the thyroid medication guide.
At the Quest center
JustLabs tests are drawn and run by Quest Diagnostics (Sonora Quest in Arizona). Testing is not available in New York, New Jersey, Rhode Island or Hawaii.
After you order, book a time online through JustLabs or walk in during the center's hours. Bring a photo ID and your lab order, which is in your JustLabs dashboard and your email. Most visits take 10 to 15 minutes.
Results are usually ready in 1 to 2 business days, in your JustLabs account.
Ordering for someone under 18? You will need a parent or guardian's details at checkout.
Frequently Asked Questions
Sources
- Li D, Ferguson A, Cervinski MA, Lynch KL, Kyle PB. AACC Guidance Document on Biotin Interference in Laboratory Tests. J Appl Lab Med. 2020 May 1;5(3):575-587. doi: 10.1093/jalm/jfz010. PMID: 32445355. PubMed
- Li D, Radulescu A, Shrestha RT, Root M, Karger AB, Killeen AA, Hodges JS, Fan SL, Ferguson A, Garg U, Sokoll LJ, Burmeister LA. Association of Biotin Ingestion With Performance of Hormone and Nonhormone Assays in Healthy Adults. JAMA. 2017 Sep 26;318(12):1150-1160. doi: 10.1001/jama.2017.13705. PMID: 28973622. PubMed
- Moerman A, Delanghe JR. Sense and nonsense concerning biotin interference in laboratory tests. Acta Clin Belg. 2022 Feb;77(1):204-210. doi: 10.1080/17843286.2020.1780770. PMID: 32567529. PubMed
- Ylli D, Soldin SJ, Stolze B, Wei B, Nigussie G, Nguyen H, Mendu DR, Mete M, Wu D, Gomes-Lima CJ, Klubo-Gwiezdzinska J, Burman KD, Wartofsky L. Biotin Interference in Assays for Thyroid Hormones, Thyrotropin and Thyroglobulin. Thyroid. 2021 Aug;31(8):1160-1170. doi: 10.1089/thy.2020.0866. PMID: 34042535. PubMed
Content reviewed for accuracy by the JustLabs editorial team.
Educational information, not medical advice. Don’t stop or change a prescribed medicine without talking to your prescriber. If your lab order gives different instructions, follow your lab order.