Passing a blood test comes down to preparation: following fasting instructions, staying on your normal medication schedule unless told otherwise, and avoiding a handful of common mistakes that can skew results or force a redraw. Most routine blood work requires minimal effort on your part, but small details like what you eat the night before, supplements you take, or even a hard workout can meaningfully change your numbers.
Fasting: When You Need It and When You Don’t
The standard fasting window for blood work is 8 to 12 hours before your appointment. During that time, you consume nothing except water. Blood glucose tests and lipid panels (cholesterol and triglycerides) are the most common tests that require fasting, because recent food intake directly raises blood sugar and fat levels in your bloodstream.
That said, fasting requirements have loosened for cholesterol screening. The latest joint guidelines from the American College of Cardiology and American Heart Association state that nonfasting samples work fine for most people, since fasting and nonfasting LDL cholesterol levels have similar value for predicting heart disease risk. Fasting is still preferred if you have a history of high triglycerides (especially above 400 mg/dL) or a family history of early heart disease or genetic cholesterol disorders. If your doctor’s order says “fasting lipid panel,” follow it. If the order doesn’t specify, a nonfasting draw is usually acceptable.
A practical tip: schedule fasting blood work first thing in the morning. You sleep through most of the fasting window, and the lab is typically less crowded.
Water, Heat, and Getting a Good Vein
You’ve probably heard that drinking lots of water before a blood draw makes your veins easier to find. The research tells a different story. A controlled trial found that oral hydration actually reduced vein diameter in both major arm veins used for blood draws, shrinking the median cubital vein by 0.57 mm and the cephalic vein by 0.33 mm compared to a control group. The promotion of water consumption to improve blood draw success is not supported by the evidence.
What does help is warmth. Applying localized heat to your arm made veins significantly larger and closer to the skin surface in the same study. If you tend to have difficult draws, place a warm towel or heating pad on the inside of your elbow for 10 to 15 minutes before your appointment. You should still drink water normally to avoid dehydration, which can make you feel lightheaded during the draw. Just don’t overdo it thinking it will plump your veins.
Supplements and Over-the-Counter Drugs That Skew Results
Biotin is the biggest offender most people don’t know about. The FDA has issued safety warnings that biotin, found in many hair, skin, and nail supplements, can significantly interfere with lab tests and produce incorrect results that go undetected. The interference is well documented for troponin tests (used to diagnose heart attacks), thyroid panels, and other hormone assays. If you take a biotin supplement, stop it at least 48 to 72 hours before blood work.
Vitamin C (ascorbic acid) can also distort a surprising number of results. It artificially lowers cholesterol, triglyceride, uric acid, and bilirubin readings while raising creatinine levels. Acetaminophen (Tylenol) can cause falsely elevated glucose readings on continuous glucose monitors, and some of that interference carries over to certain lab assays. Common antibiotics in the cephalosporin family inflate creatinine and bilirubin numbers, while fluoroquinolone antibiotics can swing glucose readings in either direction.
Antidepressants and antipsychotic medications can trigger false positives on several tests, including pregnancy tests run on blood samples. If you’re taking any prescription medication, your doctor should know before ordering labs. Don’t stop prescribed medications to “clean up” your results unless specifically instructed to do so.
Exercise, Alcohol, and Timing
A hard strength training session or intense workout causes muscle enzymes to leak into your bloodstream. Creatine kinase, a marker doctors use to assess muscle or heart damage, peaks about 24 hours after strenuous exercise and can stay elevated for up to 7 days. If your blood work includes a metabolic panel or your doctor is checking for muscle or liver issues, a tough workout the day before can make it look like something is wrong when it isn’t. Avoid heavy exercise for at least 24 hours before your draw, and ideally 48 hours if you’re being tested for creatine kinase specifically.
Alcohol affects liver enzyme readings, particularly GGT (gamma-glutamyl transferase), a marker commonly used to screen for liver problems or heavy drinking. After you stop drinking, GGT levels take 2 to 3 weeks to return to normal. A single night of heavy drinking won’t necessarily push your GGT above the reference range, but it can nudge borderline numbers into abnormal territory. Skipping alcohol for at least 48 hours before a blood test is a reasonable minimum, and longer is better if liver enzymes are the focus of your testing.
Timing Matters for Hormone Tests
Some blood tests need to happen at a specific time of day because the hormones they measure fluctuate on a 24-hour cycle. Cortisol is the clearest example: it’s highest in the early morning and drops through the afternoon. Blood for cortisol is typically drawn between 8 and 9 AM, with a second draw around 4 PM when doctors are evaluating adrenal function. Testosterone follows a similar morning peak pattern. If your appointment is in the afternoon for one of these tests, the results may come back misleadingly low. Confirm the timing with your doctor’s office when you schedule.
Drug Screening in Blood vs. Urine
If your blood test includes a drug screen, the detection window in blood is much shorter than in urine. Blood testing typically detects substance use that occurred within 2 to 12 hours of the test, making it a snapshot of very recent use rather than a look back over days or weeks.
For comparison, urine detection windows are considerably longer:
- Cannabis: 1 to 30 days in urine, depending on frequency of use
- Opiates (codeine, morphine): 2 to 5 days in urine
- Amphetamines and methamphetamine: 2 to 5 days in urine
Heavy, long-term cannabis users face the longest urine detection windows because THC is highly fat-soluble and accumulates in body tissue. In blood, even cannabis clears within hours for most people. The type of test your employer or doctor orders determines which window applies to you.
The Morning of Your Test
If you’re fasting, drink a normal amount of water but skip coffee, tea, juice, and gum (even sugar-free gum can trigger a small metabolic response). Take your regular prescription medications with a sip of water unless your doctor has specifically told you to skip them. Wear a short-sleeved shirt or one with sleeves that roll up easily past your elbow. If you know you have difficult veins, warm your arm beforehand.
Let the phlebotomist know if you’ve had problems with blood draws in the past, if you tend to feel faint, or if one arm tends to work better than the other. Lying down during the draw is a perfectly normal request and reduces the chance of lightheadedness. Stay seated for a few minutes afterward, especially if you’ve been fasting, and have a snack ready for when you leave.