A single mistake before a blood draw can inflate your LDL, distort a cortisol reading, or make your kidney function look completely wrong — and most people have no idea they’re making it. Here are seven common mistakes worth knowing about before your next blood test, why each one happens, and what to do differently. (Based on the insights of Dr. Mitch Rice)
Key Takeaways
- Eating before a fasting lipid panel can inflate triglycerides and, by extension, your calculated LDL number — the standard fasting window is 8-12 hours.
- Intense exercise within 24 hours of a blood draw can temporarily raise liver enzymes, white blood cell counts, and even LDL, sometimes creating a false positive on cardiac or muscle injury tests.
- Dehydration causes hemoconcentration, which can falsely elevate kidney markers (BUN, creatinine) and mask real anemia by making hemoglobin look artificially high.
- Biotin supplements are well documented to interfere with certain hormone and cardiac marker tests — always disclose every medication and supplement to your clinician.
- Standard lab panels aren’t designed to catch heavy metal accumulation, even though research links elevated lead and mercury exposure to real effects on blood cells and inflammation markers.
Mistake #1: Not Fasting Correctly (or for the Wrong Tests)
Most people are told to fast before a blood draw without being told exactly why, or which tests it actually matters for. Eating before a fasting lipid panel can cause postprandial lipemia — fat from your last meal still circulating in your blood — which directly inflates your triglyceride reading. Since LDL cholesterol is often calculated using a formula that includes triglycerides, an inflated triglyceride number can throw off your LDL result too, unless your doctor specifically orders a directly measured LDL instead of a calculated one. The standard window is 8 to 12 hours of fasting for a lipid panel, with a similar window recommended for iron testing, since eating can artificially raise detected iron levels.
Plain water is always fine during a fast, and it’s genuinely worth drinking some — 8 to 16 oz (roughly 250-500 ml) the day of your blood draw — since it also makes it easier for the person drawing your blood to find a vein.
Mistake #2: Exercising Before Your Blood Draw
This one rarely occurs to people, since exercise generally feels like a healthy thing to do before anything — and it is. But intense exercise shortly before a blood draw can temporarily raise several markers, including certain liver enzymes, white blood cell counts, and in some cases LDL itself. This matters most for cardiac or muscle injury workups specifically, since exercise can elevate the same enzymes used to detect actual muscle or heart damage, creating a confusing false positive. The fix: avoid intense exercise in the 24 hours before a blood draw.
Mistake #3: Getting Blood Drawn at the Wrong Time of Day
Many blood markers follow a natural daily rhythm, known as circadian rhythm, and testing at the wrong