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 point in that rhythm can produce a misleading result. Cortisol is the clearest example — naturally highest in the early morning (roughly 6-8 a.m.) and dropping steadily throughout the day to its lowest point around midnight. A cortisol test drawn in the afternoon can look low even in someone with completely normal adrenal function, simply reflecting the expected afternoon pattern rather than a real problem. Iron, glucose, and several hormones, including testosterone, follow similar daily patterns, which is part of why doctors generally recommend scheduling blood work in the morning — it’s both easier to maintain a fast and produces more standardized, comparable results over time.
Mistake #4: Showing Up Dehydrated
Dehydration has more significant consequences than being slightly over-hydrated. When you’re dehydrated, the fluid portion of your blood (plasma) shrinks, and since many blood substances are measured as a concentration within that fluid, a smaller fluid volume makes everything else appear more concentrated than it actually is — an effect called hemoconcentration. This can falsely elevate markers like BUN and creatinine (used to assess kidney function), and can make hemoglobin and hematocrit look artificially high, potentially masking a real underlying anemia. Sodium, potassium, and other electrolyte readings can shift as well. The fix is straightforward: stay adequately hydrated during your fast, including that same 8-16 oz of water the morning of your draw.
Mistake #5: Not Disclosing Every Medication and Supplement
This isn’t about anything you do right before the blood draw — it’s about what you tell (or don’t tell) the clinician ordering your labs. Certain medications, including thyroid medication and cholesterol drugs, can meaningfully shift lab values depending on timing and dosage. Many supplements — biotin in particular — are well known for interfering with certain hormone and cardiac marker tests, sometimes producing dramatically inaccurate results. GLP-1 medications (like low-dose Ozempic) and exogenous testosterone can also significantly affect blood test results. If your clinician doesn’t know everything you’re currently taking, they can’t properly interpret an unusual number — and in some cases, an inaccurate result driven by an undisclosed supplement can lead to unnecessary additional testing or even a wrong diagnosis.
Mistake #6: Drinking Alcohol the Night Before
Alcohol is generally fully metabolized within about 12 hours for most people, which is why having one or two drinks more than 12 hours before a blood draw is typically fine for most tests. But some people don’t metabolize alcohol as quickly, and if your blood work includes liver enzymes (commonly part of a comprehensive metabolic panel), drinking can temporarily elevate those markers as your liver actively processes it. For a liver panel specifically, avoiding alcohol for at least 24 hours beforehand produces a more accurate baseline reading.
Mistake #7: Never Checking What’s Actually Accumulating in Your Body
Standard lab panels weren’t designed to catch environmental exposures like heavy metals, even though these can directly interfere with how blood cells function. Research has linked elevated lead exposure to disruptions in red blood cell production, sometimes contributing to unexplained anemia, and elevated mercury exposure has been associated with increased inflammatory markers like hs-CRP. Various heavy metals have also been shown to interfere with normal enzymatic and metabolic function at the cellular level. A standard panel your doctor typically orders simply isn’t built to screen for this kind of exposure — it generally requires asking for it specifically.
What to Do If You Made One of These Mistakes
Before assuming a strange result means something is seriously wrong — or that a normal one means everything is fine — it’s worth honestly reviewing the conditions around your blood draw. If you recognize one of these mistakes, it’s not the end of the world; simply repeating the blood draw a week or two later, correcting for that factor, is a reasonable next step.
Frequently Asked Questions
Is it okay to drink water during a fasting blood test?
Yes — plain water is always fine during a fast and doesn’t break it. In fact, drinking 8-16 oz the morning of your draw is genuinely encouraged, since it helps prevent dehydration-related distortions and makes it easier to draw blood.
Why does exercising before a blood draw matter?
Intense exercise can temporarily raise liver enzymes, white blood cell counts, and LDL. This matters most for cardiac or muscle injury testing specifically, since exercise can elevate the same enzymes doctors use to detect real heart or muscle damage.
Why does it matter what time of day I get my blood drawn?
Many markers, including cortisol, iron, glucose, and testosterone, follow a natural daily rhythm. A cortisol test in the afternoon can look artificially low even with completely normal adrenal function, simply reflecting the expected daily pattern rather than a real issue.
What should I do if I realize I made one of these mistakes?
It’s not a major problem — simply repeating the blood draw a week or two later, correcting for whatever factor affected it, is a reasonable next step rather than assuming the original result reflects a real health issue.
Before Your Next Blood Draw Checklist
- ☐ Fast 8-12 hours for a lipid panel; drink water freely during the fast
- ☐ Avoid intense exercise for 24 hours beforehand
- ☐ Schedule your draw in the morning when possible
- ☐ Stay well hydrated, including 8-16 oz of water that morning
- ☐ List every medication and supplement for your clinician, including biotin
- ☐ Avoid alcohol for at least 24 hours before a liver panel
Disclaimer: This article is for general informational purposes only and is not a substitute for professional medical advice, diagnosis, or treatment. Always disclose all medications and supplements you’re taking to the clinician ordering your bloodwork. Consult your doctor about the correct fasting window and preparation for your specific tests, and before making any changes based on lab results.