Fasting: when, how long, and what counts
Fasting is the most commonly discussed preparation variable. The primary reason for fasting is to get accurate readings for glucose, insulin, and triglycerides — all of which are directly affected by recent food intake. Triglycerides can rise by 20-30% or more after a meal, and postprandial glucose is an entirely different measurement than fasting glucose[1].
How long: Fast for 10-14 hours before your blood draw. Overnight fasting (last meal at 8 PM, draw at 8 AM) is the easiest approach.
What breaks the fast: Any caloric food or drink. This includes juice, milk, sugar in coffee, and energy drinks.
What does not break the fast: Water (drink plenty), black coffee (without sugar or milk), plain unsweetened tea, and prescribed medications (unless your doctor specifically says to hold them).
Hydration: the underappreciated variable
Dehydration is one of the most common causes of falsely abnormal blood test results — and the most preventable. When you are dehydrated, plasma volume decreases, concentrating everything dissolved in your blood. This can falsely elevate hemoglobin, hematocrit, albumin, total protein, sodium, creatinine, and BUN[2].
Conversely, overhydration (rare in practice) can dilute blood values. The goal is normal hydration — not bone dry, not waterlogged.
Practical advice: Drink 2-3 glasses of water in the evening before your test and 1-2 glasses the morning of. Water does not break a fast. Being well-hydrated also makes the blood draw easier — veins are more prominent and blood flows more freely, reducing the chance of hemolysis (red cell destruction during draw), which can falsely elevate potassium and LDH.
Exercise: the 24-48 hour rule
Vigorous exercise causes significant, predictable changes in blood work. These are temporary physiological responses, not disease, but they can trigger false alarms or mask real problems if not accounted for[3].
| Marker | Effect of intense exercise | Duration of effect |
|---|---|---|
| CK (creatine kinase) | +200-1000% (2-10x increase) | 24-72 hours |
| AST | +50-200% (from muscle, not liver) | 24-48 hours |
| LDH | +30-100% | 24-48 hours |
| White blood cells | +50-100% (exercise leukocytosis) | 6-24 hours |
| Cortisol | +30-100% (acute stress response) | 2-6 hours |
| Creatinine | +10-20% | 24 hours |
| Potassium | Transiently elevated, then low | 1-6 hours |
| Testosterone | Acute increase, then suppression | 24-48 hours |
The most clinically significant is CK. A CrossFit session or intense leg workout 24 hours before a blood draw can produce CK levels of 1,000-5,000 IU/L (normal range: 30-200 IU/L). This is sometimes mistaken for rhabdomyolysis, cardiac damage, or autoimmune myositis — leading to unnecessary emergency workups[4].
Alcohol: the 72-hour window
Alcohol affects multiple markers on standard blood panels. Even moderate drinking the night before can elevate triglycerides by 20-50%, acutely raise AST, and alter glucose metabolism[5].
Minimum: No alcohol for 24 hours before a blood draw. Better: 72 hours. For a clean baseline on liver enzymes and GGT: 2-4 weeks. See our detailed article on alcohol and blood markers for full recovery timelines.
Medications: take them unless told otherwise
The general rule: take your regular medications as prescribed on the morning of your blood test, unless your doctor specifically instructs you to hold them. Stopping medications without guidance can be dangerous and can also distort the test results your doctor is looking for[6].
However, some medications directly affect specific tests. If you are being tested for a condition that your medication treats, your doctor may want a trough level (just before your next dose) or may ask you to hold a dose:
- Thyroid medication (levothyroxine): Take it after the blood draw, not before. Taking it right before can falsely elevate free T4.
- Testosterone replacement: Timing matters. Your doctor will typically want a trough level (just before injection) or a mid-point level.
- Statins: Take as usual. Your doctor wants to see your lipid levels on the medication.
- Blood pressure medication: Take as usual unless specifically told otherwise.
Supplements to pause before blood work
This is where many people get tripped up. Several common supplements directly interfere with blood test accuracy[7]:
Biotin (vitamin B7) — the biggest offender
Biotin is the most clinically significant supplement interference. Many immunoassay-based lab tests use biotin-streptavidin chemistry. Excess circulating biotin competes with the assay, producing falsely high or falsely low results. Affected tests include thyroid hormones (TSH, free T4, free T3), troponin, vitamin D, cortisol, ferritin, sex hormones (testosterone, estradiol), and PSA[8].
The interference is dose-dependent. Standard multivitamin doses (30-100 mcg) are generally not problematic. But "hair, skin, and nails" supplements often contain 5,000-10,000 mcg — doses that can cause clinically meaningful assay interference. Stop biotin for at least 72 hours before any blood work.
Other supplements to consider pausing
- Creatine: Raises creatinine and falsely lowers eGFR. Pause for 3-4 weeks if kidney function is being assessed, or request cystatin C instead.
- High-dose vitamin C (>1000 mg): Can interfere with glucose measurement on certain point-of-care meters and falsely lower some hemolysis markers.
- Iron supplements: Affect serum iron and transferrin saturation. Pause for 24-48 hours if iron studies are ordered.
- Fish oil (high-dose): Can affect triglycerides and bleeding time markers. Pause for 48 hours if lipids or coagulation are being tested.
- Vitamin D supplements: Do NOT pause before a vitamin D test. Your doctor wants to know your level on supplementation[9].
Timing: when to schedule your draw
Many hormones and biomarkers have significant circadian variation — they are higher or lower depending on the time of day[10]:
- Cortisol peaks at 6-8 AM and is lowest at midnight. Morning draws give the most useful clinical reading.
- Testosterone peaks at 7-10 AM and declines 20-30% by late afternoon. Always draw in the morning for accurate assessment.
- Iron and TIBC are highest in the morning and decline throughout the day. Morning draw is recommended for iron studies.
- TSH peaks in the early morning and is lowest in the afternoon. A late-afternoon TSH can be 30-50% lower than an early-morning value.
The best time: 7-10 AM, fasted, well-hydrated, well-rested, 48+ hours after intense exercise, 72+ hours after alcohol.
Consistency matters: If you are tracking trends over time, draw at approximately the same time of day each time. A testosterone measured at 8 AM and compared to one drawn at 4 PM is not a valid comparison.
The day-before checklist
- 48 hours before: No intense exercise. Stop biotin supplements.
- 72 hours before: No alcohol (ideally).
- The evening before: Eat a normal dinner by 8 PM (if fasting is required). Drink 2-3 glasses of water. Get a normal night's sleep.
- Morning of: Drink 1-2 glasses of water. Take medications as usual (except thyroid medication — take after the draw). Skip supplements. Arrive rested and calm.
- At the lab: Sit for 5-10 minutes before the draw. Posture affects results — lying down vs. sitting can shift albumin, hemoglobin, and calcium by 5-8%.