How to Prepare for Blood Tests

Which Tests Require Fasting?

Not all blood tests require fasting. Here's what you need to know:

✅ Tests That DO Require Fasting (8-12 hours)

•Fasting glucose: Must be done after overnight fast
•Lipid panel: Total cholesterol, LDL, HDL, triglycerides (triglycerides are most affected by food)
•Fasting insulin: Used to assess insulin resistance
•Comprehensive metabolic panel (CMP): Includes glucose

🍽️ Tests That DON'T Usually Require Fasting

•HbA1c: Reflects 2-3 month average blood sugar—food doesn't matter
•Thyroid panel: TSH, T3, T4 are not affected by food
•Vitamin D (25-OH): Eat normally
•Iron, ferritin: Eat normally (though morning is better for consistency)
•Complete blood count (CBC): No fasting needed
•hs-CRP: Inflammation marker—no fasting needed
•Liver enzymes (ALT, AST): Eat normally

Fasting Guidelines

What “Fasting” Means

✅Water is ALWAYS allowed (and encouraged for easier blood draw)
✅Plain black coffee or tea (no sugar, no cream)—technically breaks fast but typically allowed
❌No food for 8-12 hours (most tests require 8-10 hours)
❌No beverages with calories (juice, milk, soda, energy drinks)
❌No gum or mints (can contain sugar)

What About Medications?

Always follow your doctor's instructions. In general:

•Take essential medications (blood pressure, thyroid, etc.) as usual with water
•Hold diabetes medications until after test (unless directed otherwise)
•Ask your doctor if unsure—some meds can affect test results

Timing Matters: When to Get Tested

🌅 Morning Is Best (But Not Always Required)

Why morning:

•Easier to fast overnight than during the day
•Hormone levels are more standardized in the morning
•Reduced variability = more accurate baselines

Tests that MUST be done in the morning:

•Cortisol (peaks at 8-9 AM)
•Testosterone (peaks at 7-10 AM)

📅 Consistency for Tracking Trends

For biomarkers you track regularly (like thyroid hormones), test at the same time of day each time to minimize variability.

What Can Affect Your Results (Avoid Before Testing)

•Intense exercise: Can temporarily elevate liver enzymes (ALT, AST), creatine kinase, and glucose. Avoid hard workouts 24 hours before testing.
•Alcohol: Elevates liver enzymes and triglycerides. Avoid for 24-48 hours before a lipid panel or liver function test.
•Biotin (B7) supplements: Can interfere with many lab tests (thyroid, troponin, vitamin D). Stop 72 hours before testing if taking >5 mg/day.
•Dehydration: Concentrates blood, affecting creatinine, electrolytes, and hematocrit. Drink plenty of water the day before.
•Stress and sleep deprivation: Can affect cortisol, glucose, and inflammatory markers. Get a good night's sleep.
•Recent illness or infection: Will elevate white blood cells and hs-CRP. Wait until you've recovered.

Day-of-Test Tips

✅Hydrate: Drink 16-24 oz of water before your appointment (makes veins easier to find)
✅Wear short sleeves or loose clothing for easy access
✅Bring your test order form and insurance card
✅Bring a list of medications and supplements you're taking
✅Bring a snack for after your test if you've been fasting
✅Relax: Anxiety can temporarily elevate blood pressure and glucose

When to Retest

How often should you retest biomarkers? It depends:

•Baseline screening (healthy adults): Annually
•After starting new medications or interventions: 6-8 weeks (for lipids, glucose, thyroid)
•Iron/ferritin after supplementation: 8-12 weeks
•Vitamin D after supplementation: 8-12 weeks
•Thyroid medication adjustments: 6-8 weeks
•Chronic conditions (diabetes, high cholesterol): Every 3-6 months

Getting Your Results

Always request a copy of your results. Even if your doctor says “everything is normal,” reference ranges are wide, and optimal may differ from standard.

Upload your results to Toowit to visualize trends, understand what's optimal (not just “normal”), and get AI-powered insights.