What to Do Before Lab Work (So Your Results Are Actually Accurate)

If you’re going to spend time, money, and emotional energy on labs, you deserve results you can trust.

You don’t want markers to show as “abnormal” when it’s really just a reflection of how you prepared (or didn’t), what you did the day before, or the fact that you showed up dehydrated.

I see it all the time.

Below is my simple, practical checklist to reduce noise and get the most accurate lab data possible.

The goal: Let’s reduce Variables

Labs are a snapshot, one point in time. The goal is to make that snapshot as consistent and repeatable as possible.

We want to control the big levers:

• Timing (time of day, cycle day if you’re cycling, fasting window)

• Food and alcohol

• Exercise

• Sleep and stress

• Supplements and meds (with guidance)

• Hydration

• Illness and travel

  
  

Ready to dig in? Let’s go.

1) Schedule your lab draw at the right time

Morning is best (for most labs). Many markers follow a daily rhythm. A morning draw helps standardize results.

• Aim for first thing in the morning when possible

• Try to test at the same time of day if you’re repeating labs later
  

If you’re cycling: pick the right cycle day

Hormones change across the month. If you’re comparing results over time, cycle timing matters.

• If you’re still cycling, ask your provider which day(s) they want Common examples: mid-luteal for progesterone or early follicular for baseline sex hormones

• If you’re on hormonal birth control, your labs can reflect the medication more than your baseline physiology
  

If you’re on HRT/TRT: timing matters

Your results depend on when you last dosed.

• Ask your prescribing clinician whether they want a peak or trough reading

• Keep your timing consistent between lab rounds
  

2) Know whether you should fast (and how)

Fasting isn’t always required for every lab, but it can reduce variability for certain markers. For the most consistent data, I typically recommend fasting unless your ordering provider tells you otherwise.

When I order lab panels for functional interpretation, I prefer:

• 12 hour fast (water is fine)

• No coffee

• Avoid gum, mints, and flavored drinks
  

A simple rule:

• Fasting = no calories, just water
  

Markers commonly affected by food:

• Glucose and insulin

• Triglycerides

• Some inflammatory markers
  

3) Hydrate like a normal person who cares about their health (not a camel, not a raisin)

Hydration changes blood concentration. You would not believe the number of people who spend hundreds on lab work, show up dehydrated, and then a bunch of markers are thrown off. Please don’t let that be you.

• Drink ample water the day before and the morning of

• Avoid overdoing it right before the draw Chugging can dilute certain markers
  

  

4) Don’t do a hard workout 24–48 hours before

This one surprises people. Hard training can temporarily shift:

• CK (muscle breakdown marker)

• AST/ALT (can rise from muscle stress, not just liver)

• Inflammation markers

• Glucose regulation
  

Best practice:

• Avoid intense lifting, long endurance sessions, or HIIT for 24–48 hours before labs If you want the cleanest read, lean closer to 48

• Light movement is fine Walking, easy bike, gentle mobility, basic cardio if your body is used to it
  

5) Avoid alcohol for 48–72 hours (longer if you can)

Alcohol can alter:

• Triglycerides

• Glucose

• Liver enzymes

• Sleep quality (which impacts cortisol and glucose)
  

If you want the cleanest read:

• Skip alcohol 2–3 days before labs

• If you’ve had a heavier-than-usual weekend, consider rescheduling if possible
  

6) Prioritize sleep the night before

Sleep impacts:

• Fasting glucose

• Cortisol

• Appetite hormones

• Inflammation
  

Aim for:

• A normal bedtime

• A normal wake time

• No revenge bedtime procrastination the night before labs
  

7) Keep your diet consistent for at least 2–3 days

This is not the time for:

• A brand-new experiment

• Trying to eat “super healthy” all of a sudden

• A 36-hour fast

• A cleanse

• A massive refeed
  

Big swings create noisy data.

Best practice:

• Eat your normal pattern for at least 2–3 days leading into labs

• Keep sodium and carbs relatively consistent Especially if you’re tracking blood pressure, glucose, or lipids
  

8) Supplements: don’t guess, but do be intentional

Some supplements can meaningfully change lab values.

Common examples:

• Biotin can interfere with certain thyroid and hormone immunoassays

• Creatine can increase creatinine (not always a kidney problem)

• Iron can affect iron studies depending on timing

• B vitamins can shift B12 and related markers
  

Best practice:

• Ask your clinician whether they want you to hold specific supplements

• My general rule: if it’s optional, I pause it for 48 hours before labs unless your prescribing clinician tells you not to

• If you do take something, note it so you can repeat the same approach next time
  

Important: Do not stop prescription meds unless your prescribing clinician tells you to.

9) Reschedule if you’re sick, inflamed, or freshly vaccinated

Acute illness and immune activation can temporarily alter:

• CRP and other inflammatory markers

• White blood cell count

• Glucose

• Ferritin (it’s an acute phase reactant)
  

If you’re actively sick, just recovered, or recently had a vaccine, ask whether you should delay.

10) Make a lab context note in your phone (this is elite-level)

If you want to interpret labs like a pro, make note of:

• Time of draw

• Fasting hours

• Sleep duration and quality

• Exercise in the prior 48 hours

• Alcohol in the prior 72 hours

• Any supplements and meds taken (or not)

• Cycle day (if applicable)

• Recent illness, travel, or unusual stress
  

This makes your results actionable, not just interesting.

Quick checklist (screenshot this)

• Book labs for morning

• Confirm fasting requirements

• Ensure you are hydrated

• No hard training 48 hours before

• No alcohol 48–72 hours

• Ensure proper timing of cycle and/or HRT administration

• Sleep like it’s your job

• Keep diet consistent 2–3 days

• Don’t stop meds unless instructed to; ask about supplements

• Reschedule if sick or inflamed

• Write down your context notes
  

The best labs are the ones you can actually use

The best labs are the ones you can gather data from and use to make decisions about your health and lifestyle.

If you’ve ever had labs done and walked away with:

• Everything is normal but you still feel off

• A bunch of flagged markers with zero context

• The bare bones basics from your doctor but you know you don’t feel right. 

• A PDF you don’t know how to translate into real life

  
  

That’s exactly why I created ESN Personalized Lab Insights.

It’s a great way to get:

• A more thorough lab panel than any annual physicals

• A custom interpretation based on your intake and goals

• A clear, practical action plan: Food, supplements, movement, sleep, stress

I’ve interpreted a handful of panels so far and the feedback has been awesome! If you’re ready to dig deeper, I’d love to partner with you!  Order your panel here.

If you’re not sure which panel fits or you have questions, email me at  erin@erinstimac.com  and I’ll be happy to direct you.