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Should I drink a lot of water before blood work?

Quick Answer

It is generally recommended to drink plenty of water in the 24 hours leading up to having blood drawn. Being well hydrated can help make it easier to access veins and get an adequate blood sample. However, there is no need to excessively over-hydrate before blood work. Drinking a normal amount of fluids is fine.

What Are the Requirements for Fasting Before Blood Work?

Most routine blood tests require fasting for 8-12 hours beforehand. This means no food or drink other than water. Fasting ensures that food intake does not affect test results.

However, fasting requirements can vary. Some tests require a longer fast, while others don’t require any fast. It’s important to follow your healthcare provider’s specific instructions on fasting.

Some examples:

  • A lipid panel, glucose test, or metabolic panel typically requires a 8-12 hour fast.
  • A complete blood count (CBC) does not require any fast.
  • A fasting blood sugar test requires a 8-hour fast.

When fasting before blood work, you should:

  • Drink plenty of water.
  • Avoid all food and drinks other than water.
  • Avoid caffeinated drinks like coffee and tea.
  • Avoid chewing gum or mints.
  • Take any regular medications as prescribed with water.

So while fasting limits food and drink, increased hydration with water is recommended.

Why Stay Hydrated Before Blood Work?

Drinking extra water leading up to blood work can help in a few key ways:

Easier Vein Access

Well hydrated veins are easier to access than dehydrated veins. The phlebotomist may have an easier time locating a vein and drawing blood if you are hydrated.

Adequate Blood Sample

Hydration provides greater blood volume, making it easier to collect the amount of blood needed for testing. With dehydration, smaller blood samples may be drawn.

Reduce Risk of Dizziness/Fainting

Some people feel lightheaded or faint when having blood drawn. Staying well hydrated can help minimize this risk.

Dilute Substances That Could Impact Results

Hydration can help dilute dietary supplements, medications, or other substances that could potentially affect test results.

So while not excessively overdoing fluid intake, proper hydration before blood work is beneficial.

How Much Water Should I Drink Before Blood Work?

There are no universal guidelines on exactly how much water to drink before blood work, but here are some general tips:

– Drink about 16-20 oz (500-600 ml) in the 1-2 hours before your blood draw

– Drink an extra 1-2 glasses (8-16 oz or 250-500 ml) in the 2-4 hours before your appointment

– Drink a total of 3-4 liters (12-16 cups) of water evenly spread throughout the day before your test

Again, there’s no need to excessively over-hydrate. Stick to normal hydration along with a couple extra glasses in the few hours leading up to your appointment. Avoid downing a gallon of water right before, as this can potentially dilute results.

What About Morning Blood Work?

For morning blood draws after fasting overnight:

– Drink 1-2 glasses (8-16 oz or 250-500 ml) of water when you wake up

– Sip water on your way to the appointment

This will provide hydration after fasting without significantly impacting results.

Some experts suggest weighing yourself before and after sleep. Drink an amount of water equal to the pounds lost overnight to rehydrate.

Will Drinking Too Much Water Before Blood Work Dilute Results?

Excessive water intake can potentially dilute blood samples, causing abnormally low results for certain tests like:

  • Electrolytes (sodium, potassium, chloride)
  • Protein
  • Glucose
  • Hormones

However, for most people drinking normally in the day or so before blood work, significant dilution is unlikely.

Factors that increase dilution risk include:

  • Drinking an excessive amount of water just before the test
  • Having high fluid intake due to medical conditions like diabetes insipidus
  • Using intravenous fluids or water pills (diuretics)

So avoid drinking more than 1-2 liters within the last 2-3 hours before blood draw. But hydrating normally in the day before is beneficial without skewing results.

Tips to Prepare for Blood Work

Here are some tips to optimize preparations before having blood drawn:

  • Follow your provider’s recommendations on fasting or other preparations.
  • Drink plenty of water the day before and morning of the test.
  • Avoid excessive alcohol, which can dehydrate you.
  • Continue taking any medications as prescribed unless directed otherwise.
  • Get a good night’s sleep.
  • Wear a short-sleeve shirt or something with sleeves that can easily be rolled up.
  • Arrive early to your appointment to complete any check-in procedures.
  • Relax during the blood draw by looking away and breathing normally.

These simple steps can help ensure the process goes smoothly.

What If My Veins Are Hard to Access?

Some people have veins that are more difficult to access for blood draws. Factors include:

  • Small, deep, or rolling veins
  • Scarring from prior blood draws
  • Age-related loss of vein elasticity
  • Obesity
  • Dehydration

If your veins are difficult to access, the following can help:

  • Drink extra water before tests to hydrate your veins.
  • Avoid caffeine, which can constrict veins.
  • Apply a warm compress to the area for 5 minutes before the draw.
  • Do arm exercises prior to the test to dilate the veins.
  • Ask for an experienced phlebotomist.
  • Request a butterfly needle, which is smaller and may be easier to insert.
  • Get blood drawn from hand or wrist veins if accessible.

Communicating any prior difficulties can allow staff to prepare accordingly with devices or techniques to obtain an adequate blood sample with minimal discomfort.


Maintaining good hydration is important before having blood drawn. Drinking extra water the day before and morning of the test can help ensure successful blood collection. While excessive fluid intake can potentially dilute results, normal hydration is beneficial. Let your phlebotomist know if you have any difficulties accessing veins so they can utilize techniques to obtain a quality sample. With the right preparation, blood work can be quick and uncomplicated.

Test Typical Fasting Requirement
Lipid panel 9-12 hours
Glucose test 8-12 hours
Complete blood count (CBC) None
Liver function tests None
Thyroid panel None

Sample Hydration Schedule Before Morning Blood Work

Time Amount to Drink
Day before test 3-4 liters water throughout the day
Morning of test 16-24 oz (500-700 ml) water upon waking
On way to appointment 8 oz (250 ml) water