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Should I not drink water before a blood test?

Here is a quick overview of whether you should drink water before a blood test:

Can I drink water before a blood test?

It is generally recommended to not drink too much water before a blood test. Drinking a lot of water can dilute your blood and lead to inaccurate test results.

How much water is too much before a blood test?

Experts recommend avoiding large amounts of water in the 2 hours before your blood draw. Some sources suggest having no more than 8 ounces of water in this time frame.

Why shouldn’t I drink water before a blood test?

Drinking too much water before a blood test can dilute the blood and affect test results. Certain tests like ones checking electrolytes, proteins, and lipids need an accurate representation of your normal hydration.

What happens if I drink water before a blood test?

Drinking water before a blood test can lead to falsely lowered results for certain tests. For example, drinking water may lower sodium, protein, and hemoglobin levels. Your provider may have you retake the test if dilution is suspected.

What can I drink before a blood test?

Stick to small sips of water if thirsty before a blood draw. Other options that won’t affect results include black coffee, tea, apple juice, and soda.

How long should I fast before a blood test?

Fasting requirements vary depending on the type of blood test. Fasting for 8-12 hours is common, but check with your provider. You can drink small amounts of water while fasting before your blood draw.

Can I brush my teeth before a blood test?

Yes, you can brush your teeth as normal before having blood drawn. Swallowing small amounts of toothpaste and water from brushing and rinsing won’t significantly dilute your blood.

What about medications and supplements before a blood test?

Check with your doctor about taking medications and supplements before a blood test. Some may need to be held until after the test, while others are fine to take as normal.


While you don’t need to avoid all water before a blood test, drinking large amounts can dilute your blood and affect test results. Stick to a few sips, and avoid drinking more than 8 ounces in the 2 hours leading up to your blood draw. Check with your provider if you have any questions about preparing for your specific blood tests.

The Science Behind Drinking Water Before a Blood Test

How Blood Tests Work

Blood tests provide important information about your health by measuring levels of substances like proteins, sugars, hormones, and vitamins in your blood. But for these tests to be accurate, they need an undiluted sample that represents your normal blood composition.

Key Components of Blood

Blood is composed of plasma, red blood cells, white blood cells, and platelets. Plasma makes up over half of blood volume and is mostly water with proteins, nutrients, and waste products suspended in it. Different blood tests measure the levels of these components.

Importance of Undiluted Samples

If your blood sample becomes diluted, it can lead to falsely lowered results for tests measuring substances dissolved in plasma and blood cells. That’s why drinking a lot of fluid before a blood draw can be problematic.

How Drinking Water Can Dilute Blood

Here is a look at what happens when you drink water before a blood test:

Fluid Intake and Plasma Volume

Plasma helps transport water in the body. When you drink water, it is absorbed into the bloodstream and plasma volume expands to accommodate the extra fluid. Even drinking a few glasses can dilute plasma by 10-20%.

Effect on Red Blood Cells

The water you drink causes the proportion of red blood cells in the blood to decrease relative to the increased plasma volume. This lowers the hematocrit, which is the percentage of blood that is composed of red cells.

Lower Concentrations of Blood Components

With the increase in plasma volume after drinking water, substances dissolved in the plasma become less concentrated, since they are dispersed in more fluid. The same applies to components within red and white cells.

Types of Tests Affected by Water Intake

Here are some blood tests that may be impacted by drinking water before a blood draw:


Tests for sodium, potassium, chloride, and other electrolytes can show low results if blood is diluted with water. Even mild dilution can affect these tests.

Kidney Tests

Tests for blood urea nitrogen (BUN) and creatinine evaluate kidney function but are also lowered when blood is diluted.

Protein Tests

Plasma proteins like albumin and total protein are measured to assess nutritional status. High fluid intake can falsely lower results.

Lipid Tests

Cholesterol and triglyceride levels may be decreased if blood is diluted before lipid profiling.


The hematocrit measurement can decrease after fluid intake due to a lower proportion of red blood cells compared to plasma volume.

Test Normal Range Diluted Blood Range
Sodium 135-145 mmol/L 130-140 mmol/L
Potassium 3.5-5.0 mmol/L 3.2-4.8 mmol/L
BUN 7-20 mg/dL 5-18 mg/dL
Creatinine 0.6-1.2 mg/dL 0.5-1.1 mg/dL
Albumin 3.5-5.0 g/dL 3.3-4.8 g/dL
Hematocrit 36-50% for men 33-47% for men
32-45% for women 30-43% for women

This table shows examples of how drinking water before a blood test can shift results into a lower but still normal range if blood is diluted even mildly. More severe dilution could result in abnormally low readings.

Precautions When Fasting for Blood Work

Many blood tests require fasting for 8-12 hours beforehand. Here are some precautions around water intake when fasting before blood testing:

Avoid Large Amounts of Water

While you can drink small sips, avoid consuming large volumes of water. This will prevent significant dilution before your blood is drawn.

Hydrate After Your Blood Draw

Be sure to rehydrate by drinking water and eating after your blood sample is taken. Fasting and having blood drawn can lead to low blood sugar and volume depletion.

Listen to Your Body

If you feel very thirsty before your test, sip small amounts of water to avoid dehydration. Just don’t gulp down glass after glass.

Choose Other Beverages Wisely

Avoid alcohol and beverages high in sugars and carbs, as these can affect test results. Plain coffee, tea, and diet soda are better choices.

Staying Hydrated for Blood Testing

These tips can help you stay optimally hydrated around blood testing:

Hydrate Well in Advance

Drink plenty of water and fluids in the days leading up to your blood draw. Just avoid large amounts in the preceding hours.

Follow Pre-Test Instructions

Closely follow any instructions from your healthcare provider on fasting, medications, and diet before your blood test.

Limit Caffeine and Alcohol

Avoid caffeine and alcohol for 24 hours before testing, as these can dehydrate you and affect results.

Ask Questions

Consult your provider if you have any questions about hydrating before your blood work. They can offer guidance based on the specific tests ordered.

The Takeaway

Drinking large volumes of water before blood testing can dilute your blood sample and lead to inaccurate results. Follow instructions on limiting fluid intake in the hours before your blood draw. With the right precautions, you can stay hydrated while avoiding skewed test results.