It is often recommended to drink water before having blood drawn, but some people wonder if this will affect the results of the blood tests. Here is a look at whether drinking water before a blood test is OK or if it should be avoided.
How Drinking Water Can Affect Blood Tests
Drinking water before having your blood drawn for testing can affect certain blood tests. Here are some of the potential effects:
- Diluting blood analytes – Drinking a lot of water before a blood draw can dilute the concentration of certain substances in the blood like electrolytes, proteins, hormones, and metabolites. This can lead to falsely low results.
- Altering hematocrit – Hematocrit measures the volume percentage of red blood cells in the blood. When a high volume of water is consumed before a blood draw, it can artificially lower the hematocrit.
- Changing blood viscosity – Hydration status impacts blood viscosity or thickness. Drinking water before a blood test may temporarily decrease blood viscosity.
- Affecting certain chemistry tests – Water intake can impact tests like total protein, albumin, calcium, potassium and sodium if enough water is consumed to substantially dilute the blood.
However, the effect depends on the volume of water consumed. Drinking a glass or two of water before blood testing generally does not significantly impact results. Excessive water intake is what poses the biggest problem.
Recommendations for Water Intake Before Common Blood Tests
Here are some general recommendations on water intake before common blood tests:
- Complete blood count (CBC) – It is fine to drink normal amounts of water before a CBC.
- Metabolic panel – Avoid excessive fluid intake. Up to 8 ounces of water is OK.
- Lipid panel – You can drink water normally before this test.
- HbA1c – Water intake will not affect results for this blood sugar test.
- Thyroid function tests – Avoid large amounts of water which can lower protein levels.
- Hormone tests – Do not drink excessive amounts of water before these tests.
Tips to Prepare for a Blood Test
Here are some tips if you are wondering about drinking water before having bloodwork done:
- Ask your doctor if you have any specific tests that could be affected by water intake. Follow their recommendations.
- Avoid drinking more than 1-2 glasses of water in the 2-3 hours before your blood draw.
- Drink normally in the days leading up to your test – don’t suddenly increase or restrict fluids.
- Avoid excess alcohol intake before testing as this can also impact results.
- Don’t do strenuous exercise right before the test which could change electrolyte and protein levels.
- Eat normally before the test.
- Tell the phlebotomist if you drank a large amount of water so they can take that into account.
The Amount of Water Matters
Consuming a normal amount of water before routine blood testing is generally not a problem. For example, drinking up to 8 ounces of water before a blood draw will not significantly dilute blood analytes in most cases. However, excessive fluid intake can artificially lower results for certain tests like electrolytes, proteins, liver enzymes, lipids, and more. That is why it is recommended to avoid drinking more than 1-2 glasses of water in the 2-3 hours before blood testing unless a medical provider recommends otherwise.
Common Blood Tests Affected by Fluid Intake
While most routine blood tests will not be significantly impacted by normal water intake, excessive hydration can affect results for the following common blood tests:
| Test | Normal Range | Effect of High Fluid Intake |
|---|---|---|
| Sodium | 135-145 mmol/L | Lower result |
| Potassium | 3.5-5.0 mmol/L | Lower result |
| Chloride | 98-108 mmol/L | Lower result |
| BUN (blood urea nitrogen) | 7-20 mg/dL | Lower result |
| Creatinine | 0.6-1.3 mg/dL | Lower result |
| Total protein | 6.0-8.3 g/dL | Lower result |
| Albumin | 3.5-5.5 g/dL | Lower result |
| Calcium | 8.6-10.3 mg/dL | Lower result |
When to Restrict Fluid Intake Before a Blood Test
There are some situations when your doctor may specifically advise you to restrict fluids for a period before your blood draw, such as:
- If you are getting an electrolyte panel
- If your doctor needs an accurate measurement of blood proteins
- If you are getting a test to evaluate kidney function
- If you are getting a fasting blood test
In these cases, you may be told to avoid drinking any fluids for up to 8-12 hours before your test. Be sure to follow your doctor’s recommendations about restricting water or other beverages.
Does Drinking Coffee Before a Blood Test Affect Results?
Drinking coffee before having blood drawn should not significantly impact test results as long as you do not add excessive amounts of water, cream or milk to the coffee. Black coffee or coffee with small amounts of fixings is unlikely to distort blood tests for most people.
However, some doctors may recommend avoiding coffee for 1-2 hours before certain fasting blood tests. Coffee can also potentially cause dehydration which concentrates blood analytes. As always, it’s best to check with your doctor about coffee intake before your specific blood tests.
Can Children Drink Water Before a Blood Test?
It is generally fine for children to drink normal amounts of water leading up to and before having their blood drawn. However, avoid giving children excessive amounts of water right before their blood test. Stick to their normal fluid intake.
Dehydration can also affect blood test results in children, so you don’t want to restrict their water either. The best approach is to have children drink normal amounts of fluid in the days and hours leading up to their bloodwork.
Does Water Intake Affect Pregnancy Blood Tests?
For routine blood testing during pregnancy like the glucose screening test, complete blood count (CBC) and blood type, normal water intake is fine and won’t significantly dilute results. However, for your glucose tolerance test where blood sugar levels need to be carefully measured, your doctor may advise fasting with no fluid intake about 8-12 hours beforehand.
It’s always best to check with your obstetrician about any special instructions to prepare for pregnancy bloodwork. Staying well hydrated during pregnancy is important, so balance this with any needed precautions before blood draws.
Conclusion
Drinking water before a blood test is generally OK as long as intake is not excessive. Consuming up to 8-16 ounces of water in the 2-3 hours before most routine bloodwork will not significantly distort results for healthy adults. However, clear any water intake with your doctor beforehand and follow their recommendations about restricting fluids if needed for certain blood tests. Being properly hydrated is important, but excessive water consumption right before blood draws can artificially lower results of analytes like electrolytes and proteins. With some simple precautions, you can stay hydrated while still getting accurate blood test results.