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Is Drinking Water Acceptable Before A Glucose Test?

Imagine, you are at the doctor’s office and he says, “I need to do a glucose test, don’t eat or drink for 8 hrs.” You smile and nod but secretly you’re wondering does that include water too. 

 A glucose test checks your blood sugar but there are many different types of glucose tests. Each has different requirements that you should understand. 

Are you familiar with a glucose tolerance test, a random glucose test, fasting test or A1c? Can I drink water before a glucose test?

If you have wondered about any of this then continue reading to learn more.   

3 bottles of Glucose drink

What are the different glucose tests? 

Oral Glucose Tolerance Test 

A common test to screen for diabetes. A person needs to stop eating 8-12hours before the test.  On the day of the test, your doctor will give you a drink with 75gm of glucose.  It usually tastes very sweet.  Then you wait at least 1 hr, sometimes 2 hours and a blood test is taken at the 1 hr mark or at the 1 and 2nd hour mark.  

Please do not alter what you usually eat, fast before the test or restrict your carbohydrate intake as it can lead to a misdiagnosis.     

Your doctor is checking to see if your  blood sugar is over  200mg/dL.  If it is, then your doctor may order a different glucose test such as fasting, random glucose test or check your A1c before giving a diagnosis of diabetes. 

People with these condition should be tested for diabetes with the above  testing method (1):

  • Pregnant woman 
  • Cystic Fibrosis 
  • After organ transplant 

Random Glucose Test 

A random glucose test is when glucose is tested at any time of the day.  If your blood sugar is >200mg/dL then that is an indication to your doctor that you may have diabetes (1).  Any one at high risk for diabetes can be screened with this test.

Nurse checking a patients blood sugar

Fasting Test 

A fasting test is done in the morning, because you need to fast 8-12 hours before the test.  A blood sugar over 126mg/dL is another indication to your doctor that you may have diabetes.  

A fasting blood sugar is the recommended screening test for diabetes for a person with HIV (1):

  • Before starting antiretrovial therapy 
  •  3-6 months into antiretroviral therapy
  • Switching antiretroviral therapy

Blood sugar taken in the morning before food is known as a fasting blood sugar.  This indicates how well a person with diabetes is managing their blood sugar.  The recommendation on the ADA website is for the fasting blood glucose to be between 80-130mg/dL (2).  

A vial of blood to be assessed for a glucose test


A1c is a test that is used to determine how the blood sugar has been over the period of 3 months.  The goal for a person with diabetes is to have an A1C of 7 or lower.  For a person who does not have diabetes an  A1c >6.5 is considered high (1). This is not the best test to use to diagnose a person with diabetes. 

This is a very common test people with diabetes get tested for every 3-4months. There are no requirements before getting blood work before an A1C test.  No fasting or abstaining from water is necessary. 

Can I drink water before a glucose test?  Yes! Inadequate water intake before any glucose test can lead to unreliable lab results.  

Dehydration and Misinterpreted Lab Results

Man drinking water

Dehydration can lead to blood sugar levels being too high or too low than they actually are.  Factors that affect hydration status (3):  

  • Physical activity 
  • Hot, dry weather 
  • Inadequate water intake 
  • Diuretics such as coffee 

Can I drink water before a glucose test?  Yes! Inadequate water intake before any glucose test can lead to unreliable lab results.  

Final Thoughts 

There are 4 main tests used to either screen for diabetes or to evaluate how effectively blood sugars are being managed.  

The oral glucose tolerance test, random glucose test and the fasting test are used to screen for diabetes. A1c can also be used but is not the best test.  aA1c is used by the physician to evaluate how effectively the blood sugar is being managed.  The fasting blood sugar lets people with diabetes know how well blood sugar was managed last night.   

Only the fasting test and the oral glucose tolerance test needs a person to fast ~8-12 hours before blood work is taken for these tests.  Otherwise, it is very important for all of these tests to drink sufficient water to avoid dehydration. Dehydration can lead to inaccurate results.  

Do you have any experience taking these tests?  Share your experience below.

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