Frequently Asked Questions

Do I need to fast before I have a blood test?

Some blood tests require the patient to fast overnight before blood samples are collected in the morning. 

Fasting means that, from the night before the test, you must not eat anything and must drink only water. 

Substances in tea, coffee and cola can affect some blood results, so even black coffee and sugar-free (diet) drinks must be avoided. Likewise, chewing gum or smoking must be avoided on the morning of the test. 

Fasting guidelines 

  • Fasting glucose: please fast for at least 8 hours. 
  • Oral glucode tolerance test: please fast for at least 8 hours. Note: this test takes a minimum of 2 hours. You must sit quietly and not eat or smoke during the test. 
  • Lipids / triglycerides / fasting cholesterol: please fast for at least 10 - 12 hours. 
  • Cholesterol: Fasting is not essential when the test is requested for screening or monitoring but may be recommended for diagnosis. 

What if I am diabetic? 

Fasting lipid results are rarely needed in diabetic patients. If you are diabetic DO NOT fast before this test, and do not stop taking, or change, your existing medication. 

What if I have to take medicine in the morning?  

You should not alter your medication unless instructed to do so by your doctor. If your medicine needs to be taken with food and this will interfere with the timing of your fasting blood test, please discuss this with your doctor.

More information

More information about blood tests can be found at Lab Tests Online UK. This is a resource to help you better understand the many tests that are part of routine care, as well as diagnosis and treatment of a broad range of conditions and diseases. 

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