Fasting Instructions for diagnostic tests


Laboratory errors may occur in three different stages: before testing, during the test procedure and after testing. The most common errors are preanalytical errors (before testing) that include patient's preparation condition, the maintenance and transfer of the sample, etc. To prevent these errors, Payvand Clinical and Specialty Laboratory has provided various guidelines for the respectable clients. Following the conditions mentioned in these guidelines  are useful in earning the correct results and comparing them with the normal values for the proper diagnosis and treatment by physicians.

Fasting Instructions

Fasting is required for some medical tests including tests for blood sugar, glucose tolerance, insulin, triglyceride, vitamin D, calcium, iron (Fe), alkaline phosphatase enzyme, growth hormone, vitamin B12, etc., due to the alteration that eating can cause on these tests results.
If your doctor or our colleague in the laboratory requests you to come to the test with fasting, please note the following points:

  1. Eight hours before visiting the lab for fasting blood sugar test, do not consume any kind of food or drink (except water). This is the recommended fasting period at night which minimizes body activity, and makes the patient in a good condition for testing in the morning.
  2. For blood lipid tests such as triglycerides requested the fasting time is 10-12 hours.
  3. Avoid drinking juice, tea and coffee during fasting. Drinking water is permissible unless noted by the doctor.
  4. During the fasting time avoid smoking, having chewing gum and These may lead to stimulation of the digestive and muscular system and alter the test results.
  5. After sampling, you can start your normal daily diet again.
  6. For 24-hour urine VMA test, it is necessary to avoid foods such as coffee, vanilla, chocolate, banana and cocoa for 3 days prior the test.
  7. The fasting time for babies is 4 hours after the last lactation.













Why should I fast?

Some test results would alter after eating, for instance, the glucose level increases after eating. When you are fasting, the amount of sugar is set at the base and comparable to your next test.

Can I drink water while I am fasting?

Drinking during fasting is not needed to be avoided unless the doctor limits the amount of water intake. However, drinking other fluids during fasting should be avoided.

Should I continue to use medications?

Yes, unless your doctor tells you otherwise. As taking some medications may interfere with the results in some tests, if you are taking medication, you will need to declare the type of medication you are taking when you are in Administration Department. Among the most important tests that may be affected by consuming some medications are tests such as cortisol, aldosterone, amylase, steroid hormones, alkaline phosphatase, growth hormone, lithium and prolactin, etc.

Note: Laboratory is not able to provide any instruction on the prohibition or discontinuation of medication. Be sure to consult with your physician.

Can I drink juice?

No, you are only allowed to drink water.

Am I allowed to drink coffee or tea?

No, even sugar-free coffee or sugar-free tea is not permitted.

Am I allowed to smoke?

No, smoking may alter the test results.

Am I allowed to do my usual exercise?

No, exercise may result in a change in the test results. Prolonged heavy exercise and muscular activity increase the amount of some test results, such as prolactin, growth hormone, and CPK enzymes. Thus, exercising should be avoided prior the test.

What time should I go to the lab?
For some tests that require fasting it is advised to start fasting between 7 to 9 pm after eating the usual meal and  then next morning, go to the lab to have your blood test. For some tests, such as cortisol level, which differs throughout the day, you need to follow your doctor timing instruction (for example, 8 am) and be in lab at the exact time to be tested. Therefore it is advised to be at lab earlier to complete the administration process on time and can do your test on the exact time.



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