One of the most important factors in determining the accuracy and reliability of your lab test is you, the patient. After all, it is a sample from your body (blood, urine, or some other specimen; see Collecting Samples for Testing) on which the test will be performed. Therefore, it is essential that you do the following to ensure that the results will be useful and interpreted correctly by your health care provider:

  • Follow instructions, if there are any, to prepare for the specific test you are having performed.
  • Alert the person collecting your sample if you have deviated from the instructions and how.
  • Inform the health care practitioner of any medications (including vitamins and supplements) you are taking. If you are taking certain prescribed medications, such as anticoagulants (blood-thinners) or seizure medication, you may want to write down the exact time at which you took your dose and when your blood was drawn. This information will be useful if your health care provider has any questions about your test results.

Certain behaviors may affect some test results, such as recent or excessive exercise, not drinking enough fluids (dehydration), excessive eating, or recent sexual activity. You may be asked to refrain from some of these activities for certain tests.

It should be noted that many tests require no special preparation. But for those that do, be certain to adhere to the instructions provided. If you are ever unclear about the instructions, be sure to ask the person ordering the test for clarification. If you are not given any instructions, you should ask if there are any special instructions needed to prepare for the test.

To help you remember what you need to do before having a lab test, your provider should give you a written copy of instructions, if there are any. In fact, all laboratories that perform moderate and/or highly complex tests are required by the federal government under the Clinical Laboratory Improvement Amendments (CLIA) to provide patients with such written policies. You also may find information about preparations needed for specific tests on websites like However, you should always follow your health care provider’s instructions, as the procedures for a particular test might vary from one lab to another.

One of the more common types of preparation required for testing is fasting (to go without all or certain foods) for several hours before the test or even overnight. Certain tests may require you to increase or decrease the amount you drink for 10 to 12 hours prior to the test. There may be specific foods and medications you will need to avoid. Or you may be asked not to smoke before the test or even not to drink your favorite herbal tea. If you are collecting the specimen at home (such as urine, stool, or semen), you could be asked to follow certain procedures to transport the sample from home to the lab.

Examples of some common laboratory tests that require advance preparation include:

  • Glucose tolerance, fasting, and two-hour post-prandial blood glucose tests: fasting or eating meals at specific times may be required.
  • Lipids profile (triglycerides, cholesterol, etc.): fasting for 9-12 hours may be required.
  • Creatinine: fasting overnight or refraining from eating cooked meat may be required since some studies have shown that eating cooked meat prior to testing can temporarily increase the level of creatinine.
  • Fecal occult blood test: certain food and/or medication restrictions may be required.
  • Urine culture: a patient may be instructed not to urinate for at least one hour before the test and/or to drink a glass of water 15-20 minutes before sample collection.
  • 5-HIAA: foods such as avocados, bananas, pineapples, plums, walnuts, tomatoes, kiwi fruit, and eggplant can interfere with 5-HIAA measurement and should be avoided for 3 days prior to and during urine collection; there are also a variety of drugs that can affect the 5-HIAA test.
  • Cortisol: resting before sample collection may be required and, if a saliva sample is to be collected, it may be necessary to refrain from eating, drinking, or brushing teeth for a period of time prior to the test.
  • Pap test (pap smear): a woman may be instructed not to douche or tub bathe for 24 hours before the Pap test is to be performed and not to use any vaginal creams or foams for 48 hours prior to the exam; she also may be asked to refrain from sexual intercourse for 24 to 48 hours before the test and not schedule the test during her menstrual period.

On, we provide some information about test preparations you may need to follow before taking certain tests. However, be sure to check with your health care provider for specific instructions rather than relying on the information on this or any other web site, as different labs may have varying testing protocols.

Finally, with laboratory testing, like other aspects of medical care, it is crucial that you are open and honest with your healthcare provider. Just as you should give them your complete personal, medical, and family history, you may need to report deviations from preparation instructions and/or any medications that you are taking at the time of testing, including vitamins and supplements, as these may affect the results. You also may be asked about the amount of alcohol you consume, tobacco products you smoke, or any illicit drugs you may have recently taken. Providing complete, accurate information will help to ensure the reliability of your test results and prompt diagnosis and/or treatment.

Ask a Laboratory Scientist

Ask A Laboratory Scientist

This form enables patients to ask specific questions about lab tests. Your questions will be answered by a laboratory scientist as part of a voluntary service provided by one of our partners, American Society for Clinical Laboratory Science. Please allow 2-3 business days for an email response from one of the volunteers on the Consumer Information Response Team.

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