College of American Pathologists
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  Patient Preparation Prior to Blood
  and other Laboratory Testing


Posted May 6, 2009


Since the vast majority of medical decisions are based upon results obtained from laboratory tests, it is critical to the well-being of patients to obtain the most accurate information possible. Patients can help to ensure that their lab tests are accurate by following pre-testing instructions carefully and by providing complete medical histories, including lists of medications and natural supplements, to their health care providers.

Information Highlights

The vast majority of medical decisions are based upon results obtained from laboratory tests, therefore, it is critical to the well-being of patients to obtain the most accurate information possible.

Variables that could affect test results

  • Patient variables including exercise, diet, age, sex, circadian variation, posture, obesity, stress, smoking and medication may affect laboratory test results.
  • An individual’s diet and lifestyle may affect laboratory test results. It is generally recommended that the night before laboratory tests patients avoid high-fat foods, alcohol and strenuous exercise.
  • Patients should ask their doctors if certain medications should be stopped prior to lab testing as certain medications may interfere with the laboratory test results.

Blood Tests

  • Patients may need to fast prior to certain blood tests. For example, patients should not eat or drink anything except water for 9 to 12 hours prior to glucose and lipid profile tests.
  • The amount of blood drawn at the time of collection for laboratory testing depends on the tests that are ordered. Usually the amount collected is very small (around 3-6 teaspoons.)
  • Some patients become anxious when they have their blood drawn. Patients should tell the health care professional who is drawing the blood if they feel faint or sick. Slow deep breaths prior to the needle stick may help to alleviate anxiety.
  • After a blood draw, the phlebotomist makes sure that all signs of bleeding have stopped. A bandage is applied to the arm for a minimum of 15 minutes.
  • Aspirin or other anticoagulant (blood thinners) drugs can prolong bleeding. In such cases, patients may need continued applied pressure until the bleeding has stopped. A cold pack may be necessary to reduce swelling and bruising.
  • After a patient has blood drawn, even when bleeding has stopped, patients should not carry or lift a purse or child (or other heavy object) with that arm for a minimum of one hour.

Collecting Specimens at Home

  • Patients must follow all instructions exactly for collection of specimens performed at home then brought to the laboratory for testing.
  • Special containers with a powder or liquid preservative may be provided for urine or stool collection. Patients should never empty or discard any powder or liquid from the container before beginning the collection of a specimen.
  • Specimens should be delivered to the laboratory in the prescribed timeframe in order to assure accurate results.


  • Depending on the laboratory work performed, test results may be available within a few hours to as long as several weeks.
  • Laboratory test results are often reported with a reference interval to assist the clinician in interpreting them. These reference intervals reflect the values in the majority of healthy individuals; however, a small number of healthy people (5%) may have results that are higher or lower than those in the reference range. Therefore, laboratory results should interpreted by clinicians who can decide whether or not the results indicate a medical condition.
  • Clinicians consider personal medical history, family history, and results from physical examination when interpreting an individual patient’s laboratory test results.