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Factors That May Influence Your Spirometry Results

Spirometry Results Highly Dependent Upon Patient and Technician

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Updated October 26, 2012

Written or reviewed by a board-certified physician. See About.com's Medical Review Board.

If you've recently had a spirometry test and your results were less than favorable, you're probably wondering if there was something you did—or didn't do—that may have influenced your spirometry results. Accuracy of test results is, in fact, highly dependent upon you, the patient, and the technician administering the test. There are a number of factors that are known to influence test results, the most common of which are:

  • Inaccurate Demographic Data — When you take a spirometer test, your test results are compared to those that are expected for a normal, healthy population. These "normal, predicted values" are determined during population-based research studies using subjects who have normal-functioning lungs. Each patient has her own set of predicted values based on demographic data from someone of the same age, height, weight and sex (and sometimes ethnicity). If for some reason a patient gives inaccurate demographic data, or demographic data is incorrectly entered by the technician, test results will be affected. A good doctor will review this information for accuracy before interpreting the test.
  • Results That Aren't Acceptable — After spirometry is performed, test results are evaluated for acceptability and reproducibility (see below). Generally, the FVC maneuver is acceptable if the patient has made a good effort--a factor that's pre-determined using specific criteria. If the patient is ill, or something else impairs his effort to forcibly inhale or exhale during the test, the results won't be accurate and will therefore be deemed unacceptable.
  • Results That Aren't Reproducible — The spirometry test should be repeated a minimum of three times. In order to meet reproducibility criteria, all three FEV1 measurements, and all three FVC measurements, must be within 200 milliliters (ml) of each other. The test with the greatest FEV1 and FVC represent the patient's test results for that particular test. If any one of these conditions isn't met, the test has failed to meet reproducibility criteria.

Common Reasons For Unacceptable or Non-Reproducible Results

Test results that aren't acceptable or reproducible aren't considered accurate, nor will they reflect a patient's true, underlying lung impairment. Here are a few reasons why this might occur:

  • Failing to provide sufficient effort (often due to illness or pain that worsens with each maneuver).
  • Not understanding test instructions (possibly due to either a language or cognitive barrier, or a hearing deficit).
  • Coughing at any time during the maneuver, but especially during the first second.
  • A Valsalva maneuver (bearing down) or hesitation during the maneuver that causes air to cease flowing.
  • Inadequate seal around the spirometer mouthpiece.
  • Obstruction of the mouthpiece (occurs when the tongue or teeth are incorrectly placed in front of the mouthpiece during the test).
  • Deformation of the mouthpiece (caused by biting or chewing on the mouthpiece itself).

How to Ensure Accurate Spirometry Results

If you have COPD, spirometry will be repeated at various times to determine if your disease has progressed and how well you're responding to COPD treatment. At first, the test may seem daunting, but before long, it will become second nature.

If this is your first spirometry test, review the following tips before you take your test to ensure the most accurate results:

  • Make sure you understand the test instructions. If you don't, ask the technician to repeat them while demonstrating the correct way to inhale and exhale during the test.
  • Inform the technician if you're not feeling well. Your test may need to be rescheduled for a day when you feel better.
  • Get a good night's sleep the night before the test. Poor quality sleep may affect effort.
  • Refrain from smoking at least one hour before the test. Smoking may impact your results.
  • Give the technician your accurate weight. In general, gentlemen don't have a problem being honest about their weight, but there probably isn't a woman alive who, at some point in her life, has not lied about her true weight. An accurate weight is essential for calculating spirometry results.
  • Refrain from eating a heavy meal the morning of the test. A full stomach may impair your ability to take a deep breath.
  • Avoid drinking alcohol at least 4 hours before the test. Among other things, alcohol may impair effort and negatively impact your results.
  • Avoid heavy exercise at least 30 minutes before the test. You'll need all your energy to put forth sufficient effort during the test.
  • Follow your doctor's instructions for which medications you should and should not take on the day of the test. Using a bronchodilator or other types of medication before the test may affect your results.

Sources:

American Thoracic Society. Pulmonary Function Testing. American Journal of Respiratory Critical Care Medicine. Vol. 176, P5-6, 2007.

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