How is severity of impairment quantified in spirometry testing?

Updated: May 14, 2020
  • Author: Kevin McCarthy, RPFT; Chief Editor: Nader Kamangar, MD, FACP, FCCP, FCCM  more...
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A reduced FVC on spirometry in the absence of a reduced FEV1/FVC ratio suggests a restrictive ventilatory problem. An inappropriately shortened exhalation during spirometry can (and often does) result in an artifactually reduced FVC. Causes of restriction on spirometry include obesity, cardiomegaly, ascites, pregnancy, pleural effusion, pleural tumors, kyphoscoliosis, pulmonary fibrosis, neuromuscular disease, diaphragm weakness or paralysis, space-occupying lesions, lung resection, congestive heart failure, inadequate inspiration or expiration secondary to pain, and severe obstructive lung disease. One scheme for describing the severity of reductions in the FVC and/or the FEV1 is shown below:

  • Mild - Greater than 70% of predicted

  • Moderate - 60-69% of predicted

  • Moderately severe - 50-59%

  • Severe - 35-49% of predicted

  • Very severe - Less than 35% of predicted

The lower limit of normal for the FEF25-75% can be less than 50% of the mean predicted value, making it important to use the lower limit of normal defined by the 95% confidence limit of the mean predicted value rather than a threshold defined by a fixed percentage of the predicted value. The FEF25-75% is also very dependent on expiratory time. If expiratory times of spirometry efforts vary by more than 10%, comparisons of the FEF25-75% before and after bronchodilator challenge are difficult to interpret. Early termination of expiration shifts the middle 50% of the exhaled volume toward the start of the exhalation, artifactually raising the FEF25-75%. For these reasons, the use of the FEF25-75% to assess airway function in adults is discouraged.

The FVC is a reliable means of assessing the clinical status in idiopathic pulmonary fibrosis (IPF). A minimum clinically important difference of the FVC, expressed as a percentage of the mean predicted normal value, of 2-6% of has been established. This obviates the need to obtain a total lung capacity (TLC) measurement to assess disease progression or the effects of medical therapy.

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