Comparison of per cent predicted and percentile values for pulmonary function test interpretation

Area of Science:

• Pulmonary Medicine
• Respiratory Physiology

Background:

• Traditional pulmonary function test (PFT) interpretation relies on percentage of predicted normal values (e.g., <80% or >120%).
• Current American Thoracic Society/European Respiratory Society guidelines recommend a percentile-based approach (e.g., <5th or >95th percentiles) for defining abnormal PFT results.

Purpose of the Study:

• To compare the diagnostic accuracy of the per cent predicted method versus the percentile-based method for interpreting lung function parameters.
• To evaluate the agreement between these two common PFT interpretation methods.

Main Methods:

• Analysis of full PFTs from 2176 men and 1658 women performed between 2000 and 2004.
• Calculation of per cent predicted and percentile values using Crapo and Gutierrez equations.
• Comparison of diagnostic test characteristics using the percentile method as the reference standard.

Main Results:

• High agreement (>94%) between per cent predicted and percentile methods for most PFT parameters, except reduced residual volume (88%).
• Suboptimal sensitivity of the per cent predicted method for abnormal total lung capacity, increased residual volume, and reduced diffusion capacity.
• Suboptimal specificity observed for decreased residual volume using the per cent predicted method.

Conclusions:

• The per cent predicted and percentile-based methods yield similar results for most spirometry parameters.
• These two methods are largely interchangeable for spirometry interpretation.
• Caution is recommended when relying solely on the per cent predicted method for assessing lung volumes or diffusion capacity.