On post-bronchodilator (after administration of a bronchodilator medication) testing, do I look at the best result for the Forced Expiratory Volume in 1 second (FEV-1) to Forced Vital Capacity (FVC) ratio, or the predicted value?

Post-Bronchodilator FEV1/FVC Interpretation

You should use the best measured value (not percent predicted) of the post-bronchodilator FEV1/FVC ratio and compare it to the absolute threshold of 0.70 or the lower limit of normal (LLN), not to a predicted value. The FEV1/FVC ratio itself does not have a "percent predicted" value that is clinically meaningful for interpretation 1.

Key Interpretation Principles

The FEV1/FVC Ratio is Reported as a Decimal

• The FEV1/FVC ratio should be reported as a decimal fraction (e.g., 0.65), and the space for percent predicted value should be left blank to minimize miscommunications 1.
• This is a critical reporting standard from the American Thoracic Society because the ratio is an inherently relative measure that doesn't require normalization to predicted values 1.

What "Best Result" Means

• For post-bronchodilator testing, you use the best FEV1 and best FVC values from acceptable maneuvers to calculate the FEV1/FVC ratio 1.
• The "best" refers to the highest measured values from technically acceptable efforts, not any comparison to predicted values 1.

Clinical Application

For Diagnosis of Airflow Obstruction

• Compare the measured post-bronchodilator FEV1/FVC ratio to either the fixed threshold of 0.70 or the LLN (typically a z-score of -1.64) 1, 2.
• A post-bronchodilator FEV1/FVC < 0.70 is the GOLD guideline criterion for confirming COPD diagnosis 3, 2.
• The LLN approach may be more appropriate for older adults to avoid over-diagnosis 4.

For Assessing Severity

• Disease severity classification is based on the post-bronchodilator FEV1 percent predicted, NOT the FEV1/FVC ratio 1, 5.
• The post-bronchodilator FEV1 % predicted is a better predictor of mortality and clinical outcomes than pre-bronchodilator values 5.

Common Pitfalls to Avoid

• Do not look for a "predicted" FEV1/FVC ratio value - this is not how the ratio is interpreted and doing so leads to confusion 1.
• Do not use pre-bronchodilator values for diagnosis - COPD diagnosis specifically requires post-bronchodilator spirometry showing FEV1/FVC < 0.70 3, 2, 4.
• Do not confuse the ratio interpretation with individual parameter interpretation - while FEV1 and FVC are compared to predicted values, the ratio is compared to absolute thresholds 1.

Bronchodilator Response Assessment

• When assessing bronchodilator response itself, you evaluate the change in FEV1 and FVC (not the ratio) using both percent change from baseline (>12%) and absolute change (>200 mL) 1, 6.
• The FEV1/FVC ratio can be misleading for assessing bronchodilator response if expiratory time changes 1.