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Routine Lung Function Tests

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3 ➤ Global Lung Function Initiative (GLI) reference equations for spirometry, diffusing capacity and lung volumes should be used to define the expected range of values in healthy individuals. ➤ Changes in forced expiratory volume (FEV 1 ) and forced vital capacity (FVC) following bronchodilator responsiveness testing should be expressed as the percent change relative to the individual's predicted value. A change >10% of the predicted value indicates a positive response (see Box 1 on page 3). Interpretation Box 1. Determination of a Bronchodilator Response Bronchodilator Response (BDR) = (Post-bronchodilator value - Pre-bronchodilator value) × 100 Predicted value a A change of >10% is considered a significant BDR response. a Predicted value should be determined using the appropriate GLI spirometry equation. Box 3. Calculation of FEV 1 Q in adults FEV 1 Q is the observed FEV 1 in liters divided by the sex-specific first percentile of the FEV 1 distribution found in adult subjects with lung disease; these percentiles are 0.5 liters for males and 0.4 liters for females. The index approximates to the number of turnovers remaining of a lower survivable limit of FEV 1 . For example, a 70-year-old woman with an FEV 1 of 0.9 liters would have an FEV 1 Q of 0.91/0.4 liter or 2.25. Values closer to 1 indicate a greater risk of death. Box 2. Calculation of a Conditional Change Score in Children The change score is defined as zFEV 1t2 - (r × zFEV 1t1 ) where zFEV 1 at t 1 and t 2 are the observed z-scores at the initial and second time point, and r is defined as 0.642 – 0.04 × time(years) + 0.020 × age (years) at t 1 . Changes within +/– 1.96 change scores are considered within the normal limits. √1- r 2 ➤ There are limited data to support a single recommendation for interpreting PFT reproducibility. Two distinct approaches were identified to express natural changes in lung function: conditional change scores for children and forced expiratory volume quotient (FEV 1 Q) for adults.

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