Comparison between specified percentage and fifth percentile criteria for spirometry interpretation in Thai patients

Objectives: The present study was conducted to determine the degree of agreement between the interpretation of spirometry using a specified percentage of predicted value (SPC) and the fifth percentile (FPC) as the cut off between normal and abnormal. Methodology: Spirometric values were collected for 1754 subjects attending the respiratory service at Siriraj Hospital between February 2000 and April 2001. These values included forced vital capacity (FVC), forced expiratory volume in 1 s (FEV1), FEV1/FVC, maximal mid-expiratory flow (FEF25-75%) and peak expiratory flow (PEF). A comparison of results between SPC and FPC was performed. The SPC cut-off values for FVC, FEV1, FEV1/FVC, FEF25-75% and PEF were 80% predicted, 80% predicted, 70%, 65% predicted and 80% predicted, respectively. The FPC cut-off values were calculated from reference equations for the Thai population. Inter-rater agreement was calculated as the kappa score. Results: High kappa scores were obtained for FVC (0.84), FEV1 (0.88) and FEF25-75% (0.80). However, poor agreement was found for FEV1/FVC (0.72) and PEF (0.61). When the cut-off values for SPC were modified to 90% of predicted values for FEV1/FVC and to 65% of predicted values for PEF, agreement was substantially improved to 0.92 and 0.89, respectively. Conclusions: Interpretation by SPC using cut-off values of 80% predicted for FVC and FEV1 and 65% predicted for FEF25-75% resulted in good agreement with FPC. However, the SPC cut-off values for FEV1/FVC and PEF should be modified to 90% predicted and 65% predicted, respectively.