Objective To investigate the impact of using low limit of normal( LLN) for FEV1 /FVC ratio and fixed ratio ( 70% ) as cut-off point in the qualitative diagnosis on the prevalence of chronic obstructive pulmonary disease( COPD) . Methods An epidemiological study was carried out in preoperative patients who received pulmonary function test in Zhongshan hospital fromNovember 6, 2007 to December 30, 2007. 339 patients were enrolled and diagnosed as COPD by different diagnostic criteria as follows: ①GOLD criteria; ②FEV1 /FVC