Spirometry should be used to confirm a diagnosis of chronic obstructive pulmonary disease (COPD). This test is not always performed, leading to possible misdiagnosis. We investigated whether the proportion of patients with diagnostic spirometry has increased over time as well as factors associated with omitted or incorrectly interpreted spirometry. Data from medical reviews and a questionnaire from primary and secondary care patients with a doctors’ diagnosis of COPD between 2004 and 2010 were collected. Data were compared with a COPD cohort diagnosed between 2000 and 2003. Among 703 patients with a first diagnosis of COPD between 2004 and 2010, 88% had a diagnostic spirometry, compared with 59% (p< 0.001) in the previous cohort. Factors associated with not having diagnostic spirometry were current smoking (OR 2.21; 95% CI 1.36–3.60), low educational level (OR 1.81; 1.09–3.02) and management in primary care (OR 2.28; 1.02–5.14). The correct interpretation of spirometry results increased (75% vs 82%;p= 0.010). Among patients with a repeated spirometry, 94% had a persistent FEV1/FVC or FEV1/VC ratio <0.70.