Validity of spirometric testing in a general practice population of patients with chronic obstructive pulmonary disease (COPD).

OBJECTIVE: To investigate the validity of spirometric tests performed in general practice.
METHOD: A repeated within subject comparison of spirometric tests with a "gold standard" (spirometric tests performed in a pulmonary function laboratory) was performed in 388 subjects with chronic obstructive pulmonary disease (COPD) from 61 general practices and four laboratories. General practitioners and practice assistants undertook a spirometry training programme. Within subject differences in forced expiratory volume in 1 second and forced vital capacity (DeltaFEV1 and DeltaFVC) between laboratory and general practice tests were measured (practice minus laboratory value). The proportion of tests with FEV1 reproducibility <5% or <200 ml served as a quality marker.
RESULTS: Mean DeltaFEV1 was 0.069 l (95% CI 0.054 to 0.084) and DeltaFVC 0.081 l (95% CI 0.053 to 0.109) in the first year evaluation, indicating consistently higher values for general practice measurements. Second year results were similar. Laboratory and general practice FEV1 values differed by up to 0.5 l, FVC values by up to 1.0 l. The proportion of non-reproducible tests was 16% for laboratory tests and 18% for general practice tests (p=0.302) in the first year, and 18% for both in the second year evaluation (p=1.000).
CONCLUSIONS: Relevant spirometric indices measured by trained general practice staff were marginally but statistically significantly higher than those measured in pulmonary function laboratories. Because of the limited agreement between laboratory and general practice values, use of these measurements interchangeably should probably be avoided. With sufficient training of practice staff the current practice of performing spirometric tests in the primary care setting seems justifiable.