Volume calibration alone may be misleading.

The use of spirometry is becoming more and more widespread in non-laboratory situations such as general practice or occupational medicine. In these non-laboratory situations, volume calibration with a 3000 ml syringe is often the only feasible method to ensure that the spirometer produces valid and reproducible data. Sophisticated equipment to calibrate forced manoeuvres with standard waveforms are not present. In this study, we assessed whether volumetric calibration is a guarantee for valid and comparable spirometric results. Two portable spirometers were tested. On 8 consecutive test days, both spirometers were calibrated with a 3000 ml syringe in accordance with the American Thoracic Society (ATS) guidelines. The comparability of the spirometric results (forced expiratory volume in 1 S, FEV1) was tested in two ways. Firstly, the spirometers were compared to each other using the results from 43 volunteers on the same 8 test days. The spirometers were presented in a randomized order and volunteers were asked to perform a series of reproducible manoeuvres in both spirometers. Paired observations were analysed, using Bland and Altman plots. Secondly, the spirometers were compared to a 'gold standard', a computer-driven syringe (CDS). Calibration with the 3000 ml syringe showed that both spirometers complied with the ATS criteria for volume calibration for diagnostic spirometry. However, paired FEV1 data obtained in subjects showed a systematic, volume-dependent difference between the two spirometers (mean difference: 289 ml, P < 0.001, systematic difference: 8.6%, P < 0.0001). This systematic difference was confirmed by the comparisons with the CDS. Volume calibration may be misleading. The results from volume calibration may meet the ATS criteria, but this is no guarantee that data from forced manoeuvres are accurate. If CDS equipment to simulate standard wave forms is not available, it is recommended that biological calibration is performed regularly and, if possible, that paired data from two (or more) different spirometers are compared.