After observing large differences in FEV 1 performed with correct technic with different spirometers in selected patients, we hypothesizedthat this was due to a high dynamic back pressure in one spirometer. Methods:Ten patients performed spirometry on a Morgan Spiroflow,a Vitalograph and a MEC spirometer.The spirometry was performed by two staff members specially trained .The Morgan Spiroflow was a modified model allowing continuous sampling of volume,pressure,temperature, time and flow. The spirometers were calibrated with a 3 L syringe 1 ,a decompression calibrator 2 and a waveform generator. 3 Results: When the Vitalograph and MEC spirometerswere calibrated at three flow levels with the decompression calibrator and with 6 different ATS waveforms, the differences in FEV 1 were below ±3.0% of the Morgan reference results. In 10 patients the differencesin FEV 1 were on average 4.4% and in half of the patients the differences were >5.0% with a maximum of 12%. Conclusion: Calibrators traceable to international standards are a prerequisite for valid calibration of spirometers. If spirometers and measuring technic are complying with the ATS/ERS standard the same spirometric results should be expected even with different spirometers. In our group of patients selected on basis of previous observed large differences in FEV 1 , it was possible to reproduce the differences. Our results indicate that there might be an interaction between the patients forced expiration technic and the spirometers since the spirometers showed highly comparable results when traceable flow calibration was applied. Furthermore large non-systematic differences were observed in FEV 1 in selected patients.
Madsen et al. (Mon,) studied this question.