Supplementary Material for: Accuracy and Precision of Desktop Spirometers in General Practices

<i>Background:</i> Spirometry has become an essential tool for general practices to diagnose and monitor chronic airways diseases, but very little is known about the performance of the spirometry equipment that is being used in general practice settings. The use of invalid spirometry equipment may have consequences on disease diagnosis and management of patients. <i>Objectives:</i> To establish the accuracy and precision of desktop spirometers that are routinely used in general practices. <i>Methods:</i> We evaluated a random sample of 50 spirometers from Dutch general practices by testing them on a certified waveform generator using 8 standard American Thoracic Society waveforms to determine accuracy and precision. Details about the brand and type of spirometers, year of purchase, frequency of use, cleaning and calibration were inquired with a study-specific questionnaire. <i>Results:</i> 39 devices (80%) were turbine spirometers, 8 (16%) were pneumotachographs, and 1 (2%) was a volume displacement spirometer. Mean age of the spirometers was 4.3 (SD 3.7) years. Average deviation from the waveform generator reference values (accuracy) was 25 ml (95% confidence interval 12–39 ml) for FEV₁ and 27 ml (10–45 ml) for FVC, but some devices showed substantial deviations. FEV₁ deviations were larger for pneumotachographs than for turbine spirometers (p &lt; 0.0031), but FVC deviations did not differ between the two types of spirometers. In the subset of turbine spirometers, no association between age and device performance was observed. <i>Conclusions:</i> On average, desktop spirometers in general practices slightly overestimated FEV₁ and FVC values, but some devices showed substantial deviations. General practices should pay more attention to the calibration of their spirometer.