If PFTs are done on different machines or different places, are the comparisons valid?

Pulmonary function test results might differ slightly when performed at different facilities due to staff engagement and coaching during respiratory maneuvers and machine calibration. Another important consideration is there may be differences due to use of body box plethysmography versus helium dilution for lung volume determination.

Comparisons of PFT between equipment and locations are valid if they are consistent with the underlying condition and the caveats mentioned by Dr. @Dr. First Last are taken into account. Lung volume measured by nitrogen washout would have the same issue as helium dilution, as both would miss very poorly ventilated or non-ventilated lung regions (eg, emphysematous bullae). Attention should focus on comparing absolute measures, not percent predicted, between PFT locations, as predicted equations may differ between locations, especially for the DLCO.