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 dil...

Comparisons of PFT between equipment and locations are valid if they are consistent with the underlying condition and the caveats mentioned by Dr. @Chen are taken into account. Lung volume measured by nitrogen washout would have the same issue as helium dilution, as both would miss very poorly venti...