Ideally all spirometry tests would be performed with acceptable quality and reproducibility.  Many tests do not meet these criteria however, and it is for this reason that all spirometry tests should be assessed for possible errors.

Testing errors can cause the FVC, FEV1 and FEV1/FVC ratio to be under- or over-estimated but their importance will depend on whether or not they can actually affect the interpretation.

The place to start is with a preliminary interpretation of the results:

A spirometry test should then be assessed for errors.  It important to keep in mind that there is no certainty that a given error will affect the interpretation, only a probability.  The level of this probability however, can be judged to some extent by the magnitude and direction of the error.  An interpretation needs to address both the apparent interpretation based on a straight assessment of the results but also needs to be shaded according to probabilities.

Normal spirometry:

Although spirometry results may nominally be within normal limits, an underestimated FVC or overestimated FEV1 will affect the FEV1/FVC ratio:

Obstruction:

For spirometry results that show airway obstruction, the question is whether the presence or amount of obstruction is overestimated because of testing errors.

A reduced FVC does not alter the relationship between a reduced FEV1/FVC ratio and airway obstruction, but when both the FVC and the FEV1/FVC ratio are reduced this raises of the question as to whether this is due to obstructive gas trapping or whether there are combined obstructive and restrictive defects.  Without lung volume measurements it is not possible to be definitive about this.  It’s also not possible to differentiate between these possible causes when the FVC is reduced due to suboptimal test quality.

When the FVC quality is good however:

• When gas trapping is the cause of a reduced FVC the FVC is usually not as reduced as the FEV1 and FEV1/FVC ratio .
• When there are combined obstructive and restrictive defects the FVC and FEV1 are usually reduced more than the FEV1/FVC .

So, for a nuanced interpretation of a reduced FVC and a reduced FEV1/FVC ratio:

Bronchodilator Response:

A post-bronchodilator increase in FEV1 and/or FVC that is ≥12% and 0.20 L is considered significant.  However the magnitude of this increase may be amplified by suboptimal test quality in either the baseline or post-bronchodilator spirometry.

Restriction:

Because the FVC can be reduced for a variety of reasons restriction can never be diagnosed solely on the results from spirometry.  Lung volume measurements are always required to verify the presence of a reduced lung capacity. When restriction is present there is often a correlation between a reduced FVC and a reduced TLC, but it is not exact.  For all these reasons the severity of any suspected restriction (mild, moderate, severe) should never be based on the FVC.

Flow-volume loop:

The flow-volume loop always provides some information about the quality of the FVC and FEV1 measurements, but the contour of the flow-volume can also be associated with certain airway disorders.  These disorders are relatively uncommon but when their presence is noted, this fact should be included in an interpretation.

Recommendation:

Spirometry results can be interpreted using a simple algorithm but the accuracy of this approach towards interpretation is only fair at best and can often be misleading.  It is possible to interpret spirometry with less than optimal quality with reasonable accuracy when the possible errors and their effect on the results are recognized.

 

PFT Interpretation by Richard Johnston is licensed under a Creative Commons Attribution-NonCommercial 4.0 International License