How should I interpret spirometry results in a patient suspected of having asthma?

To interpret spirometry results in a patient suspected of having asthma, first assess the forced expiratory volume in 1 second (FEV1) and the ratio of FEV1 to forced vital capacity (FVC). A reduced FEV1/FVC ratio suggests airflow obstruction, which is a hallmark of asthma but not diagnostic on its own ¹.

Next, perform bronchodilator reversibility testing by measuring FEV1 before and after administering a bronchodilator. A significant bronchodilator response supports an asthma diagnosis if the FEV1 increases by 12% or more and at least 200 ml from baseline, or if the increase is 10% or more of the predicted normal FEV1 ¹.

Be aware that inhaled corticosteroid treatment can normalize spirometry results, potentially masking airflow obstruction, so interpret results in the context of treatment status ¹.

If spirometry is not available or inconclusive, consider serial peak expiratory flow (PEF) measurements to detect variability, with a variability of 20% or more supporting asthma diagnosis ¹.

In children aged 5 to 16, a similar approach applies, with bronchodilator reversibility defined as a 12% or more increase in FEV1 from baseline ¹.

Interpretation should also consider clinical context and other objective tests such as fractional exhaled nitric oxide (FeNO) and blood eosinophil counts, which can support diagnosis when elevated ¹.

From a pulmonary function test interpretation perspective, it is important to recognize that spirometry results can fluctuate and that a single test may not be definitive; repeated testing and integration with clinical findings improve diagnostic accuracy (Johnson and Theurer, 2014). Spirometry is essential but should be part of a stepwise diagnostic approach including history, examination, and other tests (Ayuk et al., 2017).