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 NICE NG245.

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 NICE NG245.

Be aware that inhaled corticosteroid treatment can normalize spirometry results, potentially masking airflow obstruction, so interpret results in the context of treatment status NICE NG245.

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 NICE NG245.

In children aged 5 to 16, a similar approach applies, with bronchodilator reversibility defined as a 12% or more increase in FEV1 from baseline NICE NG245.

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 NICE NG245.

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 & Theurer 2014. Spirometry is essential but should be part of a stepwise diagnostic approach including history, examination, and other tests Ayuk et al. 2017.