Spirometry in primary care practices

Matthew Stanbrook and Alan Kaplan recently discussed the merits and challenges of using office spirometry in primary care practices.1 Their theme was driven to a large extent by a CMAJ research paper by Aaron and colleagues,2 which reported that up to one-third of Canadian patients diagnosed as having asthma were misdiagnosed. The diagnosis of asthma in this study was confirmed in 16% of patients by postbronchodilator spirometry and in 72% of patients by bronchial challenge testing. Although I am able to appreciate Stanbrook and Kaplan's enthusiastic stand on this subject, I think they went too far in saying that “physicians who do not use spirometry for their asthma patients should not be managing asthma.”

Given the results of Aaron and colleagues, one is tempted to advise busy family physicians to treat suspected asthma initially on the basis of the patient's history and physical findings and refer the patient for methacholine challenge testing. Unfortunately, such a recommendation does not take into account some of the limitations of the study, including the authors' strategy to perform only a single reversibility (bronchodilator challenge) test at the first study visit.2 It is therefore not known whether more patients would have demonstrated an improvement in forced expiratory volume in 1 second (FEV₁) by at least 15% and at least 200 mL after salbutamol challenge during the period in which their asthma medications were reduced. Such information would have helped us to understand more fully the diagnostic potential of this strategy. Furthermore, the study by Aaron and colleagues included patients with an established diagnosis of asthma so their findings cannot be generalized to patients who typically present to their family physician for the first time with chest-related complaints.

I respectfully disagree with Stanbrook and Kaplan that asthma and chronic obstructive pulmonary disease can be reliably distinguished with objective testing. The study they cite to support this notion used a reduction of the ratio of FEV₁ to forced vital capacity (FEV₁/FVC) by less than 70% to establish a diagnosis of chronic obstructive pulmonary disease.3 Although the postbronchodilator FEV₁/FVC ratio provides important spirometric diagnostic information, it does not by itself confirm a specific clinical diagnosis because a reduced FEV₁/FVC ratio after bronchodilator administration can be observed in both patients with asthma and those with chronic obstructive pulmonary disease. To suggest otherwise may oversimplify the role of spirometry in clinical decision-making. The limitations of using reversibility to differentiate asthma and chronic obstructive pulmonary disease are known4 and were recently confirmed by the work of Tashkin and colleagues5 with a large cohort of patients with chronic obstructive pulmonary disease. Although a normalization of the postbronchodilator FEV₁/FVC ratio rules out chronic obstructive pulmonary disease, the data of Aaron and colleagues2 suggest that spirometry provides a much lower yield than methacholine challenge testing in terms of asthma diagnosis in treated patients.

My intuitive enthusiasm for spirometry use is dampened by the reality that there is a lack of strong evidence on how we might best use this simple and important test. The question of whether appropriate quality control of spirometry can be ensured at the primary care level requires further study. Until such data are available, we should beckon family physicians who are not yet regularly using spirometry with a more tempered call to task.