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Is spirometry necessary to diagnose and control asthma?

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Release date: August 1, 2017
Expiration date: July 31, 2018
Estimated time of completion: 1 hour

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A 41-year-old woman presented with intermittent shortness of breath that worsened with exposure to cold air and cigarette smoke. She said her symptoms got better when she used albuterol, which had been prescribed after an emergency department visit during a worsening episode.

The patient was severely obese (body mass index 48 kg/m2) and had bilateral expiratory wheezes but no other significant findings. Based on the clinical presentation, we suspected she had asthma.

To establish the diagnosis and assess the severity of her condition, we questioned her further about her symptoms, and this information increased our suspicion of asthma. Is spirometry also indicated?

SPIROMETRY’S ROLE IN DIAGNOSING ASTHMA

Asthma is a chronic inflammatory condition of the airways characterized by recurrent or persistent symptoms with evidence of variable airflow obstruction or hyperresponsiveness to certain stimuli.1 The clinical diagnosis is based on episodic symptoms of chest tightness, wheezing, shortness of breath, or cough, but we cannot reliably diagnose asthma based on symptoms alone.

Spirometry provides an objective measure of obstruction, which adds to the reliability of the diagnosis. Therefore, it should be done in all patients in whom asthma is suspected.

Spirometry provides another diagnostic measure by quantifying whether airway obstruction reverses after the patient is given a dose of a bronchodilator. Although the exact criteria for reversibility of obstruction are unclear, the American Thoracic Society defines it as an increase in the forced expiratory volume in 1 second (FEV1) of 12% or more from baseline and an absolute increase of 200 mL or more. It can also be an increase of more than 200 mL in the forced vital capacity (FVC).2,3

Spirometry can also be used to evaluate or rule out other causes of chronic shortness of breath and common asthma mimics.

Failure to perform spirometry can result in a false diagnosis of asthma in patients who do not have it, or in a missed diagnosis in patients who do.4,5 Either situation often leads to inappropriate use of medications, exposure of patients to side effects, delays in appropriate diagnosis, and ongoing morbidity.

Despite the evidence in its favor, spirometry is underused. In a 2012 Canadian study, only 42.7% of 465,866 patients with newly diagnosed asthma had any spirometry testing performed within 1 year before or 2.5 years after the diagnosis.6 Similarly, in a 2015 US study, only 47.6% of 134,208 patients had spirometry performed within 1 year of diagnosis.7 Interestingly, this study found that the use of spirometry actually decreased after publication of guidelines from the National Asthma Education and Prevention Program1 that recommended spirometry.

CASE CONTINUED

The patient's pulmonary function test results
We discussed the benefits of spirometry with our patient, who agreed to undergo the test. Her results are shown in Table 1.

Her baseline values were normal; her FEV1/FVC ratio was 73.67% (lower limit of normal 72.62%) and thus was not significant for airway obstruction. However, after 4 puffs of an inhaled short-acting beta agonist, her FEV1 increased by 15% from baseline (from 1.98 L/second before to 2.25 L/second after), a clinically significant response (defined as ≥ 12% from baseline and an absolute increase of at least 200 mL1–3). Had we not included bronchodilator testing, given the absence of underlying baseline obstruction, her shortness of breath could have been attributed to other causes, resulting in a missed asthma diagnosis.

Nevertheless, postbronchodilator measurements should not be performed in all patients with normal baseline results unless asthma is strongly suspected on clinical grounds. In one study, only 3% of 1,394 patients with normal baseline results showed improvement with a bronchodilator.8 In this patient population, bronchodilator testing would add both time and cost with little benefit.

Our patient’s reversibility of obstruction helped confirm the diagnosis of asthma. Absence of reversibility, however, does not rule out asthma, because spirometry results, like clinical symptoms of asthma, can vary. If clinical suspicion remains high and spirometry does not show clinically significant reversibility, then bronchoprovocation testing (most commonly with methacholine) could be done.

Although a positive methacholine challenge test can help identify asthma in patients with atypical symptoms or normal baseline test results, conditions other than asthma can also cause positive results. The sensitivity of methacholine challenge has been reported to be as high as 96%, while its specificity averages less than 80%.9 Given its high negative predictive value, the test can help rule out asthma, as negative results are rarely falsely negative.

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