INTRODUCTION
Confirming the diagnosis of asthma in children by demonstrating variable expiratory flow limitation, expressed by a significant bronchodilator response (SBDR) or the presence of bronchial hyperresponsiveness (BHR) to methacholine or exercise, avoids over and underdiagnosis(1–4). The finding of an SBDR in spirometry, defined as a bronchodilator change ≥ 12% in FEV1, confirms the diagnosis of asthma in a child with clinical suspicion, without the need for other studies (4).
The methacholine test (MT) measures BHR directly and has high sensitivity and low specificity for the diagnosis of asthma, which increases with lower PC20 (methacholine concentration that produces a 20% drop in FEV1) (2). A PC20 between 4 and 8 mg/dl is considered borderline, and below 4 mg/dl, the lower the PC20, the greater the possibility of diagnosing asthma in patients with clinical suspicion of this disease (5,6).
The exercise test (ET) is an indirect bronchial provocation test, which has low sensitivity and high specificity for the diagnosis of asthma. A positive ET is considered to be a drop in FEV1 ≥ 10% post-exercise (7).
Although SBDR in spirometry, a PC20 ≤ 2 mg/dl and a positive ET measure different pathophysiological aspects, all three indicate with high probability the presence of asthma (1,2,7).
In recent years, the value of FEF25-75 and forced expiratory flows have been questioned in the interpretation of spirometry, because flow values depend on the volume at which they are measured and have high variability (8).
Spirometry has low sensitivity for the diagnosis of asthma(9). Frequently, spirometry in children with suspected asthma is often normal, without SBDR in FEV1, but with SBDR in FEF25-75 (>30%), without a >5% change in FVC (which indicates that the curves pre and post bronchodilator were measured at similar volumes). This result, in a child with clinical suspicion of asthma, raises doubts in its interpretation, due to the publications that suggest that the FEF25-75 does not contribute to clinical decisions(8,10). Despite this, this parameter is still frequently used (11).
We postulate that if the only SBDR in FEF25-75 in the spirometry, of children with suspected asthma, indicates a reversible obstruction of the airway that allows to certify the diagnosis, it will be more frequently associated with a BHR in the MT and/or ET than , in children with normal spirometry, without SBDR in FEF25-75.
Due to the fact that the evidence is not conclusive regarding the role of FEF25-75 for the evaluation of spirometry, this study aims to relate SBDR to FEF25-75, when FEV1 and FEV1/FVC are normal, with MT and/ET, in children aged 5 to 15 years with clinical suspicion of asthma.