CPFT Practice Exam 2025 – Complete Test Prep Guide

For a methacholine challenge test, a minimum decrease in FEV1 (Forced Expiratory Volume in one second) of 20% from the baseline value is necessary to indicate significant bronchial hyperreactivity. This threshold is well-established in clinical practice for diagnosing conditions such as asthma. The rationale behind using a 20% decrease is tied to its sensitivity for identifying asthma and other reactive airway diseases. Such a decline is indicative of an exaggerated bronchoconstrictive response to methacholine, which is a potent bronchoconstrictor used in this testing process.

Lower percentages, such as 10% or 15%, may not yield adequate specificity for making a diagnosis of asthma, as they could lead to false positives, especially in individuals with other respiratory conditions. A 25% decrease, while indicative of severe bronchoconstriction, is rarely necessary for the diagnostic criteria and is more stringent than what is generally accepted in practice. Therefore, the chosen threshold of 20% provides a balanced and clinically useful benchmark for evaluating bronchial responsiveness.