Methacholine Challenge Testing: Comparison of FEV1 and IOS Parameters in Adult Asthma Patients

Asthma is a common chronic disease that is characterized by a history of variable respiratory symptoms and variable expiratory airflow limitation, and usually associated with airway hyperresponsiveness (GINA).

In general, FEV1 is used to test for airway hyperresponsiveness (AHR), and a 20% decrease after methacholine challenge is considered a positive test (Coates 2017; ERJ).

There is increasing recognition that the small airways are involved in 40-80% of patients with asthma, and the function of these airways is overlooked (the "silent zone") when measuring FEV1, which mainly reflects the function of the central airways (Cottini M, J Allergy Clin Immunol Pract 2019; Cottini M, 2022; Postma 2019 Lancet Respir Med).

Small airway dysfunction (SAD) can be assessed by IOS where pressure applied to the airways at a range of frequencies, and components of respiratory resistance and reactance are measured. Resistance at 5 Hz (R5) and 20 Hz (R20), respectively, represent total airway resistance and proximal airway resistance. The difference between these two values can be calculated (R5-R20). High R5-R20 and low reactance at 5 Hz (X5) indicate the presence of SAD.

A recent study showed that patients with a negative methacholine test measured by FEV1 may report asthma-like symptoms and may have a positive test when measured by IOS (Urbankowski; 2021). The question is whether these patients are a subgroup of patients with SAD (based on IOS at baseline) or are characterized in another way.