Background The assessment of small airways obstruction (SAO) using spirometry is practiced in population-based studies. However, it is not clear what are the most used parameters and cut-offs to define abnormal results. Methods We searched three databases (Medline, Web of Science, Google Scholar) for population-based studies, published by 1 May 2021, that used spirometry parameters to identify SAO and/or provided criteria for defining SAO. We systematically reviewed these studies and summarised evidence to determine the most widely used spirometry parameter and criteria for defining SAO. In addition, we extracted prevalence estimates and identified associated risk factors. To estimate a pooled prevalence of SAO, we conducted a meta-analysis and explored heterogeneity across studies using meta regression. Results Twenty-five studies used spirometry to identify SAO. The most widely utilised parameter (15 studies) was FEF25–75, either alone or in combination with other measurements. Ten studies provided criteria for the definition of SAO, of which percent predicted cut-offs were the most common (5 studies). However, there was no agreement on which cut-off value to use. Prevalence of SAO ranged from 7.5% to 45.9%. As a result of high heterogeneity across studies (I2 = 99.3%), explained by choice of spirometry parameter and WHO region, we do not present a pooled prevalence estimate. Conclusion There is a lack of consensus regarding the best spirometry parameter or defining criteria for identification of SAO. The value of continuing to measure SAO using spirometry is unclear without further research using large longitudinal data. PROSPERO registration number CRD42021250206

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IntroductionIn recent years, there has been increasing interest in the use of spirometry for the assessment of small airways obstruction (SAO) driven by the idea that these changes occur prior to development of established obstructive lung disease. Maximal mid-expiratory and distal flow rates have been widely used despite a lack of agreement regarding parameter selection or definition of an abnormal result. We aim to provide evidence from population-based studies, describing the different parameters, definitions of normal range and the resulting impact on prevalence estimates for SAO. Summarising this evidence is important to inform development of future studies in this area.Methods and analysisA systematic review of population-based studies will be conducted. MEDLINE, Web of Science and Google Scholar will be searched from database inception to May 2021. Primary outcomes will include the spirometry parameter used to define SAO, and the definition of an abnormal result. The Preferred Reporting Items for Systematic Reviews and Meta-Analyses guidelines will be followed for study selection. Study methods will be assessed using the Newcastle-Ottawa scale and the Grading of Recommendations Assessment, Development and Evaluation working group methodology. Narrative synthesis will be conducted for all included studies. Meta-analysis will also be conducted for prevalence estimates and associated risk factors where data quality and availability allow. Random effects models will be used to conduct the meta-analysis and I2 statistics will be used to assess heterogeneity across studies. Where appropriate subgroup analysis will be conducted to explore heterogeneity.Ethics and disseminationThere is no requirement for ethical approval for this project. Findings will be disseminated via peer-reviewed publications and other formats, for example, conferences, congresses or symposia.PROSPERO registration numberCRD42021250206.

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Objectives: The impulse oscillometry system (IOS) provides an alternative method of lung function testing for patients in whom forced expiratory manoeuvres are contraindicated, such as those with inherited vascular connective tissue disorders. Here we examine the role of IOS in the diagnosis and monitoring of asthma in such patients through a clinical case series and literature review. Methods: The clinical case series comprised of data from 12 patients with inherited connective tissue disorders representing 32 clinical encounters. Of these, 11 encounters were for asthma diagnosis and 21 were for asthma monitoring. Symptoms, exhaled nitric oxide (FeNO) and IOS were assessed at each encounter. Results: In the clinical case series, 5 of 6 patients with likely asthma (as determined by physician review and exhaled nitric oxide testing) had abnormal IOS parameters compared with 0 of 5 of those with unlikely asthma. In the monitoring group, 11 encounters resulted in treatment escalation (demonstrating suboptimal control), and 8 resulted in no change to treatment (good control). Six of 11 of those with suboptimal control had abnormalities in ≥3 IOS parameters, with R5 and R5-20 most frequently affected. Only 1 of 8 of those with good control had abnormalities in ≥3 IOS parameters. Conclusions: IOS can be used as an alternative to conventional lung function testing to support the diagnosis and monitoring of asthma when forced expiratory manoeuvres are contraindicated. Larger studies are required to establish severity and treatment escalation thresholds and provide clearer comparisons with spirometry values.