Abstract
BackgroundThe Severe Asthma Questionnaire (SAQ) is a health related quality of life (HRQoL) questionnaire validated for use in severe asthma. It is scored using the mean value of 16 items (SAQ score) in addition to a single item global rating of HRQoL (SAQ-global). The aim was to validate clinically relevant subscales using exploratory factor analysis (EFA).MethodsThe SAQ was completed, along with measures of asthma control and EQ5D-5L by patients attending six UK severe asthma centres. Clinical data were included in the analysis. EFA using principal axis factoring and oblimin rotation was used to achieve simple structure of data.Results460 patients with severe asthma participated, 65% women, mean age 51 (16–83) years. A three factor solution achieved best fit and showed that the SAQ items formed three distinct but inter-correlated groups of items where items were grouped in a way that was consistent with item content. The three subscales were differentially associated with clinically relevant variables (lung function and mood). Males and females interpreted the question of night disturbance in different ways.ConclusionsThis paper provides a template for best practice in the use of EFA when validating HRQoL subscales. The SAQ can be scored as three subscales with content reflecting three different constructs people with severe asthma use when making judgements about their lives. The subscale ‘My Life’ assesses the impact of severe asthma on different life activities, ‘My Mind’ assesses the perceived emotional impact and ‘My Body’ the impact of extra-pulmonary symptoms and side effects.
Highlights
The Severe Asthma Questionnaire (SAQ) is a health related quality of life (HRQoL) questionnaire vali‐ dated for use in severe asthma
The respiratory conclusion is that exploratory factor analysis (EFA) provides evidence to interpret the items of the SAQ as clustering into three meaningful subscales, subscales that are linked to three different types of cause affecting severe asthma
The methodological conclusion is that best practice guidelines for EFA that should be adopted in preference to default values in statistical packages, and that construct validation of HRQoL questionnaire subscales requires more than just running an EFA and reporting how items load on factors
Summary
The Severe Asthma Questionnaire (SAQ) is a health related quality of life (HRQoL) questionnaire vali‐ dated for use in severe asthma. Validated health-related quality of life (HRQoL) questionnaires are used in clinical practice and research to evaluate the impact of disease and/or treatment responses. They consist typically of several items the responses to which are aggregated to form an overall HRQoL score. Lanario et al Health Qual Life Outcomes (2020) 18:336 questionnaires subscales are validated by both content validity and construct validity This is done by showing that items initially grouped on the basis of content have similar statistical properties, using either principal component analysis [4, 5] or principal factor analysis [6]. The advantage of construct validation is that it can show whether patients’ interpretation of the meaning of items is the same as that of the researchers
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