Validation of Subscales of the Severe Asthma Questionnaire (SAQ) Using Exploratory Factor Analysis (EFA)
Total Page:16
File Type:pdf, Size:1020Kb
Lanario, J. W., Hyland, M. E., Menzies-Gow, A., Mansur, A. H., Dodd, J. W., Fowler, S. J., Jones, R. C., & Masoli, M. (2020). Validation of subscales of the Severe Asthma Questionnaire (SAQ) using exploratory factor analysis (EFA). Health and Quality of Life Outcomes, 18(1), [336]. https://doi.org/10.1186/s12955-020-01593-9 Publisher's PDF, also known as Version of record License (if available): CC BY Link to published version (if available): 10.1186/s12955-020-01593-9 Link to publication record in Explore Bristol Research PDF-document This is the final published version of the article (version of record). It first appeared online via BMC at https://doi.org/10.1186/s12955-020-01593-9 .Please refer to any applicable terms of use of the publisher. University of Bristol - Explore Bristol Research General rights This document is made available in accordance with publisher policies. Please cite only the published version using the reference above. Full terms of use are available: http://www.bristol.ac.uk/red/research-policy/pure/user-guides/ebr-terms/ Lanario et al. Health Qual Life Outcomes (2020) 18:336 https://doi.org/10.1186/s12955-020-01593-9 RESEARCH Open Access Validation of subscales of the Severe Asthma Questionnaire (SAQ) using exploratory factor analysis (EFA) Joseph W. Lanario1, Michael E. Hyland1,2* , Andrew Menzies‑Gow3, Adel H. Mansur4, James W. Dodd5, Stephen J. Fowler6, Rupert C. Jones1 and Matthew Masoli7 Abstract Background: The Severe Asthma Questionnaire (SAQ) is a health related quality of life (HRQoL) questionnaire vali‑ dated 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). Methods: The 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. Results: 460 patients with severe asthma participated, 65% women, mean age 51 (16–83) years. A three factor solution achieved best ft 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 diferentially associated with clinically relevant variables (lung function and mood). Males and females interpreted the question of night disturbance in diferent ways. Conclusions: This 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 refecting three diferent constructs people with severe asthma use when making judgements about their lives. The subscale ‘My Life’ assesses the impact of severe asthma on diferent life activities, ‘My Mind’ assesses the perceived emotional impact and ‘My Body’ the impact of extra‑pul‑ monary symptoms and side efects. Keywords: Asthma, Quality of life, Measurement, Exploratory factor analysis Background items as subscales provide information that can be useful Validated health-related quality of life (HRQoL) ques- in clinical trials and clinical practice. tionnaires are used in clinical practice and research Guidelines for validating questionnaires recommend a to evaluate the impact of disease and/or treatment two stage process where content validity is followed by responses. Tey consist typically of several items the construct validity [1], but these recommendations were responses to which are aggregated to form an overall published after the publication of four asthma specifc HRQoL score. Subscales can be formed from groups of HRQoL questionnaires. Validation of the asthma qual- ity of life questionnaire’s (AQLQ) subscales is based only *Correspondence: [email protected] on content validity [2, 3] as the subscales are formed by 1 University of Plymouth, Plymouth, UK grouping items on the basis of an examination of content Full list of author information is available at the end of the article alone. However, in three other asthma specifc HRQoL © The Author(s) 2020. Open Access This article is licensed under a Creative Commons Attribution 4.0 International License, which permits use, sharing, adaptation, distribution and reproduction in any medium or format, as long as you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons licence, and indicate if changes were made. The images or other third party material in this article are included in the article’s Creative Commons licence, unless indicated otherwise in a credit line to the material. If material is not included in the article’s Creative Commons licence and your intended use is not permitted by statutory regulation or exceeds the permitted use, you will need to obtain permission directly from the copyright holder. To view a copy of this licence, visit http://creat iveco mmons .org/licen ses/by/4.0/. The Creative Commons Public Domain Dedication waiver (http://creat iveco mmons .org/publi cdoma in/zero/1.0/) applies to the data made available in this article, unless otherwise stated in a credit line to the data. Lanario et al. Health Qual Life Outcomes (2020) 18:336 Page 2 of 10 questionnaires subscales are validated by both content detail about the patient’s experience, and this greater validity and construct validity. Tis is done by show- detail facilitates communication with the patient as well ing that items initially grouped on the basis of content providing more nuanced information about HRQoL have similar statistical properties, using either principal change in clinical trials. Te aim of this study is to pro- component analysis [4, 5] or principal factor analysis [6]. vide construct validation of the subscales of the 16 items Te advantage of construct validation is that it can show of the SAQ by showing that the diferent subscales are whether patients’ interpretation of the meaning of items associated with diferent constructs. In this study, con- is the same as that of the researchers. struct validity is achieved by providing evidence that (a) Te number of subscales in existing asthma specifc the 16 items fall into statistically distinct clusters; (b) that HRQoL varies between three [5], four [2], fve [6] and items in the statistically formed subscales are consistent six [4], but despite this variation, there is consensus that with classifcation based on content; (c) that the subscales activity restriction and mood should be measured in dif- formed from these clusters have diferent associations ferent subscales. Activity-related items are assessed in with clinically relevant variables and therefore provide subscales labelled ‘activity limitation’ [2], ‘activity’ [5], additional information compared to the overall scores. and ‘activities’ and ‘avoidance’ [6]. Items relating to the emotional impact of asthma are assessed in subscales Methods labelled ‘emotional function’ [2], ‘mood’ [4] and ‘distress’ Design and ‘preoccupation’ [6]. Tis was a cross-sectional study with questionnaire and Te Severe Asthma Questionnaire (SAQ) is the only clinic data each collected at one time point. validated HRQoL questionnaire for specifcally severe asthma [7]. It comprises 16 inter-correlated items meas- Participants uring the impact of disease and medical interventions, Patients aged 16 years of age and diagnosed with severe and the mean of those items forms the SAQ score [8]. In ≥ asthma as defned by the ERS/ATS guidelines were addition, the questionnaire has a single question meas- invited to participate [12]. Participants were recruited uring the impact of disease and its treatment on the from six UK severe asthma centres and were excluded if patient’s overall perception of quality of life, the SAQ- they were diagnosed with another condition that signif- global score. Content validity for the questionnaire was cantly contributed to their respiratory health, e.g. lung established in two qualitative studies [7, 9], and construct cancer, heart disease or chronic obstructive pulmonary validity for the SAQ score demonstrated by factor analy- disease. sis. Te content of the16 items fall into three categories. Items 1–7 ask patients to rate the impact of their asthma Patient reported outcome measures and its treatment on seven diferent types of life activ- Severe Asthma Questionnaire (SAQ) ity, and have content consistent with items in the activ- ity subscales of earlier questionnaires. Items 8–11 ask Te SAQ consists of 16-items scored from 1 to 7, with a patients about various aspect of mood and have content higher score indicating better quality of life. Te mean consistent with that in the emotional subscales of earlier of the 16 items is calculated to provide the SAQ score. questionnaires. Items 12–16 assess the impact on life of Te SAQ also contains a separate, Borg-type scale rang- extra-pulmonary symptoms and side efects. Tese last ing from 0–100 and based on the Global Quality of Life fve items measure quality of life defcits that are typically Questionnaire [13] which provides the SAQ-global score found only in severe asthma where qualitative research [8]. shows them to play a major role [7, 9–11]. Tey are rare in mild and moderate asthma and have limited represen- Asthma Control Test (ACT) tation in earlier asthma specifc HRQoL questionnaires Te ACT consists of fve asthma symptom and medica- [9]. Content derived subscales for the SAQ and one con- tion use items, which are totalled to provide an indication sistent with earlier scales would therefore be based on of asthma control. Te sum of the fve items, scale-5, is three subscales (a) impact on life’s activities (b) impact calculated to give the ACT score, with a higher number on emotional well-being, and (c) impact of extra-pulmo- indicating better asthma control [14].