The construction and validation of the Severe Asthma Questionnaire (SAQ).
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BACKGROUND: The USA's Food and Drug Administration's procedure for scale validation requires a documented stepwise process of qualitative and quantitative data. The aim of this paper is to provide the final quantitative validating data. METHODS: The severe asthma questionnaire (SAQ), asthma control test (ACT), MiniAQLQ, and EQ-5D-5L were completed by 160 patients attending a severe asthma clinic; 51 patients completed the SAQ on two occasions for test-retest reliability analysis. The SAQ produces two scores, an SAQ score based on the average of 16 items and a SAQ-global score from a single 100-point global quality of life scale. RESULTS: Construct validity was demonstrated through factor analysis of the 16 items, convergent validity by correlations of > 0.6 between the SAQ, SAQ-global and other questionnaires, and discriminant validity by the ability of the SAQ and SAQ-global to distinguish between different treatment levels. Test-retest reliability (intra-class correlation) was 0.93 for the SAQ and 0.93 for the SAQ-global, and the alpha coefficient for the SAQ was 0.93. CONCLUSIONS: The SAQ was developed using recommended qualitative and quantitative procedures for scale development, and can be used to gain insight into patients' perceptions of the impact of severe asthma and its treatment on their lives.
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