The patient experience with fatigue and content validity of a measure to assess fatigue severity: qualitative research in patients with ankylosing spondylitis (AS)

BACKGROUND Ankylosing spondylitis (AS) is an autoimmune disorder characterized by inflammation of the spine and large joints. Fatigue is a common symptom that many AS patients find significantly impacts their health-related quality of life. The Worst Fatigue - Numeric Rating Scale (WF-NRS) assesses the severity of this symptom during the previous 24-hour period. The objective of this study was to perform qualitative research to support the development and content validity of the WF-NRS. METHODS Patients with AS were recruited from clinical sites in the U.S. for a qualitative study which first entailed concept elicitation interviews to gain understanding of the patients' experience with AS and fatigue. Subsequently, cognitive debriefing interviews were undertaken to assess the understandability, clarity, and appropriateness from the patient's perspective, of the content of a measure of fatigue severity. RESULTS Thirteen patients with AS participated in concept elicitation interviews and cognitive debriefing of the Brief Fatigue Inventory (BFI) fatigue severity subscale. The WF-NRS was developed from the worst fatigue item of the BFI as patients generally reported it to be understandable and covered an important concept, the completion instructions were modified, but the response scale remained as it was familiar and readily completed, and the recall period was appropriate. CONCLUSIONS Patient responses resulted in the development of and supported the content validity of the WF-NRS. Further quantitative evaluation of the WF-NRS is warranted in order to assess its psychometric properties and confirm its usefulness as a clinical trial tool.