Measurement properties of the Flu-Like Symptom Index from the Hepatitis Physical Symptom Severity Diary
Chronic Hepatitis C (CHC) Virus infection is a serious health issue in the US. Standard treatment involves peginterferon alpha and ribavirin, often associated with adverse side effects including flu-like symptoms. These adverse effects are common reasons for the discontinuation of treatment and therefore represent a major obstacle in the effective treatment of CHC. The Hepatitis Physical Symptom Severity Diary, a newly developed patient-reported outcome measure for assessing physical symptoms in CHC patients, was recently developed. It contains four questions addressing flu-like symptoms [the Flu-Like Symptom Index (FLSI)]. Measurement properties of the FLSI in CHC patients were assessed using data from two randomized clinical trials. Exploratory factor analysis using data from baseline and the last visit while on treatment supported a single-factor solution for the FLSI. Internal reliability and test–retest reliability are acceptable (Cronbach’s alpha range 0.73–0.81; intraclass correlation coefficient range 0.85–0.97), and correspondence to several similar constructs was acceptable. The FLSI score was higher among those with investigator-reported flu-like symptoms (mean = 4.1) versus those without (1.4), although not statistically significant (p = 0.12). Responsiveness of the FLSI was moderate, as measured by standardized effect sizes and response means, and the minimum important difference (MID) was estimated at 2.5–3.0 points. While additional research should be conducted to evaluate validity with more closely related constructs and to utilize anchor-based methods for estimating the MID, data suggest that the FLSI has acceptable measurement properties and can be an effective tool in assessing flu-like symptoms in CHC patients.