Measurement properties of the Flu-Like Symptom Index from the Hepatitis Physical Symptom Severity Diary

PURPOSE: 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. METHODS: 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. RESULTS: 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. CONCLUSIONS: 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.