Validation of the systemic lupus erythematosus activity questionnaire in a large observational cohort

Abstract

Objective: To examine the reliability, construct validity, and responsiveness of the Systemic Lupus Erythematosus Activity Questionnaire (SLAQ) in a large observational cohort of persons with systemic lupus erythematosus (SLE).

Methods: We evaluated the reliability of the SLAQ using Cronbach's alpha and principal factor analysis and ascertained construct validity by studying the association of the SLAQ with other clinically relevant, validated patient assessments of health. We estimated responsiveness by calculating standardized response means and analyzing the association of changes in SLAQ scores with changes in other patient assessments of health.

Results: The SLAQ had excellent reliability, as reflected by Cronbach's alpha (0.87) and principal factor analysis (one factor accounted for 92% of the variance). SLAQ scores were strongly correlated with other health indices, including the Short Form 12 Physical Component Summary and the Short Form 36 Physical Functioning subscale. Scores were significantly higher for respondents reporting a flare, more disease activity, hospitalization in the last year, concurrent use of immunosuppressive medication, and work disability. The SLAQ demonstrated a small to moderate degree of responsiveness; standardized response means were 0.66 and -0.37 for those reporting clinical worsening and improvement, respectively. Across a range of other patient assessments of disease status, the SLAQ had a response in the direction predicted by these other measures.

Conclusion: The SLAQ demonstrates adequate reliability, construct validity, and responsiveness in our large, community-based cohort and appears to represent a promising tool for studies of SLE outside the clinical setting.

Figures

Figure 1

Distribution of Systemic Lupus Erythematosus Activity Questionnaire (SLAQ) scores in 830 participants in the Lupus Outcomes Study. Shapiro-Wilk test of normality. W = 0.97 (P < 0.0001).

Figure 2

Correlation between Systemic Lupus Erythematosus Activity Questionnaire (SLAQ) scores in years 2 and 3 of the Lupus Outcomes Study. r = 0.81 for correlation between consecutive SLAQ scores, using Spearman's correlation coefficient. Weighted regression line reflects lowess smoother.