Measurement of disease activity in systemic lupus erythematosus (SLE) is central to evaluating outcomes, differences among SLE patient groups, responses to a new drug proposed, and also for assessing disease longitudinally for observational and clinical trials. Several validated and updated instruments have been available since the early 1980s, but more recent studies gauging reliability and validity for classifying and monitoring groups of patients in the research setting are now available. Two cardinal features of SLE have challenged investigators refining these tools: first, the complex multisystem nature of this disease with fluctuating levels of disease activity, which may vary between patients and within the same patient over time; second, the absence of a “gold standard” for determining the psychometric properties of each proposed scale limits comparisons to expert opinion using a physician’s visual analog scale or by comparing one scale against other to assess performance across proposed instruments. However, these strategies do not eliminate bias based on personal experience, nor do they differentiate between different opinions on the relative importance of disease manifestations in different systems. Therefore, an experience-based evaluation may be subject to greater interrater variability than the use of the disease activity instrument itself. Furthermore, psychometric properties should be influenced by the length of the scale (number of items and scoring scale), number of patients included, or disease severity of patients under study.

Two main types of activity measures in SLE have been developed: global score systems (for example, the European Consensus Lupus Activity Measurements, Systemic Lupus Activity Measure [SLAM], and Systemic Lupus Erythematosus Disease Activity Index [SLEDAI]), which provide an overall measure of activity, and individual organ/system assessment scales that assess disease activity in single organs (such as the British Isles Lupus Assessment Group Index [BILAG]). The Systemic Lupus International Collaborating Clinics/American College of Rheumatology Damage Index score is a measure for chronic damage; it has been included due to its prognostic value in clinical and research basis. The SLEDAI, SLAM, and BILAG have performed in effective and reliable manners in studies; furthermore, they correlate with one another (1–3). The SLEDAI, Safety of Estrogens in Lupus Erythematosus National Assessment (SELENA)–SLEDAI, SLEDAI 2000 (4–7), and BILAG (8–10) have been successfully used in observational trials and case studies, although baseline disease activity index (DAI) scores were not always predictors of subsequent damage or other outcomes (11, 12). These DAIs were validated in the context of long-term observational trials studies and not in randomized clinical trials (RCTs)(1, 9, 10, 13–15). The few RCTs conducted have shown that improvement in DAI scores correlates with response rates, disease remission, and flare prevention; however, a threshold of clinically meaningful change has not been established (1, 13, 16, 17). Current work has focused on developing a responder index developed in collaboration with the Food and Drug Administration–defined response as improvement and/or no deterioration in patient-and