Disease activity refers to potentially reversible aspects of a disease. Measurement of disease activity in paediatric rheumatic diseases is a critical component of patient care and clinical research. Disease activity measures are developed systematically, often involving consensus methods. To be useful, a disease activity measure must be feasible, valid, and interpretable. There are several challenges in quantifying disease activity in paediatric rheumatology; namely, the conditions are multidimensional, the level of activity must be valuated in the context of treatment being received, there is no gold standard for disease activity, and it is often difficult to incorporate the patient’s perspective of their disease activity. To date, core sets of response variables are defined for juvenile idiopathic arthritis, juvenile systemic lupus erythematosus, and juvenile dermatomyositis, as well as definitions for improvement in response to therapy. Several specific absolute disease activity measures also exist for each condition. Further work is required to determine the optimal disease activity measures in paediatric rheumatology. 1. Introduction Measurement of a health state is the cornerstone of clinical practice and medical research. As stated in 1883 by Lord Kelvin, “when you can measure what you are speaking about, and express it in numbers, you know something about it; but when you cannot, your knowledge is of a meagre and unsatisfactory kind” [1]. When caring for children and youth with rheumatic diseases, standardized and validated definitions of disease activity, as well as of change in disease activity in response to treatment, are critical for making decisions in clinical practice as well as for conducting clinical research studies. Use of standardized measures can also facilitate comparisons between trials through meta-analyses. Over the past several years, an international collaborative effort has been made to develop and validate measures of disease activity, treatment response, inactive disease, and flare of disease in several paediatric rheumatic conditions [2]. These measures are numerous, and several new ones are emerging on an annual basis. This paper will discuss the theoretical basis of disease activity measures, methods for their development and validation, and an overview of available measures within paediatric rheumatology. 2. Definition of Disease Activity in Paediatric Rheumatic Diseases In order to measure disease activity, it is necessary to define it and to identify its measurable components. “Disease activity” may be defined as
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