• Spondyloarthritis
• Psoriatic Arthritis
• Outcomes research
• Epidemiology

Measuring is an essential part of medicine, both in research as well as in clinical practice. Clinical reasoning itself is a mental exercise based on a succession of tests and assessments (signs and symptoms and their quantification, impact on the patient, findings from the physical examination, results from laboratory and imaging investigations, among others) that result in making a diagnosis and making therapeutic decisions.1 Lord Kelvin’s (1824–1907) quote from 1883 extracted from a lecture on ‘Electrical Units of Measurement’ also applies to modern medicine: ‘When you can measure what you are speaking about, and express it in numbers, you know something about it; but when you cannot measure it, when you cannot express it in numbers, your knowledge is of a meagre and unsatisfactory kind; it may be the beginning of knowledge, but you have scarcely, in your thoughts, advanced to the stage of science, whatever the matter may be’.

The use of general or disease-specific measuring tools has become routine in rheumatology. In addition to the critical need of these tools for drug development, they are also valuable for patient management. These tools vary from simple visual analogue scales or numerical rating scales, for example, for pain or global assessment of disease activity, to patient-reported outcomes based on multiple questions, such as the Health Assessment Questionnaire (HAQ) or the Bath Ankylosing Spondylitis Functional Index, to composite indices such as the 28-joint count Disease Activity Score (DAS28) and the Ankylosing Spondylitis Disease Activity Score, which combine patient-reported and/or physician-reported variables with the laboratory measurement of acute phase reactants (C reactive protein (CRP) or erythrocyte sedimentation rate). Composite measures can be particularly helpful and are more likely to give more complete and reliable information about a certain health outcome such as disease activity.

Cut-offs increase the interpretability of a measurement …