The validity of a rheumatoid arthritis medical records-based index of severity compared with the DAS28

In the present issue of Arthritis Research and Therapy, Sato and colleagues report on their effort to determine the convergent validity of the Rheumatoid Arthritis Medical Record-Based Index of Severity (RARBIS), a newly developed measure of severity in rheumatoid arthritis (RA) [1]. The authors claim moderate convergent validity with the disease activity score based on 28 joints (DAS28), and they propose the RARBIS as a tool to adjust for confounding by indication, a treatment bias that is introduced in observational studies where the severity of the disease determines the intensity of the treatment [2].In the present commentary we shall discuss two issues. The first addresses the question of when a particular measure can be called 'validated'. The second issue discussed argues whether a statistical correlation of the RARBIS with the DAS28 adds to the validity of the RARBIS as a measure of severity in RA.Validation of outcome assessments is a vague playing field where clear guidance is lacking. People hardly, if ever, speak the same language if they claim that a measure is validated, and definitions of subcategories of validation show wide overlap. A good example is the confusion about content validity versus construct validity. The former refers to the user's perception of the content of an instrument ('does it make sense?'), the latter pertains to the underlying construct (DAS28 and its association with inflammation in the joint), and very often both are used to describe the same thing.The Outcome Measures in Rheumatoid Arthritis Clinical Trials (OMERACT) initiative has provided a useful alternative to avoid such confusion [3]. The three-step OMERACT filter, which is OMERACT's framework for validation of outcome measures, prescribes that a measure should be truthful, discriminatory and feasible before it should be used.Truth here refers to scientific evidence that the RARBIS really reflects what it is intended to measure – the severity of RA. It requires some ass