An index is a composite measure, created by combining two or more distinct measures (or instruments) into one. The distinct instruments should address the same concept or construct. Why would we want to create an index? In clinical research, the combined information from different sources can reduce complexity, easing interpretation. In clinical care, study results can be made interpretable on the individual level. Also, an index can be applied to guide treatment decisions. Many different types of indices exist (diagnostic, prognostic, and evaluative), but here we are concerned with indices that are to be used in trials, thus evaluative.
The word “validated” is (ab)used in many different ways, and thus is meaningless without context. Within the Outcome Measures in Rheumatology (OMERACT) initiative, we prefer the use of the word “applicable” to label instruments which have passed the OMERACT Filter  of Truth (“does the instrument measure what it's supposed to?”), Discrimination (“does it distinguish between situations of interest?”) and Feasibility (“is it acceptable in terms of easy of use, cost, and interpretability?”). All outcome measurement instruments used in trials must be applicable, and this goes for indices, too.
Once the applicability of an index is established, it becomes acceptable for use. Whether it IS actually used, depends on the advantages it brings. The ACR20 (American College of Rheumatology) RA response index, for example, was immediately implemented (especially by industry) because it proved to be more powerful than its components in discriminating between active and placebo, and was acceptable for regulatory authorities. The DAS (Disease Activity Score) and DAS28 became popular after authorities in several European countries adopted a DAS value as threshold for reimbursement. More recently, the SDAI (Simple Disease Activity Index) is gaining in popularity because it is better than the DAS at defining remission, and has become part of the ACR/EULAR remission definition.
OMERACT has recently developed OMERACT Filter 2.0, a new conceptual framework for core areas of measurement, and an explicit process to develop core outcome measurement sets for clinical trials. Briefly, core domains are chosen from core areas (death, life impact, resource use and pathophysiological manifestations), and then instruments are chosen to represent these domains, based on research of their applicability. Such instruments can include indices. Choices are made in data-driven consensus processes that involve all stakeholders, notably patients.
In conclusion, getting an index accepted for use in clinical trials means proving its applicability, documenting its advantages over existing measures, and if possible, getting it into a core set.
1. Boers M, et al. The OMERACT Filter for outcome measures in rheumatology. J Rheumatol 1998;25:198-9.
2. Boers M, et al. Developing Core Outcome Measurement Sets for Clinical Trials: OMERACT Filter 2.0. J Clin Epidemiol 2014. DOI 10.1016/j.jclinepi.2013.11.013.
Disclosure of Interest None declared
Statistics from Altmetric.com
If you wish to reuse any or all of this article please use the link below which will take you to the Copyright Clearance Center’s RightsLink service. You will be able to get a quick price and instant permission to reuse the content in many different ways.