Protocol for the development of a core outcome set for pelvic girdle pain, including methods for measuring the outcomes: the PGP-COS study.

Background: Pelvic Girdle Pain (PGP) is an important cause of disability and economic cost worldwide. There is a need for effective preventative and management strategies. Emerging studies measure a variety of outcomes rendering synthesis and translation to clinical practice difficult. A Core Outcome Set (COS) can address this problem by ensuring that data are relevant, useful and usable for making well-informed healthcare choices. The aim of this study is to develop a consensus-based PGP-COS, including agreement on methods (e.g. instruments) for measuring the construct outcomes in the COS for use in research and clinical practice. Furthermore, as there is uncertainty as to whether incorporating stakeholder interviews in addition to conducting a systematic review to determine an initial list of outcomes for the Delphi survey, or, whether using different rating scales in a Delphi survey impacts on the final COS, we propose to embed two methodological studies within the PGP-COS development process to address these questions.

Methods: The PGP-COS study will include five phases: (1) A systematic review of the literature and semi-structured interviews with 15 patients (three countries) to form the initial list of outcomes for the Delphi survey; (2) A 3-round Delphi including patients, clinicians, researchers and service providers; (3) A systematic review of methods for measuring the outcomes in the preliminary PGP-COS identified in the Delphi survey; (4) A face-to-face consensus meeting to agree on the final PGP-COS and methods for measuring the COS; (5) Global dissemination. To address the methodological questions, we will assess the number and type of outcomes, in the final PGP-COS, that were exclusively derived from the interviews. Secondly, we will randomise Delphi survey participants to either a 5-point or 9-point importance rating scale, and examine potential differences in 'important' ratings between the groups.

Discussion: There is currently no COS for measuring/monitoring PGP in trials and clinical practice. A PGP-COS will ensure that relevant outcomes are measured using appropriate measurement instruments for patients with PGP globally.

Core Outcome Set Registration: This PGP-COS was registered with COMET (Core Outcome Measures for Effectiveness Trials) in January 2017 ( http://www.comet-initiative.org/studies/details/958 ).