Outcome measurement instruments for the core outcome sets on genital gender-affirming surgery: the GenderCOS project

Outcome assessment in genital gender-affirming surgery (gGAS) has long been a heterogenous practice. Although two core outcome sets (COS) for masculinizing and feminizing gGAS have been previously established, the absence of standardized and validated outcome measurement instruments (OMIs) limits consistent reporting. The second phase of the GenderCOS project aimed to identify, evaluate, and recommend OMIs to standardize outcome assessment and facilitate adoption of the COS in gGAS research, ultimately enabling comparability and evidence synthesis.

The project followed the Core Outcome Measures for Effectiveness Trials (COMET) initiative standards and the COnsensus-based Standards for the selection of health Measurement INstruments (COSMIN) guideline for selecting OMIs for a COS. Phase 2 of the GenderCOS project was conducted from September 2024 to June 2025. Potential OMIs were identified through systematic reviews: PROSPERO: CRD42022347400 (inception to September 2023) and CRD42020223430 (inception to November 2020), clinical guidelines, and international expert consultations. All instruments relevant to at least one core outcome underwent quality and feasibility assessment. Consensus on the most appropriate OMIs and essential supplementary information was achieved through an international consensus process involving professional experts across multiple disciplines.

A total of 380 potential OMIs were identified through systematic searches. After domain- and COS- matching, 152 patient-reported outcome measures (PROMs) and 53 clinical OMIs were evaluated on quality and feasibility aspects. The consensus process among professional experts in gender-care, resulted in measurement recommendations for 19 of the 20 unique core outcomes. These recommendations endorse the use of validated PROMs for the included patient-reported outcomes (PROs) and adherence to existing clinical guidelines for clinical outcomes and adverse events. For the remaining outcome relating to feminizing gGAS, a measurement recommendation was made for the subgroup that underwent vaginoplasty only. Development of an OMI suitable for all feminizing gGAS is also recommended.

The GenderCOS provides the first consensus-based standardized measurement framework for core outcomes in gGAS. Its modular structure and inclusion of validated instruments enable harmonized reporting, data synthesis and evidence-based improvement in gGAS research.

None received.