Psychometric Validation of the German Version of the Endometriosis Impact Questionnaire (EIQ)

Background Endometriosis is a chronic gynecological condition affecting an estimated 10–15% of women of reproductive age and is associated with chronic pain, fertility-related difficulties, and substantial impairments in health-related quality of life. In addition to a high symptom burden, endometriosis is characterized by considerable diagnostic delays, often lasting several years, which may contribute to symptom chronicity and significant psychosocial distress. The impact of endometriosis is commonly assessed using both generic and disease-specific measures of health-related quality of life. The Endometriosis Health Profile (EHP-30) and its short version (EHP-5) are among the most widely used disease-specific instruments. However, these measures primarily focus on current or short-term quality-of-life impairments. Given the chronic and recurrent nature of endometriosis, there is a need for instruments that more comprehensively capture the long-term subjective impact of the condition. The Endometriosis Impact Questionnaire (EIQ) was developed based on qualitative research and systematic scale development to assess the multidimensional and long-term impact of endometriosis across different domains of life. As no validated German-language version currently exists, the present study aims to evaluate the psychometric properties of a German translation of the EIQ. Study Aim The primary objective of this study is to examine the psychometric properties of the German version of the Endometriosis Impact Questionnaire (EIQ). Specifically, the study will evaluate reliability, test–retest reliability, factorial validity, convergent validity, and nomological validity. In addition, established measures of health-related quality of life, emotional distress, fertility-related quality of life, and coping strategies will be administered to assess external validity. Methods Data will be collected through a web-based survey administered via SoSci Survey. The German version of the EIQ was developed using a forward–backward translation procedure. Consistent with the original EIQ validation study, the recall periods of both the EIQ and the Endometriosis Health Profile-30 (EHP-30) will be set to 12 months to ensure conceptual comparability. The study includes two assessment points separated by a 14-day interval to evaluate test–retest reliability. Participants will complete approximately 128 questionnaire items, resulting in an estimated completion time of 20–25 minutes. In addition to the EIQ, the following instruments will be administered: * Endometriosis Health Profile-30 (EHP-30) * WHO Quality of Life – BREF (WHOQOL-BREF) * Hospital Anxiety and Depression Scale (HADS) * Fertility Quality of Life Questionnaire (FertiQoL) * Brief COPE Psychometric analyses will include item analyses, internal consistency estimates, test–retest reliability, and correlational analyses for convergent and nomological validity. Factorial validity will be examined using confirmatory factor analysis (CFA) based on the factor structure reported in the original EIQ validation study. Participants Eligible participants are adults aged 18 years or older with a self-reported physician-confirmed diagnosis of endometriosis, sufficient German language proficiency, and informed consent to participate in the study. An a priori power analysis indicated a minimum sample size of N = 67 participants for correlational analyses. However, because confirmatory factor analysis constitutes a central component of the validation process, a substantially larger sample is required. Based on recommendations for factor analytic and structural equation modeling research, a target sample size of approximately N = 240 participants is planned to ensure stable and interpretable parameter estimates. Participants will be recruited through gynecological practices, specialized endometriosis centers, and endometriosis-related online platforms and social media communities. No financial compensation will be provided for participation. De-identified data will be archived in PsychArchives (ZPID) and made available under controlled access conditions via Coscine in accordance with ethical and data protection regulations.