Validity of a single-item indicator of treatment side effect bother in a diverse sample of cancer patients

Purpose: With higher efficacy of cancer therapies, the numbers and types of side effects experienced by patients have also increased, evidencing a need for brief assessments of side effect bother. The Functional Assessment of Cancer Therapy-General (FACT-G) includes the item “I am bothered by side effects of treatment” (GP5). This study aimed to confirm GP5’s validity in a large, diverse, real-world patient sample. Methods: Real-world data were drawn from 10 Adelphi Disease Specific Programmes (DSP™) conducted between 2015 and 2019 in France, Germany, Italy, Spain, the UK and the USA, covering 10 cancer sites. We examined correlations between GP5 responses and varied measures of patient-reported global health and the number of side effects experienced. We explored whether more advanced patients and those with worse Eastern Cooperative Oncology Group Performance Status Rating (ECOG PSR) reported greater side effect bother. Finally, we conducted differential item functioning (DIF) assessment using the Mantel–Haenszel approach. Results: The sample included 6755 advanced cancer patients. GP5 responses were distributed similarly across most cancer sites. A moderate, negative correlation (r_polyserial = − 0.43) between GP5 responses and global health evidenced convergent validity. Known groups validity was evidenced by dichotomised distributions of GP5, showing expected results between cancer stage 2 vs. 3 and 4 and with ECOG PSR (p < 0.001). Little evidence of DIF was found. Conclusion: GP5 exhibited evidence of validity across cancer sites and countries and appeared to measure the same construct across these countries. GP5 has significant promise as a summary indicator of side effect bother.