Background: Systematic health monitoring with patient-reported outcome instruments may predict post-discharge complications after major surgery. The objective of this study was to conceptualize a novel patient-reported outcome instrument for detecting early adverse events within two weeks of discharge after major emergency abdominal surgery and colorectal cancer surgery. Methods: This study was conducted in two phases. (1) An exhaustive health concept pool was generated using systematic content analysis of existing patient-reported outcome measures (N = 31) and semi-structured interviews of readmitted patients (N = 49) and health professionals (N = 10). Concepts were categorized into three major domains: ‘Symptoms,’ ‘functional status,’ and ‘general health perception.’ We calculated the frequency of each health concept as the proportion of patients, who experienced the respective concept prior to readmission. (2) Colorectal cancer surgeons (N = 13) and emergency general surgeons (N = 12) rated the relevance of each health concept (1 = irrelevant, 5 = very relevant) in the context of detecting post-discharge adverse events. We selected concepts with either a high mean relevance score (≥ 4) or a combination of moderate mean relevance score and high patient-reported frequency (≥ 3 and ≥ 20% or ≥ 2.5 and ≥ 50%, respectively). Results: Content analysis of existing items with additions from patients and experts resulted in 58 health concepts, of which the majority were distinct symptoms (N = 40). The selection procedure resulted in 29 patient-reported health concepts relevant for detecting adverse events after discharge. Conclusion: The outlined framework provides content validity for future patient-reported outcome instruments detecting adverse events in the early post-discharge period after major emergency abdominal surgery and colorectal cancer surgery.