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  "path": "/t/collider-in-rct-subgroup-analysis/28689?page=2#post_31",
  "publishedAt": "2026-06-01T00:40:09.000Z",
  "site": "https://discourse.datamethods.org",
  "textContent": "s_doi:\n\n> In short, a prognostic biomarker must be orthogonal to the treatment mechanism while a predictive biomarker must be entangled with the treatment mechanism.\n\nYes, I love this framework because it is teachable to clinicians.\n\nHowever, I wonder whether orthogonality is always absolute once a “prognostic” marker is deliberately used for pathway enrichment.\n\nA predictive biomarker establishes the mechanistic pathway. A prognostic biomarker may then be used to enrich for patients in whom that pathway is sufficiently active to avoid dilution of the treatment effect. In that setting, the prognostic biomarker does not define the disease or identify the causal pathway. Rather, it measures the intensity or expression of an already established pathway.\n\nAs a result, such a marker may become associated with treatment responsiveness, not because it is itself mechanistically entangled with the treatment, but because it reflects the degree to which the targeted biological process is active.\n\nPerhaps your definition would classify such a marker as predictive once that association exists. However, I wonder whether there is value in distinguishing a third category: an “enrichment biomarker.” This would be a marker that neither establishes the pathway nor serves as a purely orthogonal prognostic marker, but instead quantifies pathway intensity within a biologically defined population.\n\nFor example, a pathogen-specific marker may establish the disease pathway, while a physiological variable such as oxygen requirement or P/F ratio may enrich for patients in whom that pathway is most active and therefore most likely to reveal a treatment effect.\n\nWould you consider such pathway-intensity enrichment markers to remain strictly prognostic, or do they occupy an intermediate position between prognostic and predictive biomarkers?",
  "title": "Collider in RCT Subgroup Analysis"
}