When One Person is the Trial: Toward Good Healthspan Practice (GHP), a Draft Good-Practice Guide for N-of-1 Evidence in Longevity Medicine

Longevity-medicine services are expanding rapidly, yet the field still lacks the quality infrastructure that mature health disciplines take for granted. The first gap is practical: practitioners have no public consensus on what competent service looks like, from inquiry to intervention review. The second is evidential: because typical clients are multi-morbid individuals receiving many interventions at once, the population-average evidence of conventional medicine answers their questions poorly, leaving individualized reasoning unregulated. We propose a working draft of Good Healthspan Practice (GHP), an open, product-neutral good-practice guide that addresses both gaps in one design. The draft is modular: five process guides cover the full service cycle (inquiry, multi-omics measurement, root-cause attribution, intervention and contribution review), with root-cause attribution as the hard requirement on which the others depend, anchoring individualized reasoning in the hallmarks of aging, the small set of mechanisms the whole field shares. A sixth guide, GHP-600, runs across all five and makes individualized causal inference itself checkable: causal claims must be registered, falsifiable and honestly reviewed, using tools regulators have already developed for N-of-1 trials, real-world evidence and learning health systems. Adoption is voluntary and by self-declaration, module by module, with no conformity assessment. Each adopted client loop yields a prediction–validation record that, aggregated across adopters, calibrates the guide itself over time. In the longer run, these records feed biomedical world models that predict how one person responds to one intervention; the guide is built to grow with them. GHP is not a standard under any national standardization law, not a certification and not an association; it is a public working document whose aim is to make competent practice definable, individualized reasoning verifiable, and the field's evidence base cumulative. The draft text (v0.2) is public at ghp-ai.com, offered as a starting point for community discussion and iteration rather than a finished code for adoption.