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Assisted primary graft patency credits maintenance of an open graft by interventions performed before it occludes. The distinction is temporal: assistance preserves uninterrupted patency, whereas successful reopening after an occlusion belongs to secondary graft patency. The 1997 reporting standard introduced this distinction explicitly when revising how preventive graft procedures were counted.^[ref]

A qualifying preventive intervention can end primary graft patency without ending assisted primary graft patency. Thus the two endpoints measure different aspects of durability: the former concerns performance without that assistance, while the latter also credits preservation work. They should share a stated time origin and compatible rules before their curves are compared.

Consider an original four-graft cohort. At month 2, one graft undergoes a preventive procedure while still open. At month 3, a different graft occludes, then is successfully reopened at month 4. The other two remain open without procedures. With complete follow-up to month 4, primary patency is $2/4$, assisted primary patency is $3/4$, and secondary patency is $4/4$, assuming the restoration qualifies for the original reconstruction’s secondary endpoint. The reopened graft does not regain an uninterrupted assisted-primary history.

Surveillance influences what is observed and when assistance is undertaken. A narrowing may be detected before occlusion, so reports need their detection and intervention rules. A high assisted-primary proportion can coexist with many preventive procedures; it is not a measure of low procedural burden. Conversely, two cohorts with different surveillance intensity are not made directly comparable merely by using the same endpoint name.

With incomplete follow-up, survival analysis uses a graft patency risk set rather than treating every graft as observed to the final study date. Clinical interpretation also requires the separately measured consequences for tissue function and the patient. Preserving a conduit is an important intermediate outcome, not a complete description of benefit.

Primary graft patency measures how long a graft remains open without an intervention that terminates the primary-patency endpoint. It concerns the history since the original reconstruction, not merely whether a bypass graft is open on today’s image. Under the 1997 reporting standard, work confined to disease beyond the graft and its anastomosis can be an exception; an intervention on the graft to prevent its failure is not.^[ref]

For an explicit time-to-event model, define $T_p$ as the first qualifying loss of primary patency. Its survival function isOpening a narrowed but still patent graft by a qualifying preventive procedure can end primary graft patency even though blood never stopped flowing. Later rescue does not put that same graft back into the original primary-patency history. Assisted primary graft patency and secondary graft patency answer different questions about preserving or restoring function.

An original five-graft example demonstrates the denominator. All enter follow-up together. One has a qualifying intervention at month 2, another is last observed event-free at month 3, and a third occludes at month 4; the other two remain event-free through month 4. The Kaplan–Meier estimator givesThe graft patency risk set falls after both the first endpoint and the later right censoring. The censored graft is neither declared a failure nor assumed patent indefinitely. Interpreting this estimate requires an appropriate independent-censoring assumption.

A report should identify the original procedure date, qualifying interventions, documented occlusions and observation limits. A status image alone cannot reconstruct every earlier event. Primary graft patency is therefore most informative alongside the intervention history and separately defined clinical outcomes, rather than as an unexplained percentage.