Medicare beneficiaries who stayed continuously enrolled in the same accountable care organization over a four-year period had 10% lower healthcare costs, a new report finds. But when beneficiaries had a catastrophic health event, costs skyrocketed.

Medicare beneficiaries who left for a new ACO, or were assigned out of one, cost 14% more from 2012 to 2016, the June Medicare Payment Advisory Commission Report to Congress finds. Beneficiaries with stable health stick with the same network, but a major health problem leads to a different group of doctors, which elevates costs, analysts note.

“We find that beneficiaries who ‘switch’ into and out of ACOs have higher spending growth than both those beneficiaries consistently assigned to an ACO and those never assigned to an ACO,” the report states.

The spikes in cost elicit “fundamental questions” as to whether or not ACOs are equipped to  manage catastrophic health events suitably, said James E. Mathews, the commission’s executive director, Bloomberg News reported.

In addition, because Dual Eligible Special Needs Plans (D-SNPs) have such low levels of integration with state Medicaid plans, report authors said changes could include prohibiting plan sponsor from enrolling partial-benefit dual eligibles or requiring the establishment of separate D-SNPs for partial-benefit and full-benefit dual eligibles.