The Affordable Care Act requires the assignment of Medicare beneficiaries based on which physician provides a beneficiary’s primary care services, and delegates the task of prescribing the assignment methodology to the Secretary of Health and Human Services.
After considering a majority rule and a plurality rule, CMS believes that the latter should be the governing rule.  CMS believes, correctly so, that the majority rule would be too strict and likely hinder the development and stability of ACOs.  Instead, the plurality rule would ensure that beneficiaries will be assigned to an ACO where they receive more primary care than any other provider.  A question arises as to whether there should be any minimum threshold of primary care services. CMS is soliciting comments on this point.  CMS also wants to use the allowable charges in order to capture the more complex primary care use, rather than a simple service count.
Under this standard, in order for a beneficiary to be assigned to an ACO, that beneficiary must have more visits or at least the threshold number of visits to the ACO physician and the value of primary care services provided by the ACO physician must be higher than other providers.
Although this standard captures the frequency and value of primary care services, it is possible that a beneficiary would not get allocated to any ACO in situations where the physician that provides more visits than others does not have the higher value of services, while the physician providing the higher value of services may not have more visits than others.
This is likely to be an exceptional situation.  However, the dual standard may make it more difficult for ACO to maintain the 5,000 beneficiaries.  Of course, if the beneficiary shops around for primary care physicians, perhaps the beneficiary should not be assigned to any particular ACO.  In any case, this presents an interesting point for CMS to ponder.