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Attribution is the process of assigning patients to groups.

This matters today because we have engaged in a decade long practice of attempting to transform care delivery in the United States by creating a proliferation of Accountable Care Organizations (ACO).  An ACO is a clinical entity (a hospital system, a collection of physician groups etc) that agree to take a per-capita payment for a group of patients and gets a bit more flexibility to organize and administer their care.  An ACO can profit if total cost of care at a given quality level comes in under the target.  An ACO can lose money if total cost of care is over the target.  The ultimate payer, such as the federal government for Medicare ACOs, can share some of the savings and losses.

One of the key questions in any ACO is who is the ACO responsible for?  This is the attribution process.  There are a lot of attribution algorithms out there that can split a universe of patients into groups of (ACO A, ACO B, not assigned) pretty well.  However, one of the key questions is when does the ACO get the final list of patients it is financially and clinically responsible for? There are two choices.  Retrospective  attribution looks at the care performed on patients during a performance year at the end of the year.  Attribution is based on who the ACO actually sees during the performance year.  The finalized list is given to the ACO after the contract year is over.   Prospective attribution looks at the care performed on patients prior to the start of the contract year.  The ACO gets the final list on the first day of the contract year.

These are big choices to make.  Prospective attribution will, by definition, miss people who move into the area or join the program for the first time in the performance year.  Retrospective attribution will include people on the list who received care from the ACO provider network in the prior year but moved three time zones away at the start of the year, but the ACO is still responsible for their costs.

In a new paper at the American Journal of Managed Care led by Dr. Brystana Kaufman, a Duke Margolis colleague and friend, in work funded by the Gordon and Betty Moore Foundation, we simulated attribution to ACOs that are already in the Medicare Shared Savings Program using both retrospective and prospective methods to look at the differences in attributed populations for Medicare ACOs among populations defined as “seriously ill”.

There are big differences depending on what method is used.

Retrospectively assigned cohorts are cheaper than prospectively assigned cohorts.

Why is that?

Dying in the first 90 days of the performance year was associated with reduced odds of retrospective ACO attribution (odds ratio [OR], 0.24; 95% CI, 0.24-0.25) relative to beneficiaries surviving 270 days or longer. Similarly, hospice use was associated with reduced odds of retrospective assignment (OR, 0.80; 95% CI, 0.79-0.80).

Retrospective attribution needs certain types of claims in the performance year. Dying in the first three months of the performance year makes it difficult for the right types and number of claims to accumulate. Hospice in Medicare also does funny things to claims accumulation.  The lack of claims makes assignment of individuals to an ACO using retrospective attribution difficult.

Prospective attribution picks up the soon to be dead as people who are in the last chunk of their life are likely to have significant utilization in the last full claim year of life.  And it is this last full claim year of life that prospective attribution is looking at to assign people to an ACO. A prospectively assigned cohort will have much higher first quarter mortality in a Medicare ACOthan a retrospectively assigned cohort just because of who is included in the assignable universe.

So what does this mean beyond a half dozen geeks geeking out?

We’ve identified the start of a business case for specific focus on serious illness. If we want to make serious illness care more central to ACOs, then our choice of attribution methods matters a lot.  If we want ACO leadership to be concerned about how the last year of life is treated in an ACO, then prospective assignment gives a very strong financial motivation to redesign care around the needs of this population.

Furthermore, a recent change to the Medicare ACO program allows for an ACO to choose which type of attribution they want.  Previously, Medicare used retrospective attribution.  Now an ACO may be able to make strategic choices to select the type of population that they think they are better at serving.  This could have federal fiscal impact without changing overall care quality or efficiency.

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