Insurance aspects of DRG outlier payments

As part of the prospective payment system, the government pays 'outlier' payments for especially long or expensive cases. These payments can be viewed as insurance for the hospital against excessive losses. They mitigate problems of access and underprovision of care for the sickest patients, and provide additional payments to the hospitals that take care of them, thereby making payments to hospitals more equitable. This paper characterizes the outlier payment formulae that minimize risk for hospitals under any fixed constraints on the sum of outlier payments and minimum hospital coinsurance rate. We then simulate per-case payments for a policy that did not include any outlier payments, the current outlier policy, and several other policies that minimize risk subject to different coinsurance constraints. The current outlier policy achieves each of its goals to at least some extent, but more insurance could be provided without lessening attainment of the other goals. We also discuss some problems with the implementation of the current policy, such as its reliance on day outliers.