An ASC must be certified and approved to enter into a written agreement with CMS. Participation as an ASC is limited to any distinct entity that operates exclusively for the purpose of providing surgical services to patients not requiring hospitalization and in which the expected duration of services would not exceed 24 hours following an admission. An unanticipated medical circumstance may arise that would require an ASC patient to stay in the ASC longer than 24 hours, but such situations should be rare. The regulatory definition of an ASC does not allow the ASC and another entity, such as an adjacent physician’s office, to mix functions and operations in a common space during concurrent or overlapping hours of operations. CMS does permit two different Medicare-participating ASCs to use the same physical space, so long as they are temporally separated. That is, the two facilities must have entirely separate operations, records, etc., and may not be open at the same time. ASCs are not permitted to share space, even when temporally separated, with a hospital or Critical Access Hospital outpatient surgery department, or with a Medicare-participating Independent Diagnostic Testing Facility (IDTF). Certain radiology services that are reasonable and necessary and integral to covered surgical procedures may be provided by an ASC; however, it is not necessary for the ASC to also participate in Medicare as an IDTF for these services to be covered.