Two-thirds of operations are performed on an outpatient basis, yet little research has assessed their quality. [1][2][3][4] Our population-based study found that 30-day revisits for clinically significant surgical site infections following ambulatory operations accounted for less than 15% of all-cause revisits to inpatient or ambulatory surgery settings. 5 The reasons for other revisits and their relationship to the index operation are unknown. This study determined the rates of all-cause, unplanned revisits (ie, not for routine medical care) within 30 days of ambulatory surgery and whether revisits were related to the operation.Methods | We performed a retrospective analysis of 6 low-to moderate-risk ambulatory operations spanning a range of specialties and complexity. We used the 2010-2011 Healthcare Cost and Utilization Project (HCUP) State Ambulatory Surgery and Services Databases, State Inpatient Databases, and State Emergency Department Databases for 7 states with unique, encrypted patient numbers (representing approximately one-third of the US population), allowing linkage of the ambulatory operations with 30-day postoperative, unplanned acute care revisits. 6 We identified index ambulatory operations performed in hospital-owned settings for adults with low surgical risk (single operation; no infection, cancer, or acute care in prior 30 days). We determined 30-day revisit rates per 1000 operations by setting (inpatient, ambulatory surgery, or emergency department [ED]). A patient with a revisit was counted once; priority was hierarchically assigned as inpatient, then ambulatory surgery, and then ED.The reason for the revisit (based on first-listed diagnosis code) was categorized as a complication related to the operation or an unrelated condition. The categories were developed iteratively using a combination of prospective and empirical methods.First, 2 of the authors (M.M-G. and G.D.S; who are surgeons) created categories a priori of known complications for each operation; revisit diagnosis codes were identified and assigned to these categories. Second, the 2 surgeons along with a third physician (C.A.S., who is an internist) reviewed the remaining revisits and empirically developed additional categories. To ensure face validity, the related complication categories were analyzed for consistency with procedures, and the unrelated conditions were reviewed to ensure that conditions were not miscategorized.We further characterized related complications by reason for revisit and present the 2 most frequent types: operation specific and pain (abdominal and other). The Agency for Healthcare Research and Quality institutional review board waived the need for informed consent because data were deidentified. Results | There were 482 034 ambulatory operations and 45 760 all-cause 30-day revisits (94.9 per 1000 operations; Table 1). Most revisits were to the ED (58.7 per 1000), followed by inpatient and ambulatory surgery settings (27.0 and 9.2 per 1000, respectively). Revisit rates to the inpatient and ED settings ...