Disadvantaged populations, as measured by zip code level socioeconomic status, are more likely to have outpatient surgery performed in hospitals than in ASCs. Patient race and ethnicity appear to modify the association between socioeconomic status and ASC use. For black and white patients, increased socioeconomic status continues to be significantly associated with ASC use. Conversely, Hispanic patients from lower socioeconomic status areas were more likely to use ASCs compared those residing in the higher groups. Because of the cost savings provided by ASC use for patients through lower overall costs and lower co-payments,[20
] ASC use would be especially important for patients of the lowest socioeconomic status groups.
Given the improved efficiencies of ASCs over hospital outpatient departments, patients could be expected to abandon hospital based outpatient surgery. However, such a trend has not developed. Instead hospital use for outpatient surgery has remained stable, and use of ASCs has grown.[21
] Barriers to the use of ASCs may exist that keep certain groups of patients in hospital outpatient departments.
One possible barrier raised by our results is patient profiling. Such profiling may be valid as in selecting patients with less comorbidity for surgery in ASCs,[22
] or may be inappropriate if barriers are created for groups based on economic status, race, or ethnicity. [23
] Since the findings of decreased ASC use in the least affluent patients were robust to control for comorbidity, sources of inappropriate profiling need to be considered.
Data from observation of physician encounters with patients supports the contention that patient profiling based on race and ethnicity may be responsible for differences in ASC use. These studies show that physicians will often recommend different procedures for the same clinical situation when the race or gender of the patient is changed.[26
] In addition, economic profiling of patients by the physician may occur. Since most ASCs are for profit enterprises with significant physician ownership,[27
] physicians have active incentives to ensure high reimbursement through these facilities. As such, similar to results seen for specialty hospitals,[29
] they may discourage the use of ASCs among patients with poor insurance and lower socioeconomic status.
In addition to physician factors, structural factors in the health care system may be responsible for the utilization patterns found. As for profit enterprises,[28
] investors in ASCs have financial incentives to avoid ventures where lack of reimbursement potential is perceived. These facilities may not be established in areas of lower socioeconomic status due to investor concerns about the insurance mix in the population. Thus, a physical barrier to ASC use based on community economic profiling may exist that limits the access of less advantaged patients to ASCs.
Regardless of the cause of the disparity, the findings in this study support the contention that more financially vulnerable groups are encountering a higher aggregate cost burden for their care than more advantaged groups. The benefits of ASCs in cost, convenience, and efficiency are not equitably distributed. Both physician and structural factors may be ultimately responsible for the association between socioeconomic status and ASC utilization found in this paper. Indeed, the finding of different effects of race on the likelihood of ASC use by socioeconomic status could be a result of either physician level or system level factors. Further research into the underlying reasons for these observations is needed to correct these biases in the delivery of health care.
Zip code tabulation areas (ZCTAs) were used to geocode the discharge records. Zip code level evaluations of socioeconomic status have been demonstrated to result in different parameter estimates than evaluations based on census block groups. With the larger population base in zip codes, estimates would likely be biased towards the null. Furthermore, since ZCTAs were used as the geographic unit of analysis in this study, spatial and temporal discrepancies between the zip codes reported in the SASD and the ZCTA from the census bureau exist. ZCTAs and zip codes may share the same 5 digit code while not representing the same geographic entity.[30
] Usually, the spatial discontinuity between these measures of geography is small.[31
] A potentially larger problem exists in zip code changes over time. While we used data with patient reported zip code information from 2005, the ZCTAs were last updated in 2002.[32
] Thus, there is potential for mismatch between the two measures of geography. However, less than one percent of our cohort was lost due to issues of missing data, suggesting that the temporal discontinuity issue was not a significant factor in our study.
A further issue to be addressed is our inclusion of only one state, Florida, in the analysis. Florida has a more elderly population than many other states, more for profit facility ownership, no certificate of need requirements, and higher per capita health care use than other states. Despite these issues, data from Florida provided a valuable substrate for our study due to the ability to gather discharges from both the ASC and hospital environments. Furthermore, the factors that make Florida a potentially unique market, including the lack of certificate of need requirements, allow us to see ASC utilization patterns independent of regulatory forces. ASC use, and disparities in use, may be lower in states with certificate of need requirements.
Finally, patients of lower socioeconomic status often carry high burdens of comorbid illness and more severe underlying disease.[33
] As such, they may be less appropriate candidates on average for surgery in ASCs. Although we correct for comorbidity in our analysis, subtle differences in severity of comorbid conditions cannot be addressed and may result is residual confounding. However, we believe the impact of these issues is minimized as all patients in the study had ambulatory surgery.