Existing research suggests that, in the United States, participation in care by PAs and nurse practitioners (NPs) does not increase overall use of medical services, but this research is from limited settings and patient populations. Hooker evaluated the effect of provider type (PA or physician) on an episode of care for four acute uncomplicated problems in a managed area setting and found that PAs did not require more expenditures or more return visits to manage the episode of care (Hooker 2002). In a rare randomized trial comparing provider types, Mundinger et al. (2000) found similar outcomes and health resource use among a predominantly female and Hispanic population randomized to receive primary care from either a NP or a physician for 1 year. The extent to which findings from these studies generalize to other settings or to care for chronic or serious conditions is unknown. Our literature review found no national study investigating the effect of PA or NP use on longitudinal health resource use.

Several studies have focused on the production efficiency of PAs and NPs as measured by the number of office visits provided per unit time or per unit of labor cost, compared with physicians. These studies have generally found that PAs provide between 76 and 100 percent as many office visits as physicians per unit time (Record et al. 1980; Hooker 1993; Larson, Hart, and Ballweg 2001), and that the use of PAs can lower labor costs (Roblin et al. 2004) and increase efficiency (Medical Group Management Association 2006).

This apparent high productivity might be misleading with respect to cost-effectiveness if the use of PAs leads to increased per person resource use. For example, it is possible that PAs may be employed to provide services that would otherwise not have been provided (complementary services) or that PAs may schedule more return visits than do physicians, thereby increasing total office visits per person. Although complementary services and additional follow-up visits could result in higher quality of care, they might not increase the overall productivity of the workforce, a pressing concern given predicted physician shortages (Association of American Medical Colleges 2006).

As health services researchers consider the impact of PAs and NPs on health care provision, it is relevant to ask whether these clinicians replace care that would otherwise be provided by a physician (substitution of services) or whether they provide care that would otherwise not have been provided (complementary care).

The substitution model is supported by Hooker's study of roles of PAs and NPs in managed care (Hooker 1993), by Gryzbicki's detailed analysis of task substitution in a single family practice/general medicine practice in Pennsylvania (Grzybicki et al. 2002), and by Mundinger's randomized trial of NP and physician care (Mundinger et al. 2000) In addition, research on staffing ratios in health maintenance organizations in the mid-1990s found an inverse relationship between the numbers of PAs and advanced practice nurses (APNs) employed and numbers of physicians employed per 100,000 enrolled (Dial et al. 1995). As the use of PAs and APNs increased, the use of physicians decreased, suggesting that the PAs and APNs were providing services that physicians would otherwise provide. Several studies reporting on the use of physician assistants to replace house staff in academic medical centers are based on the assumption that PAs replace physician services (Carzoli et al. 1994; Stoddard, Kindig, and Libby 1994; Schulman, Lucchese, and Sullivan 1995; Miller et al. 1998).

Other research lends support to the complementary care model. Laurant et al. (2004) found that random assignment of NPs to primary care practices in the Netherlands did not affect physician workload, concluding that “Nurses are not substitutes for doctors but provide a wider range of services than was available previously.” A randomized trial in Britain showed that NPs scheduled return visits more frequently than physicians (Venning et al. 2000) and a systematic review relying heavily on European experience indicated that NPs provide longer consultations and make more investigations than do physicians (Horrocks, Anderson, and Salisbury 2002). Generalizing these results to PA practice in the United States is problematic because practice patterns in the United States may differ from those in Europe, and because PA practice can vary in meaningful ways from that of NPs (Hooker and McCaig 2001).

The dichotomization of PA/NP services as either a substitute or complement for physician care is an oversimplification, and both patterns are likely to exist in practice. Physicians and PAs/NPs develop diverse practice arrangements based on personal preferences and practice needs. Some physicians may choose to hire PAs or NPs to provide preventive and counseling services that the physicians are unable to find the time to provide, leading to services that are intentionally complementary. Others may work out substitution practice arrangements in which PAs or NPs see the patients with the least complicated routine problems, while the physicians see the more complex patients. In other practices, the assignment of patients to PAs, NPs, or physicians may be random or may depend on scheduling constraints or idiosyncratic interests of the providers involved. In many practices, there will be a mix of substitute and complementary services. Practice patterns may also evolve over time, as individual physicians, PAs, or NPs develop special interests or skills.

Even though the dichotomy of substitute versus complementary services is an oversimplification, at a macro level it will be useful to assess whether, on average, addition of PAs or NPs to the national mix of providers has an effect of substituting for or complementing physician services. Information about whether increased resource use can be expected as a consequence of increasing numbers of PAs and NPs, and about the magnitude of any increased resource use, is necessary for the projection of workforce needs.

For the ADGs, the PA+ and physician-only groups differed by >4 percent (p <.05) for several important categories. The PA+ group is more like to see persons with “Time limited-minor-primary infections” (48 percent of persons in the PA+ group versus 37 percent of persons in the physician-only care group), “Likely to recur-discrete-primary infections” (27 percent for PA+ and 17 percent for physician-only), “Time limited-minor” (25 percent for PA+ and 19 percent for physician-only), and “Allergy” (18 percent for PA+ and 14 percent for physician-only) problems. The physician-only care group was more likely to see persons in the “Chronic medical: stable” (40 percent PA+, 44 percent physician-only) and “Chronic medical: unstable” (15 percent PA+, 20 percent physician-only) groups. These differences are generally in the direction of physician-only care predominating among persons with more complex problems, and again demonstrate the necessity of risk adjustment.

Table 2 shows the incidence rate ratios indicating the proportionate contrast in expected number of office visits over the 1-year period for the exposed (PA+) group compared with the control (physician-only) group. For our primary definition of substantive PA care (with a 30 percent cut-point), the risk ratio of 84 indicates that the number of visits per year is reduced by about 16 percent for persons in the PA+ group compared with persons in the physician-only group, adjusted for demographic, geographic, socioeconomic, insurance status, and health factors. When the cut-point was lowered to 10 percent, there was no difference seen between the comparison groups. Above the level of at least 25 percent of visits provided by a PA, substantive PA participation in care is associated with fewer office-based visits per year. As the proportion of PA visits used to define the PA+ group increases, the number of office visits per year consistently decreases in a dose–response pattern (Figure 1).

Table 3 shows the results of the secondary analysis examining whether reduced office visit use in the PA+ group was offset by increased number of visits in other settings. Persons in the PA+ group had about 25 percent fewer emergency department visits (p<.05). The results for hospital outpatient and inpatient settings were not statistically significant.

The decreased number of office visits that we found with substantive PA care may be due to differences between the comparison groups that were not balanced by our risk adjustment procedures. For example, it may be that, within practices or health systems, PAs are assigned patients who are expected to require fewer visits than those assigned to physicians, and that the characteristics behind these decisions are relatively intangible and not reflected by our control variables. Although the Adjusted Clinical Group® System that we used for risk adjustment has been validated to predict health resource use (Weiner et al. 1992), risk adjustment can never completely eliminate selection bias. Our results indicating that the group with substantive PA care also had fewer emergency department visits suggest that this group may have been more healthy than the physician-only group. Thus, residual selection bias may explain our finding of fewer office visits per year among the group with substantive PA care.

Another potential explanation for our finding is that, as persons have more health care visits, they are more and more likely, by chance, to encounter a PA, but less likely to stay above the threshold cut-points (10, 25, 30 percent, etc.) used in our analysis. If this is true, our primary independent variable (substantive PA care) could be endogenous with our outcome variable (number of visits per year). For a discussion of this problem, and of our efforts to address it, see Supplementary Appendix B.

When interpreting these findings, it is important to account for the strengths and limitations of MEPS data for research on PAs. Because household respondents may not accurately report the type of provider who saw a patient, the “physician-only” group likely contains some persons who were seen by a PA or another provider type that was incorrectly identified as a physician on the MEPS survey. Also, because MEPS only represents visits to a PA when a physician was not also seen, some persons in the control group were likely exposed to PAs on visits when they also saw the physician. The magnitude of this contamination is difficult to quantify. Although MEPS likely underrepresents the extent of PA participation in care (Morgan et al. 2007), MEPS still provides a relatively large national sample that is diverse with regard to patient demographics, geography, socioeconomic status, type of health plan, and health status. MEPS data provide one of the few national representative longitudinal sources of information about national patterns of health care use. The longitudinal design is well-suited to our research question because it supports the analysis of a person's care over time, rather than only at an individual health care encounter. In addition, the MEPS pattern of surveying respondents every 4–5 months minimizes recall bias more than surveys that query respondents about events over a full year. In sum, while MEPS has weaknesses for this research, it likely provides one of the best data sources available to investigate the effect of PAs on resource use.

Appendix SA1 Author Matrix.

Appendix A Additional Graphic Description of Data.

Figure A1 Frequency Distribution of Number of Visits per Year to PAs.

Figure A2 Frequency Distribution of Number of Visits per Year to Physician-Only Group and to PA+ Group.

Figure A3 Distribution of Portion of Visits Provided by PAs Among Persons Who Saw PAs.

Table A1 Distribution of Number of Visits per Year, Total Sample.

Appendix B Endogeneity Bias and Alternative Analyses.

Table B1 Quantile Regression Analysis Results.

Table B2 Propensity Score Analysis with Stratified Matching.

This material is available as part of the online article from http://www.blackwellsynergy.com/doi/abs/10.1111/j.1475-6773.2008.00874.x (this link will take you to the article abstract).