The physician assistant (PA) profession has grown dramatically in recent years, with the number of practicing PAs tripling from about 20,000 in 1991 to over 68,000 in 2008 (American Academy of Physician Assistants 2007
). There is currently about one PA for every 10 physicians in clinical practice, and soon this ratio is likely to tilt further toward PAs because there is one PA entering the U.S. workforce for every six physicians (National Center for Health Statistics 2005
). PAs are providing a growing portion of office visits for medical care in the United States (Druss et al. 2003
), and could buffer predicted physician shortages. How this change in the provider mix might impact overall use of medical services, however, is unknown. Does PA participation increase the effective supply of what have traditionally been physician services, or does it lead to provision of expanded or redundant services, thereby increasing per capita office visits per year?
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.
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.
This project addresses the research question: Is substantive inclusion of PAs in patient care associated with increased numbers of office visits per patient, adjusting for case-mix differences between patients seen by PAs and physicians? If PAs are functioning as substitutes, we would expect no increase in office visit resource use per patient when PAs are included in care. Alternatively, if PAs are providing complementary care, the total number of office visits per patient would be expected to increase when PAs are added to the provider mix. Our study expands upon existing research because it uses a diverse national sample, covers a year of health care experience for each person (rather than a single encounter or episode of care), and employs a validated means of case-mix adjustment. Because the data source does not identify NPs as a distinct category of office visit providers, this study does not include NPs.