According to the 2010 Council on Graduate Medical Education report, the United States has a shortage of primary care physicians (PCPs), and the shortage is likely to worsen.1 The shortage is attributed to a low supply of physicians in rural and underserved areas with high proportions of low-income and minority residents, as well as new physicians' low interest in primary care. The shortage also may be associated with the aging of the physician workforce, heavy workload, and administrative burden faced by PCPs.2,3 This situation will worsen as the US population ages, particularly as 80 million baby boomers become Medicare-eligible, and provisions of the Affordable Care Act expand health insurance coverage.4
Increased use of physician assistants (PAs) and nurse practitioners (NPs) has been proposed to offset declining numbers of PCPs.5,6 Between the mid-1990s and mid-2000s, the per-capita number of NPs increased by more than 9% annually, and PAs increased 4% annually. During the same time period, the per-capita number of PCPs increased about 1%.7 NP and PA scope of practice and levels of autonomy are regulated by state medical statutes. Over the past 20 years, the increased supply of PAs and NPs, increased reimbursement for PA and NP services, and state laws expanding the scope of PA and NP practice have expanded the roles for both types of clinicians in primary care.5,6,8
Greater use of PAs and NPs in primary care practices, as well as new models of primary care, such as accountable care organizations and patient-centered medical homes, requires overcoming several barriers.5 NP scope-of-practice laws vary widely; states with more restrictive laws limit the practice and access to NPs.9 Lack of reimbursement for certain NP or PA services, as well as lack of physician training in team-oriented care, also may reduce fuller use of PAs and NPs.10 Previous studies found that states favorable for PA practice had higher PA supply relative to other states.11-13 Other studies found that NPs serving rural populations practiced more often in states with the most practice autonomy than in states with less autonomy.14,15
An estimated 43% of PAs and 52% of NPs practice in primary care settings.3,16 Much of what is known about PAs and NPs at the state level is for a limited number of states. In this study, we identify states where PCPs are more likely to have PAs or NPs working in their offices. We also examine the association between PCPs with PAs or NPs in their practices and selected practice characteristics, practice location, and the effect of state scope-of-practice laws using multivariate analysis. Although selected national estimates on availability of PAs or NPs among office-based PCPs have been published, results of this multivariate analysis may be informative to policymakers at the state level.17,18
Data for this study were based on the 2012 National Ambulatory Medical Care Survey (NAMCS)-Electronic Health Record (EHR) Survey, a nationally representative mail survey monitoring physician adoption of HER systems.19 NAMCS is an annual probability survey of nonfederal, office-based physicians providing direct patient care (excluding radiologists, anesthesiologists, and pathologists), conducted by the CDC's National Center for Health Statistics. The sample of physicians was taken from the master files of the American Medical Association and the American Osteopathic Association. The NAMCS two-stage sample design includes 112 geographic primary sampling units. Within each primary sampling unit, physicians were stratified by specialty, and then a sample was selected according to each stratified specialty. The NAMCS-EHR survey was designed as a supplemental mail survey to the in-person NAMCS. The 2008-2009 NAMCS-EHR surveys used the same sample design as NAMCS; combined estimates from both surveys have been published.19 For more information about NAMCS and the NAMCS-EHR surveys, see www.cdc.gov/nchs/ahcd.htm.
Starting in 2010, the NAMCS-EHR survey sample size was increased fivefold to allow for state-level estimates. The 2012 NAMCS-EHR survey included a sample of 10,302 physicians selected from the 50 states and the District of Columbia. Nonrespondents to the mail survey received follow-up telephone calls. The NAMCS-EHR survey collected information on physician and practice characteristics, such as specialty and practice size, as well as information on adoption of EHR systems, such as availability of selected computerized capabilities and electronic exchange of clinical data. In 2012, 4,545 physicians responded to the survey, for a weighted response rate of 65%. Including the District of Columbia, an average of 89.1 physicians responded per state.
The 2012 NAMCS-EHR survey study population consisted of PCPs in the specialties of general or family practice, internal medicine, geriatrics, and pediatrics with information on nonphysician clinicians in their practice (n=1,951). Estimates of PA or NP availability in PCP practices are based on the question: “How many mid-level providers (that is, nurse practitioners, physician assistants, and nurse midwives) are associated with you at this reporting location?” Reporting location is the site at which most ambulatory patients were seen.
In this study, we examine state variability of the percentage of PCPs with PAs or NPs, including nurse midwives, in their practice after controlling for physician practice characteristics previously associated with the presence of PAs and advanced practice registered nurses in physician practices.17,18 These characteristics include practice size as measured by the number of physicians in the office where the physician saw the most patient visits; multispecialty practice status; percentage of revenue from Medicaid, and urban-rural classification of the practice location (Table 1).
We also examined the association between state scope-of-practice laws and use of PAs or NPs in PCP practices. NP autonomy was classified by the extent to which physician oversight of NPs is required by state law or regulation. As classified by the 2012 Pearson Report, NP scope-of-practice laws were categorized as:
- Physician oversight to diagnose, treat, and prescribe (least independent)
- Physician oversight to prescribe
- No physician oversight required (most independent).20
We used the PA state practice characterization scheme developed by the American Academy of Physician Assistants to classify PA scope of practice.21 This scheme includes six elements (licensure, scope of practice determined at the practice level versus by state regulation, adaptable physician supervision, full prescriptive authority, no requirement for chart cosignature, and physician-to-PA ratio determined at practice level versus state regulation) in state laws that enable physician-PA teams to treat patients. In general, the higher the number of elements that are present, the more favorable the state law is for PA scope of practice. In this article, we use the three-category grouping of AAPA's classification as described in Sutton, Ramos, and Lucado: states with one to two elements were least favorable for PA practice; three to four elements, moderately favorable; and five to six elements, most favorable.11
Because the NAMCS-EHR survey was based on a complex sample survey of physicians, compound sampling weights were applied to make national estimates of nonphysician clinician use and corresponding estimates of sampling error. The statistical analysis software SUDAAN was used to account for the sample design when calculating the standard errors. All estimates presented were reliable (relative standard error less than 30%) due to the large sample size. We conducted bivariate analysis using t-tests (P<0.05) to examine whether physician practice, location, and scope-of-practice categories were associated with availability of PAs or NPs in PCP practices. We used multivariate logistic models to examine availability of PAs or NPs in PCP practices (dependent variable) while controlling for physician practice, location, and scope-of-practice characteristics.
Two analytic questions were addressed in multivariate logistic models: whether PA or NP employment among PCP practices varied by state, and whether PA or NP employment among PCP practices varied by PA and NP scope-of-practice laws. For the first analysis, a logistic regression model for percentage of PCPs with PAs or NPs in their practice was computed, controlling for practice characteristics and state and using Texas as the reference state. Although Texas had the third-largest percentage of PCPs (Table 1), it was chosen as reference state because employment of PAs or NPS in this large state (47%) was roughly similar to the national percentage (53%) (Table 2).
The second question investigated the availability of PAs or NPs in PCP practices, controlling for practice characteristics, PA scope of practice, and NP scope-of-practice indicators. State was omitted from this model because both scope-of-practice variables were defined by state.
Because effect size cannot be directly inferred from coefficients of logistic models, we estimated marginal effects as the change in the predicted probability of a one-unit change in the independent variable, holding all other covariates at observed values. For comparability, differences with reference categories based on bivariate estimates (unadjusted) also are presented.
In 2012, 53% of PCPs worked with PAs or NPs in their practice. Employment of PAs and NPs in PCP offices varied by state and the District of Columbia, ranging from 33.4% in Washington, D.C., to 89.1% in Montana (Table 2). In unadjusted analysis, the percentage of PCPs with PAs or NPs in their practice was greater than Texas (47%) in 19 states (Alaska, Arizona, Idaho, Iowa, Kansas, Maine, Massachusetts, Minnesota, Montana, Nebraska, New Hampshire, New Mexico, North Carolina, North Dakota, South Dakota, Tennessee, Vermont, Wisconsin, and Wyoming). These results are similar to differences with the national average of 53%.22
State results may have been affected by physician practice characteristics. Table 2 indicates practice characteristics associated with PCP employment of PAs or NPs. In unadjusted analysis, the percentage of PCPs working with PAs or NPs increased with practice size, from 36.3% among solo and partner practices to 80% among practices with 11 or more physicians. Availability of PAs or NPs was higher among PCPs in multispecialty practices (74.9%) than in single-specialty practices (45.3%), and was higher among PCPs in practices with more than 7% revenue from Medicaid (54.5%) than physicians in practices with less than 7% revenue from Medicaid (44.7%). The unadjusted percentage of PCPs working with PAs or NPs increased as practice location became less urban; from 41.9% in practices located in large central metropolitan areas to 65.7% in nonmetropolitan areas.
The multivariate model produced similar results as the bivariate analysis. After controlling for practice characteristics, the percentage of PCPs working with PAs or NPs increased as practice size increased. The marginal effect of PA or NP use in PCP practices with 3 to 10 physicians was 16.5 percentage points higher relative to solo and partner practices. Among PCP practices with 11 or more physicians, PA or NP use was 33.2 percentage points higher than among solo and partner PCP practices. Multispecialty practices also were associated with higher use of PAs or NPs; the marginal effect for multispecialty practices was 14.7 percentage points higher than in single-service practices. As PCP office locations became more rural, use of PAs and NPs increased. Relative to PCPs located in large central metropolitan areas, use of PAs or NPs in offices located in medium or small metropolitan areas was 11.1 percentage points higher, and increased to 18 percentage points higher in offices located in nonmetropolitan areas.
In the same model, higher availability of PAs or NPs in PCP offices persisted in only three states (Minnesota, Montana, and South Dakota) relative to PCPs in Texas, after adjusting for physician practice size, multispecialty status, percentage of revenue from Medicaid, and urban status of office location, with all else constant (Table 2). No state had significantly lower availability of PAs or NPs relative to Texas, with all else remaining constant.
In the unadjusted analysis, employment of PAs or NPs in PCP offices varied by NP and PA scope-of-practice laws (Table 3). The percentage of PCPs with PAs or NPs in their practice was higher in states where no physician oversight for NPs was required (63.2%) compared with states that required physician oversight to diagnose, treat, and prescribe (50%). The percentage of PCPs with PAs or NPs in their practice was higher in states with laws favorable to PA practice (70%) compared with states in the least favorable environment for PA practice (49.8%).
In a separate model adjusting for the same physician practice characteristics as in Table 2, the percentage of PCPs with PAs or NPs in their practice was unrelated to NP scope-of-practice laws, all else remaining constant. Availability of PAs or NPs in PCP offices, however, was 9.6 percentage points higher in states with favorable PA scope-of-practice laws relative to least favorable environment for PA practice, all else remaining constant.
Our study found use of PAs or NPs in PCP offices was significantly associated with practice size, multispecialty practices, and metropolitan status of the office location in multivariate analysis. The largest marginal effects occurred among PCPs in practices with 11 or more physicians relative to solo and partner practices, and among PCPs in nonmetropolitan locations relative to PCPs in large central metropolitan areas.
The association between higher PA or NP use and multispecialty practices may reflect the greater resources of large multispecialty practices. On average, the practice size (17.9 physicians) among PCPs in multispecialty practices was larger than the average (4.2 physicians) among PCPs in single-specialty practices (data not shown). In addition, previous studies have documented increased use of NPs and PAs as primary care providers in managed care organizations such as HMOs and multispecialty clinics since the l990s.23
Although the availability of PAs or NPs in PCP practices varied by state, after controlling for practice characteristics, use of PAs or NPs in PCP offices was significantly higher in three states (Minnesota, Montana, and South Dakota) relative to Texas (reference state), all else remaining constant. This suggests that characteristics other than practice size, multispecialty status, percentage of revenue from Medicaid, and urban status of practice location may be associated with higher use of PAs or NPs in PCP practices in these states.
In a second model controlling for the same practice characteristics, higher use of PAs or NPs in PCP offices was associated with states having favorable PA scope-of-practice laws, all else remaining constant. This finding may be related to a higher supply of PAs in states with favorable PA practice laws relative to other states.11,12 According to a previous study, a PA practice index (measuring legal standing and requirements for physician oversight, prescriptive authority, and reimbursement) was positively correlated with per-capita supply of PAs during the period from 1992 to 2000.12
No association, however, was found between state scope-of-practice laws for NPs and availability of PAs or NPs in PCP offices after adjusting for practice characteristics in the same model. The wording of the question in which NPs and PAs were combined may have made it difficult to interpret the effect of NP scope-of-practice laws. Previous research using the same NP scope-of-practice variable but a more direct NP outcome measures (patients with NPs as their primary care provider) found a strong association between the percentage of patients with NPs as primary care providers and the degree of state restriction.13 Public and private payment policies recognizing NPs as primary care providers affects use of NPs in office practices because their recognition as primary care providers permits NPs to bill claims directly for their patients.24-26 Current reimbursement policies may create incentives for physician involvement in care provided by NPs in order to bill at a higher rate.27 Further research is needed to investigate the association between physician employment of NPs and state scope-of-practice laws taking NP recognition as primary care provider into account, as well as other factors noted in the literature, such as reimbursement rates and the effect of local supply of PCPs and/or PAs on the availability of NPs.10,13 Finally, physicians may underreport NPs in their practices if NPs are employed as administrators rather than as care providers. The 2008 National Sample Survey of Registered Nurses found that 14.6% of NPs in ambulatory or primary care reported principally working in management.28
The findings are subject to certain limitations:
- Because the findings are based on a cross-sectional study, causation should not be inferred.
- Conclusions about PCPs reporting PAs and NPs in their office practice may not be generalizable to other healthcare settings.
- The study was unable to specify type of nonphysician clinician associated with PCPs due to question wording that grouped nonphysician clinician use (NPs, PAs, or nurse midwives). Thus, although some PCP practices may employ nurse midwives, most nurse midwives are associated with obstetrics and gynecology practices. In addition, reporting joint availability of NPs and PAs may have influenced the inability to detect associations between NP scope-of-practice laws and availability of PAs or NPs in PCP practices. This limitation also made comparisons with other state-based studies of PA or NP availability problematic. Presence of a PA or NP within a practice is subject to respondent recall bias; estimates could have varied if the respondent were a PA or NP.
- Findings also may vary if NP and PA scope-of-practice variables were defined differently (for example, included public and private reimbursement policies, or supply of PCPs and PAs at the local level).
- Estimates for practice characteristics (practice size, multispecialty status) as defined by the reporting location may be inaccurate if the sampled location is different from other locations.
- Findings in certain states may have been limited by sampling errors due to small sample size.
Although much state variation in use of PAs and NPs in PCP offices was associated with physician practice characteristics, higher use of PAs or NPs in primary care physician offices was associated with state scope-of-practice laws favorable to PA practice. Uniformity in PA and NP scope-of-practice laws across states could expand access in primary care shortage areas.
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Keywords:Copyright © 2015 American Academy of Physician Assistants
physician assistant; nurse practitioner; primary care practice; scope of practice; multispecialty; metropolitan