The United States Chiropractic Workforce: An alternative or complement to primary care?

In the United States (US) health care policymakers continue to be concerned about the ability of the primary care workforce to meet the future primary care needs of an aging and expanding population. Some authorities encourage increasing the general physician workforce to address the shortage [1‐3]. However, others argue that, because of market forces, expansion would only perpetuate the overabundance of specialists and continue to widen the divide between high and low supply of primary care physicians [4, 5]. Indeed, recent studies suggest that, after accounting for population differences, a 2- to 3-fold variation in physician supply exists across geographic regions [4]. And only primary care physician supply [4] appears to be related to improvements in population health outcomes [6].

To meet the nation’s future primary care needs, it might be reasonable to expand the scope of practice of ancillary practitioners such as nurse practitioners and physician assistants. However, there is concern that the number of new graduates from nurse practitioner and physician assistant programs is not keeping pace with the required needs to mitigate future primary care physician shortages [7]. In addition, there is evidence that physician assistants are providing more specialty care [8, 9].

Many Americans seek care for common health conditions outside the offices of primary care physicians [10, 11]. More specifically, neck and lower back conditions are among the most common conditions encountered in the primary care setting [12‐15]. Chiropractors in the US provide a substantial portion of satisfactory care for these complaints [16]. In this regard, chiropractors may already be offsetting primary care shortfalls (serving as an alternative) by merely being located in areas of lesser primary care supply. If chiropractors are indeed located in areas of lesser primary care physician supply, they might be serving as points of primary contact for patients, thereby improving access. On the other hand if chiropractors are located in areas of adequate primary care physicians supply, it might suggest chiropractors function in more complementary roles (i.e. treating cases that would otherwise have been treated by primary care physicians as opposed to serving as a point of primary contact). Depending on their existing role in the larger health care system, chiropractors may be positioned to play an even larger part in addressing the nation’s growing health care needs.

However, little is actually known about the relationship between the chiropractic and primary care physician workforces [17‐19]. We hypothesized chiropractors might be distributed across the nation in a manner, either directly or inversely, related to that of the nation’s primary care physician workforce. Therefore, the purpose of our study was to examine the distribution of the chiropractic workforce and compare it to that of primary care physicians.

To examine the chiropractic workforce and compare it to that of primary care physicians, we used data from the Center for Medicare and Medicaid Service’s Unique Physician Identification Number Directory, the US Census Bureau, and data on medical physician supply previously collected by the Dartmouth Atlas of Health Care for 2006. The Dartmouth Atlas of Health Care provides national information on the distribution and use of medical services across the US according to predefined geographic units.

We estimated that there were 74,623 chiropractors in the US who held 87,237 state chiropractic licenses (a mean of 1.2 licenses per practitioner) in 2006. The national chiropractic workforce has been relatively stable from 2002 to 2008 (Additional file 1: Appendix 1). There were about twice as many active primary care physicians (169,843) in 2006.

The per capita supply of chiropractors ranged from 10.7 per 100,000 capita in Bryan, Texas to 126.4 per 100,000 capita in Mason City, IA, a 10.8-fold variation in supply. Although New Orleans had an even lower supply of chiropractors (6.8 per 100,000 capita) than Bryan, because the low numbers in New Orleans could have been an effect of chiropractors relocating after Hurricane Katrina in 2005, we were concerned that the New Orleans results were spurious and eliminated New Orleans from further analysis. The median supply of chiropractors was 36.6 per 100,000 capita. The supply of primary care physicians ranged from 46.0 to 123.8 per 100,000, with a median supply of 74.0 per 100,000, a considerably lower 1.7-fold variation across HRRs (Figure 1).

The supply of chiropractors and primary care physicians was correlated (Pearson correlation coefficient = 0.17, p-value < 0.01). However, the 20% of HRRs with a high supply of chiropractors per capita (defined as those greater than or equal to 50 per 100,000 capita) were roughly equally distributed across high and low primary care physician supply locales (Figure 1).

We found that variation in the supply of chiropractors across the US exhibited a specific spatial pattern (Figure 2). The Midwestern and Western regions contained considerably more chiropractors than expected after accounting for differences in population age and size — a similar but more pronounced pattern than that of the crude supply of chiropractors per 100,000 capita (Additional file 1: Appendix 2). Conversely, the Southern portion of the US had considerably fewer chiropractors than expected. Considering the high supply of medical physicians in the Northeast, we were surprised to find that few HRRs in the Northeast exceeded expected supply of chiropractors. New Hampshire and Vermont Hospital Referral Areas were an exception and some of these HRRs trended towards higher concentrations of chiropractors (a ratio of 1.26 to 1.50 of observed to expected).

This is one of the few recent studies to examine the national supply of chiropractors [17, 23‐25] and, to our knowledge, the first to compare regional variation in the supply of chiropractors to that of primary care physicians. We estimate the total chiropractic workforce to be over 74,000 practitioners, almost half of our estimate of 169,843 active primary care physicians in 2006. We found greater than a 10-fold variation across the country in the supply of chiropractors, a higher concentration of chiropractic services than expected in the Midwest, and a lower one in the South which collaborates with a recent report that examined the influence of supply on utilization [25]. Finally, we found a statistically significant correlation with the per capita supply of primary care physicians.

Our study suggests that chiropractors tend to establish their practices in areas that also have higher supply of primary care physicians. This is likely explained by the influence of market conditions on both groups: both are likely to locate in areas that will support their practices – however, there are many other personal and professional factors that our data did not account for. This finding may suggests that chiropractors function in a complementary role, as opposed to an alternative role, to primary care physicians. Whether that complementary role is substitutive or additive is unknown.

When chiropractors and physicians are asked independently to define the role primary care plays in population health there is significant agreement between the two professions [26]. However, chiropractors in the US remain largely considered to be neuromuscular specialists. In addition to government and private insurances designating chiropractors' as specialists in health care systems, chiropractors role as neuromuscular specialists is also is due to patients’ impressions of their services – patients tend to view chiropractors as “back doctors” not as primary care providers [27]. Many within the US chiropractic profession are content with their role as neuromuscular specialists and those who wish to provide more primary care roles likely have numerous barriers to overcome [28].

Previous studies have examined the professional relationship between primary care physicians and chiropractors and uncovered an overall lack of inter-professional cooperation [29‐31]. There is considerable overlap in the patient population treated by primary care physicians and chiropractors - neck and back conditions are among the most common complaints in the primary care setting [15]. This, coupled with our finding that chiropractors tend to locate in areas with higher primary care physician supply, suggests there is considerable potential for inter-professional collaboration. Health service use for neck and back conditions has risen in recent years and are attributed with substantial costs to the US economy [12, 13, 32]. Novel, interdisciplinary strategies are needed to address the growing needs of this population. Improved collaboration and coordinating care between chiropractors and primary care physicians has potential to help improve population health and reduce health care costs.

The relatively strong influence of population characteristics on the regional supply, of chiropractic care suggests that chiropractors are more responsive to market conditions than primary care physicians. Indeed, the chiropractic profession in the US continues to be predominately a “cottage industry”, and therefore most chiropractors are also small business owners. It is likely that chiropractors establish their practices in areas of higher socioeconomic status, health care access, and overall demand for their services.

Also, there appears to be a relationship between rurality and chiropractic supply, and previous reports have explored the relationship [17, 24, 33]. For instance, one study found that chiropractors practicing in a Health Professional Shortage Area (which tend to be more rural) had larger patient panels than those not in Health Professional Shortage Areas, suggesting chiropractors may be playing a role in improving access to health services in certain locales [24]. Our results align with most previous reports as we found more rural areas to be predictive of a higher supply of chiropractors. Nevertheless, while chiropractors tend to be located in more rural areas (which may improve access to care in some populations), they are less likely to be located in poorer areas and areas with high proportions of racial and ethnic minorities.

These findings will inform health care policymakers as they decide how chiropractors will be part of US health care reform efforts, such as Accountable Care Organizations [34] or the potential expansion of coverage of chiropractic services. The size of the chiropractic workforce is relatively large, with higher concentrations in the Midwest, a pattern not exhibited among medical physician groups [21]. The extent to which chiropractors might be able to supplement medical care should be considered as a way to enhance the primary care workforce. Specifically, even though chiropractors might not be serving as points of primary contact role, chiropractors may be contributing to addressing the continued health care related to back and neck conditions particularly for populations residing in rural locales.

As the US health care system begins to recognize and reimburse more for preventive interventions, services such as chiropractic care may find more opportunities for integration with primary care. On a per visit basis, chiropractic care is inexpensive when compared to medical care [35] but larger studies are required to more thoroughly evaluate the costs associated with specific episodes of care as well as any potential indirect effects on overall cost. In light of increasing demands on an already overextended primary care workforce, it is an important time for health care policymakers to consider the direct and indirect effects of decisions regarding future coverage of chiropractic care [34]. Health care policymakers and other stakeholders should consider these findings when planning for the future health care needs of the nation.