Table 2.

Adjusted Rate Ratios^* (95% Confidence Intervals) for the Association between Quintile of Chiropractic Care Supply and Annual Number of Visits to Primary Care Physicians for Back and/or Neck Pain

Independent Variable	Visits to any PCP for back and/or neck pain	Visits for Back and/or Neck Pain, By PCP Specialty
		Family Medicine PCP	Internal Medicine PCP
Supply of health services
Quintile of chiropractic care
1 (lowest)	1.00 (Reference)	1.00 (Reference)	1.00 (Reference)
2	1.08 (1.07–1.09)	1.07 (1.06–1.08)	1.08 (1.07–1.09)
3	0.98 (0.97–0.98)	0.97 (0.96–0.98)	0.94 (0.93–0.94)
4	0.97 (0.97–0.98)	0.86 (0.86–0.86)	1.02 (1.01–1.02)
5 (highest)	0.92 (0.91–0.92)	0.85 (0.84–0.86)	0.90 (0.89–0.91)
PCP services,† number per 10,000	1.00 (1.00–1.00)	1.02 (1.02–1.02)	1.01 (1.01–1.01)
Patient characteristics
Age, years	1.00 (0.99–1.00)	0.99 (0.99–0.99)	1.01 (1.00–1.01)
Sex
Male	1.00 (Reference)	1.00 (Reference)	1.00 (Reference)
Female	1.36 (1.35–1.36)	1.30 (1.30–1.31)	1.41 (1.40–1.41)
Race/ethnicity
Non-Hispanic white	1.00 (Reference)	1.00 (Reference)	1.00 (Reference)
Other	1.11 (1.10,–1.11)	1.00 (0.99–1.00)	1.15 (1.14–1.16)
Rurality of residence
Urban	1.00 (Reference)	1.00 (Reference)	1.00 (Reference)
Rural	0.99 (0.99–0.99)	1.39 (1.38–1.40)	0.72 (0.71–0.72)
Charlson Comorbidity Score
0	1.00 (Reference)	1.00 (Reference)	1.00 (Reference)
1	1.13 (1.12–1.13)	1.10 (1.09–1.11)	1.15 (1.14–1.16)
2	1.25 (1.25–1.26)	1.20 (1.19–1.21)	1.30 (1.30–1.31)
≥3	1.29 (1.29–1.30	1.22 (1.21–1.24)	1.35 (1.34–1.35)

For each of the 3 models, n = 14.9 million Medicare patients.

^*All rate ratios are highly significant at P <.001 (except for race/ethnicity) in predicting visits to family medicine physicians.

^†Variable used corresponds to primary care physician (PCP) specialty for the respective model (eg, for prediction of visits to family medicine PCPs, number of family medicine physicians per 10,000 capita was used).