Use of chiropractic spinal manipulation in older adults is strongly correlated with supply.

Study Design. Retrospective cross-sectional analysis of administrative data. Objective. To examine the relationship between regional chiropractic supply and both use and utilization intensity of chiropractic services among Medicare beneficiaries. Summary of Background Data. Numerous studies have documented trends and patterns in the utilization of chiropractic services in the United States, but little is known about geographic variation in the relationship between chiropractic supply and utilization. Methods. We analyzed Medicare claims data for services provided by chiropractic physicians in 2008. We aggregated the data to the hospital referral region level and used small area analysis techniques to generate descriptive statistics. We mapped geographic variations in chiropractic supply, use and utilization intensity (treatments per user), and quantified the variation by coefficient of variation and extremal ratio. We used Spearman rank correlation coefficient to correlate use with supply. We used a logistic regression model for chiropractic use and a multiple linear regression model for chiropractic utilization intensity. Results. The average regional supply of chiropractic physicians was 21.5 per 100,000 adult capita. The average percentage of beneficiaries who used chiropractic was approximately 7.6 (SD, 3.9). The average utilization intensity was 10.6 (SD, 1.8). Regional chiropractic supply varied more than 14-fold, and chiropractic use varied more than 17-fold. Chiropractic supply and use were positively correlated (Spearman rho, 0.68; P < 0.001). A low back or cervical spine problem was strongly associated with chiropractic use (odds ratios, 21.6 and 14.3, respectively). Increased chiropractic supply was associated with increased chiropractic use (odds ratio, 1.04) but not with increased chiropractic utilization intensity. Conclusion. Both the supply of chiropractors and the utilization of chiropractic by older US adults varied widely by region. Increased chiropractic supply was associated with increased chiropractic use but not with increased chiropractic utilization intensity. Utilization of chiropractic care is likely sensitive to both supply and patient preference.