Financial incentives, inherent in self-referral, "seem to have an influence on physician behavior that we can''t ignore, and an impact on patient care in the long run," said Jacqueline Baras Shreibati, M.D., of Stanford University School of Medicine, lead study author.
There is no definitive evidence that either MRI or surgery for low back pain improve outcomes, the authors say.
The study compared scan rates for Medicare patients of 1,033 primary care physicians and 1,271 orthopedists, before and after the physicians acquired MRI equipment - either by purchase, or by entering into lease arrangements whereby they billed directly for scans they ordered.
As earlier research has shown, when physicians begin self-referral for MRI they become significantly more likely to order scans: in this study the rate increased by 13 percent for orthopedists'' patients, and 32 percent for patients of primary care doctors.
What was more, patients who visited an orthopedist after he or she had acquired an MRI were 34 percent more likely to undergo back surgery within six months.
The study intent was not to assess whether the additional surgeries were necessary, Shreibati said. Yet, it did show "a direct relationship from imaging to surgery - in a group of patients where surgery is very controversial."
Jean M. Mitchell, Ph.D., professor of public policy at Georgetown University, said, "the main contribution of this research is what it shows about 'cascade effects''" of self-referral - its impact "not just on the procedure itself, but things that happen as a result. If the patient hadn''t had the MRI, they never would have had the back surgery."
Federal regulations prohibit physicians from referring Medicaid or Medicare patients for health services in which they have a financial interest, but make an exception for "in-office" procedures, which has been interpreted to include MRIs done with offsite equipment leased on a per-scan basis.
"Medicare spending alone would go down about 25 percent if self-referral were really policed or eliminated," Mitchell said.
While the findings do not indicate that patients should second-guess their doctors'' recommendations, Shreibati said, "I think it''s important for them to keep in mind that financial incentives may influence the care they receive."
The study authors presented an earlier version of the study at a research meeting in June 2010.
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Schreibati JB, Baker LC. The relationship between low back MRI, surgery, and spending:
impact of physician self-referral status. Health Services Research online, 2011.