The findings are from a Harvard Medical School study that looked at informal patient-sharing networks among doctors across the country. The researchers used 2006 data from nearly 4.6 million Medicare patients seen by more than 68,000 doctors in 51 urban and rural hospital referral regions.
The study was published in the July 18 issue of the Journal of the American Medical Association.
There was substantial variation between the doctors' informal sharing networks. The number of doctors included in a network ranged from 135 in Minot, N.D., to nearly 8,200 in Boston. The average number of other doctors each doctor was connected to per 100 Medicare patients was 27.3.
Doctors were far more likely to have connections with doctors based at the same hospital than those based at different hospitals. Connected doctors also were much more likely to be in close geographic proximity: The average distance for connected doctors was 13.1 miles, compared to 24 miles for unconnected doctors.