Between 1999 and 2009, the percentage and absolute number of ambulatory visits that resulted in a referral appear to have grown substantially in the United States. A large analysis found a 94% increase in the probability that such visits would result in a referral, increasing from 4.8% in 1999 to 9.3% in 2009. The absolute number of referrals increased by 159% during the study period, rising from 41 million to 105 million. Growth was slowest among physicians with ownership stakes in their practice.
Abstract: Archives of Internal Medicine, January 23, 2012.