UnitedHealthcare requires that providers notify them of changes, but also has a Professional Verification Outreach program to proactively request information from providers. However, providers are burdened by having to maintain their information with multiple networks (e.g., competitors to UnitedHealthcare). The total cost of maintaining these directories is estimated at $2.1b annually, and a blockchain initiative began in 2018 to share the directory. When patients receive care from doctors who are out of network, they can be subject to balance billing; this is particularly common in emergency or hospital care, where the patient may not be notified that a provider is out of network. Utilization review Utilization management (UM) or utilization review is the use of managed care techniques such as prior authorization that allow payers to manage the cost of health care benefits by assessing its appropriateness before it is provided using evidence-based criteria or guidelines. UM criteria are medical guidelines which may be developed in house, acquired from a vendor, or acquired and adapted to suit local conditions. Two commonly used UM criteria frameworks are the McKesson InterQual criteria and MCG (previously known as the Milliman Care Guidelines). Lawsuits In the 21st century, commercial payers were increasingly using litigation against providers to combat alleged fraud or abuse. Examples included litigation between Aetna and a group of surgical centers over an out-of-network overbilling scheme and kickbacks for referrals, where Aetna was ultimately awarded $37 million. While Aetna has led the initiative, other health insurance companies have engaged in similar efforts. Organizations There is a continuum of organizations that provide managed care, each operating with slightly different business models. Some organizations are made of physicians, and others are combinations of physicians, hospitals, and other providers. Here is a list of common organizations: Group practice without walls Independent practice association Management services organization Physician practice management company (PPM) Industry in the United States As of 2017, the largest commercial plans were Aetna, Anthem, Cigna, Health Care Service Corp, UnitedHealthcare, and Centene Corporation. As of 2017, there were 907 health insurance companies in the United States, although the top 10 account for about 53% of revenue and the top 100 account for 95% of revenue. Smaller regional or startup plans are offered by Oscar Health, Moda Health, and Premera. Provider-sponsored health plans can form integrated delivery systems; the largest of these as of 2015 was Kaiser Permanente. Kaiser Permanente was the highest-ranked commercial