New Yorkers are routinely shortchanged by insurance companies when they see doctors outside of their insurance networks, Attorney General Cuomo said yesterday.

Mr. Cuomo said his office plans to file a civil lawsuit against one of the country’s largest insurers, UnitedHealth Group, and is investigating 16 other insurers, including Aetna, Oxford, and Cigna. The investigation focuses on the process by which insurers reimburse medical care that patients receive outside their networks. Generally, insurers refund a percentage of what they calculate to be the reasonable price for a given type of procedure or doctor’s visit. That price — called the “usual and customary” price — is supposed to reflect actual local prices for medical care.

Mr. Cuomo said insurers are routinely low-balling those figures. The result is that the “usual and customary” cost calculated by insurers can be less than half of what medical care actually costs. Patients are often responsible for paying the difference.

In one example, Mr. Cuomo said a 15-minute visit to a doctor — with an actual market price of $200 — would be valued by UnitedHealth at $77, leaving the patient to make up the difference.

Insurers across the industry generally get data on the “usual and customary” cost of a given procedure from a subsidiary of UnitedHealth Group, Ingenix. The company manages a database of more than 1.3 billion current medical bills, from which it calculates the “usual and customary” cost in different neighborhoods.

Mr. Cuomo’s lawsuit against UnitedHealth Group will allege that Ingenix is manipulating the data to underestimate how much medical care actually costs.

A lawyer under Mr. Cuomo, Linda Lacewell, said at a press conference yesterday that Ingenix was “nothing more than a conduit for rigged information.” Ms. Lacewell, who leads health care industry investigations for the attorney general’s office, said Ingenix’s database operated through “deception, manipulation of data, and outright fraud.”

In a statement yesterday, UnitedHealth Group said Ingenix’s “reference data is rigorously developed, geographically specific, comprehensive, and organized using a transparent methodology that is very common in the health care industry.”

Ingenix is the largest such database used by insurers. Its data are used not only by UnitedHealth Group, but also by competitors such as Aetna, Empire Blue Cross Blue Shield, and Cigna.

The statement said UnitedHealth Group, which is based in Minneapolis, is cooperating with Mr. Cuomo. The company is “committed to fair and appropriate payment for physicians,” the statement said.

In 2000, the American Medical Association filed a lawsuit with many similar allegations against UnitedHealth. The suit is being litigated in U.S. District Court in Manhattan.