Don’t take no for a final answer when a health insurer rejects a claim and leaves behind an unpaid medical bill. As many as 50 percent of some appeals prompt insurers to reverse their decisions, according to a report from the Government Accountability Office.

Insurers frequently deny claims due to billing errors, missing information or judgments on whether the care or service is appropriate, the investigative arm of Congress said in a report released Wednesday.

These denials can be based on mistakes like an incorrect code on a claim submitted by a doctor’s office, said Nancy Davenport-Ennis, CEO of Patient Advocate Foundation, a not-for-profit that helps people appeal claims denials.

“You’ve got a lot of people in America who are ultimately paying a bill they don’t owe because they don’t realize it’s an incorrect code,” said Davenport-Ennis, who wasn’t involved in the government study.

The GAO studied health insurer rejection rates at the request of Congress, which wanted a better picture of the issue as part of the health care overhaul it passed last year. The overhaul aims to cover millions of uninsured people after it unfolds over the next few years.

The GAO studied data collected from a handful of states and reports done by other agencies. It found that as many as 50 percent of appeals to insurers in Maryland in 2009 led to coverage decision reversals.

In Ohio, 48 percent of appeals to insurers led to reversals last year.

Appeals to a third party can fare decently, too. The GAO cited a report from America’s Health Insurance Plans, which studied 37 state external review programs a few years ago and found that about 40 percent of external appeals led to the reversal of a claim denial.

These figures do not mean patients have nearly a 50 percent shot at success if they appeal a denial. The statistics are based on cases appealed, and only a small portion of denials are challenged, said John Dicken, a GAO health care director.

Patient Advocate Foundation, which works in all 50 states, helped more than 17,000 people deal with insurance claims denials last year. A benefit not covered by a health plan is the most frequent reason they see for claim denials.

Davenport-Ennis said coverage parameters can vary widely, and they are often determined by the insurer and the employer that provides group health coverage. For instance, some company plans may not cover Autism treatments or clinical trial enrollments.

Davenport-Ennis also said they’ve seen insurers tighten restrictions on prescription drug coverage in recent years and add cost-control wrinkles like limits on the number of surgeries covered in a year.

That means a breast cancer patient in some cases may have her biopsy and lumpectomy covered but not the reconstructive surgery that follows.

Patient Advocate Foundation also deals with claims that are denied even though they involve a benefit covered by the insurance plan.

“There are many times the claim is denied the first go-round to see if you come back and appeal,” Davenport-Ennis said.