CHICAGO — Bill Dunphy thought his colonoscopy would be free.

His insurance company told him it would be covered 100 percent, with no copayment from him and no charge against his deductible. The nation's 1-year-old health law requires most insurance plans to cover all costs for preventive care including colon cancer screening. So Dunphy had the procedure in April.

Then the bill arrived: $1,100.

Dunphy, a 61-year-old Phoenix small business owner, angrily paid it out of his own pocket because of what some prevention advocates call a loophole. His doctor removed two noncancerous polyps during the colonoscopy. So while Dunphy was sedated, his preventive screening turned into a diagnostic procedure. That allowed his insurance company to bill him.

Like many Americans, Dunphy has a high-deductible insurance plan. He hadn't spent his deductible yet. So, on top of his $400 monthly premium, he had to pay the bill.

"That's bait and switch," Dunphy said. "If it isn't fraud, it's immoral."

President Barack Obama's health overhaul encourages prevention by requiring most insurance plans to pay for preventive care. On the plus side, more than 22 million Medicare patients and many more Americans with private insurance have received one or more free covered preventive services this year. From cancer screenings to flu shots, many services no longer cost patients money.

But there are confusing exceptions. As Dunphy found out, colonoscopies can go from free to pricey while the patient is under anesthesia.

Breast cancer screenings can cause confusion too. In Florida, Tampa Bay-area small business owner Dawn Thomas, 50, went for a screening mammogram. But she was told by hospital staff that her mammogram would be a diagnostic test — not preventive screening — because a previous mammogram had found something suspicious. (It turned out to be nothing.)

Knowing that would cost her $700, and knowing her doctor had ordered a screening mammogram, Thomas stood her ground.

"Either I get a screening today or I'm putting my clothes back on and I'm leaving," she remembers telling the hospital staff. It worked. Her mammogram was counted as preventive and she got it for free.

"A lot of women ... are getting labeled with that diagnostic code and having to pay year after year for that," Thomas said. "It's a loophole so insurance companies don't have to pay for it."

For parents with several children, costs can pile up with unexpected copays for kids needing shots. Even when copays are inexpensive, they can blemish a patient-doctor relationship. Robin Brassner of Jersey City, N.J., expected her doctor visit to be free. All she wanted was a flu shot. But the doctor charged her a $20 copay.

"He said no one really comes in for just a flu shot. They inevitably mention another ailment, so he charges," Brassner said. As a new patient, she didn't want to start the relationship by complaining, but she left feeling irritated. "Next time, I'll be a little more assertive about it," she said.

How confused are doctors?

"Extremely," said Cheryl Gregg Fahrenholz, an Ohio consultant who works with physicians. It's common for doctors to deal with 200 different insurance plans. And some older plans are exempt.

Should insurance now pay for aspirin? Aspirin to prevent heart disease and stroke is one of the covered services for older patients. But it's unclear whether insurers are supposed to pay only for doctors to tell older patients about aspirin — or whether they're supposed to pay for the aspirin itself, said Dr. Jason Spangler, chief medical officer for the nonpartisan Partnership for Prevention.

Stop-smoking interventions are also supposed to be free. "But what does that mean?" Spangler asked. "Does it mean counseling? Nicotine replacement therapy? What about drugs (that can help smokers quit) like Wellbutrin or Chantix? That hasn't been clearly laid out."

But the greatest source of confusion is colonoscopies, a test for the nation's second leading cancer killer. Doctors use a thin, flexible tube to scan the colon and they can remove precancerous growths called polyps at the same time. The test gets credit for lowering colorectal cancer rates. It's one of several colon cancer screening methods highly recommended for adults ages 50 to 75.

But when a doctor screens and treats at the same time, the patient could get a surprise bill.

"It erodes a trust relationship the patients may have had with their doctors," said Dr. Joel Brill of the American Gastroenterological Association. "We get blamed. And it's not our fault,"

Cindy Holtzman, an insurance agent in Marietta, Ga., is telling clients to check with their insurance plans before a colonoscopy so they know what to expect.

"You could wake up with a $2,000 bill because they find that little bitty polyp," Holtzman said.

Doctors and prevention advocates are asking Congress to revise the law to waive patient costs — including Medicare copays, which can run up to $230 — for a screening colonoscopy where polyps are removed. The American Gastroenterological Association and the American Cancer Society are pushing Congress fix the problem because of the confusion it's causing for patients and doctors.

At least one state is taking action. After complaints piled up in Oregon, insurance regulators now are working with doctors and insurers to make sure patients aren't getting surprise charges when polyps are removed.

Florida's consumer services office also reports complaints about colonoscopies and other preventive care. California insurance broker Bonnie Milani said she's lost count of the complaints she's had about bills clients have received for preventive services.

"'Confusion' is not the word I'd apply to the medical offices producing the bills," Milani said. "The word that comes to mind for me ain't nearly so nice."

When it's working as intended, the new health law encourages more patients to get preventive care. Dr. Yul Ejnes, a Rhode Island physician, said he's personally told patients with high deductible plans about the benefit. They weren't planning to schedule a colonoscopy until they heard it would be free, Ejnes said.

If too many patients get surprise bills, however, that advantage could be lost, said Stephen Finan of the American Cancer Society Cancer Action Network. He said it will take federal or state legislation to fix the colonoscopy loophole.

Dunphy, the Phoenix businessman, recalled how he felt when he got his colonoscopy bill, like something "underhanded" was going on.

"It's the intent of the law is to cover this stuff," Dunphy said. "It really made me angry."

AP Medical Writer Carla K. Johnson can be reached at http://www.twitter.com/CarlaKJohnson