(MONEY Magazine) – You might think that joining a managed-health-care plan, such as a health maintenance organization, guarantees that your insurance bills will be paid if a health crisis sends you rushing to an emergency room. Think again. Some health plans now define an emergency by the diagnosis you receive rather than the symptoms you experienced--even if the problem appears life-threatening. For example, if your E.R. doctor determines that your chest pains were caused by heartburn rather than a heart attack, your HMO may fully reject your claim and pay nothing. Says Jane Howell, spokesman for the American College of Emergency Physicians in Washington, D.C.: "It's like having an insurer say that it won't pay for your biopsy if it turns out that you don't have cancer."

Of course, patients often don't know if their symptoms truly require emergency care. As a result, complaints of rejected E.R. claims are mounting, sometimes leading to enforcement actions against HMOs. For example, in January, the Insurance Division of the Oregon Department of Consumer and Business Services fined the PacifiCare HMO $20,000 for denying E.R. claims without conducting a reasonable investigation. Four other states, including Maryland and Virginia, have recently passed laws requiring HMOs to pay for E.R. visits as long as the patient had what a "prudent layperson" would call urgent symptoms. A similar bill has been introduced in the U.S. House of Representatives by Rep. Ben Cardin (D-Md.), but it has been stalled by election-year politics.

Meantime, if you belong to an HMO or are shopping for one, here are five questions to ask:

--How do you define an emergency? The best answer: Your E.R. visits will be covered by the managed-care plan if your symptoms are severe, regardless of the final diagnosis.

--Do you provide your own after-hours care? Most HMOs run after-hours urgent-care centers to handle emergencies. If yours doesn't, it should have a 24-hour service that will refer you to the appropriate location for care.

--Whom do I need to call before going to the hospital? In an emergency, no one--although generally you are required to call within the 24 or 48 hours following an E.R. visit. For an elective hospital admittance, most HMOs require that you phone your primary-care physician or plan representative to get pre-approval. Bear in mind, even if your HMO okays an E.R. visit, you may need additional approvals for further treatment.

--Which hospitals can I use? Managed-health-care plans generally require that you use only their member hospitals, so find out how close the nearest participating emergency room is to you. And ask how you can get care if you have a health emergency out of town.

--Do you require a payment even for approved E.R. visits? Most managed-care plans require a co-payment, which typically ranges from $25 to $50, for emergency-room trips.

Of course, if you have symptoms that need immediate care and you can't reach your doctor, you should go straight to the nearest emergency room anyway. You can always appeal a rejected claim later.

--Karen Cheney and Thomas Schwarz