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When it comes to staving off skyrocketing medical costs, having insurance is only half the battle. That's because cost-sharing — expenses you cough up in the form of deductibles, copayments and coinsurance — has been increasing in employer plans, according to the Kaiser Family Foundation. The foundation recently analyzed medical bills from large employer plans and found that nearly 1 in 5 inpatient hospital admissions include a claim from an out-of-network provider.

The cost difference can be significant when you go in-network: Doctors and facilities that are in-network have agreed with your plan to a set price for service. Your insurer may only partially cover an out-of-network expense and leave you to pay the remainder. Even going to an in-network facility won't necessarily keep you from encountering out-of-network costs. More than 15 percent of in-patient admissions who used an in-network facility still wound up with a claim from an out-of-network provider, Kaiser found. Here are four tips from Carolyn McClanahan, a certified financial planner and an M.D., to help you become an empowered patient and prepare you to contest unreasonably high bills. McClanahan is director of financial planning at Life Planning Partners in Jacksonville, Florida.

1. Understand your coverage

Know what your policy covers, including the details on out-of-pocket maximums, deductibles and co-insurance. Get familiar with your state's laws against "balance billing" — a practice in which insurers only partly cover an out-of-network cost and leave consumers to pay the remainder, which can be thousands of dollars. More than 20 states have some kind of legislation on the books to protect consumers from balance billing, according to the Commonwealth Fund. See below.

When you are admitted to the hospital in the event of an emergency, write on all of your paperwork that you will only permit in-network care, McClanahan said. Be sure to take a photo of this document or make a copy of it. "Once you get the bill, if they charge you for out-of-network care, you have ammunition," McClanahan said.

2. Ask the right questions

Ambulances are parked outside the emergency room at Good Samaritan Hospital in San Jose, California. Noah Berger | Bloomberg | Getty Images

"Will you take my insurance?" isn't enough to head off an out-of-network charge. "When you're making an appointment, don't ask if you're covered under your plan," said McClanahan. "Everyone will take your insurance." Your doctor's office may accept your insurance, but that won't spare you from a surprise bill weeks later if he or she is out of your network, McClanahan said. The correct question for your doctor's receptionist is, "Are you in my network?" Be sure to document who gave you the answer and the date so that you're prepared to fight back if you get a bill that says otherwise, McClanahan said. Speak up when your doctor recommends tests and prescriptions. Ask questions such as: "Is the testing facility in-network?" or "Are the doctors evaluating the tests in-network?"

Once you get the bill, if they charge you for out-of-network care, you have ammunition. Carolyn McClanahan director of financial planning, Life Planning Partners

Don't be shy about asking your doctor for an explanation if he or she recommends that you undergo a test. Learn more about the test and find out how the results will affect the approach to treatment. "If they can't provide a clear answer, ask, 'Is this test really necessary?'" McClanahan said. Finally, curb costs on prescriptions by asking whether your medication is covered under your plan and if a cheaper version is available. Sometimes it's cheaper to pay cash for your medication than to claim it on your insurance, McClanahan said. She suggested digging up drug price information on GoodRx.com and comparison shop. "Know the benefits and risks, how long will you be on medication, and what are the alternatives to the medication prescribed," McClanahan said.

3. Keep records

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Maintain a health-care log so that you're equipped to fight back in the event of a dispute with your insurer. Keep track of any communications you have with your insurance company and your doctor. If needed, hire an expert to act as your advocate, McClanahan said. She suggested reaching out to the Alliance of Claims Assistance Professionals.

4. Call your regulator