If you're concerned about the large, out-of-pocket costs from being on Medicare, you may consider buying additional protection through a Medicare Supplement plan. However, because these are private insurance policies and not government programs, they don't accept every applicant.

What does that mean in practice? Can you be denied a Medicare Supplement Plan? Here's how applying for these policies works — as well as how to maximize your chances of qualifying.

How Do Medicare Supplement Plans Work?

Also known as Medigap, Medicare Supplement plans pay for medical expenses not covered by the government. For example, if you need to stay in the hospital, you'll be responsible for a deductible up to $1,484 under Medicare Part A plus 20% of all doctor services under Medicare Part B. Especially for serious medical issues, these out-of-pocket expenses can really add up.

With a Medigap plan, you pay a monthly premium. In exchange, the insurance company covers large out-of-pocket expenses when you seek out health care. What your Medicare Supplement plan pays for will vary by which type you buy. They are labeled A through N, each with different coverage amounts.

In all situations, the plan is run by a private insurance company. Their private status gives them some flexibility to decide which applicants to accept based on government regulations.

Can You Be Denied a Medicare Supplement Plan?

It depends on when you apply. When you first join Medicare Part B at age 65 or older, you'll qualify for an open enrollment period. This period lasts six months, starting the month you first join Medicare Part B. During this time, you can sign up for any Medicare Supplement plan available in your state and are guaranteed to qualify.

Can you be denied a Medicare Supplement plan after this period ends? Yes, there is a chance. The insurance company will pull your medical records and ask you questions about your health — if you have serious preexisting health conditions, then the insurance company could charge you a higher rate or potentially deny your application.

Four states offer extended application periods: Connecticut, Maine, Massachusetts and New York. During this time, insurers cannot deny you a Medigap plan. Depending on the state, you can either apply any time after turning 65 or only at certain points during the year in special enrollment periods. In both cases, you would qualify even with preexisting conditions. If you live anywhere else in the country, it's possible for you to be denied a Medicare Supplement plan for health reasons.

How Can You Improve Your Chances of Qualifying?

The easiest way to qualify is by applying during your Medicare open enrollment period. That way, you can sign up for any plan you want without worrying about rejection. You can also change your mind and switch plans without being rejected during this stretch.

If you're outside the open enrollment period, the next best move is to apply as soon as possible. Applying while younger means you may have fewer serious preexisting conditions — someone in their 60s, for example, will likely have a better chance of passing the health standards compared with someone in their 80s.

Another strategy is to apply with multiple insurance companies. Each company has different rules for who qualifies, so a condition that bars you from one program might not disqualify you from another.

What Should You Do if You're Denied?

If it doesn't look like you can qualify for a Medicare Supplement plan, you have another alternative: Medicare Advantage. This is a private insurance program that covers out-of-pocket costs for Medicare enrollees.

It has a different sign-up process than you might see with Medicare Supplement plans. You can enroll in Medicare Advantage when you first join Medicare. After that, you can only sign up at certain points during the year — October 15 to December 7 — during which you can sign up and qualify no matter what, even if you have health issues.

As you do your research, consider meeting with a health insurance agent who is familiar with both programs. They can help you compare your options and also prepare your application to give you the best shot at qualifying.