You may have wondered how Medicare works with other insurance coverage, such as a group health plan from a former employer or union.

It’s an important issue because it determines whether your medical bills are paid correctly and on time.

If you have Medicare and other insurance, always be sure to tell your doctor, hospital, and pharmacy.

When there’s more than one insurance payer, certain rules determine which one pays first.

This is what’s called “coordination of benefits.”

The “primary payer” pays what it owes on your bills first – and then sends the balance to the “secondary payer” to pay.

In some cases, there may also be a third payer.

The primary payer pays up to the limits of its coverage. The secondary payer only pays if there are costs the primary insurer didn’t cover.

But keep in mind that the secondary payer (which may be Medicare) may not pay all of the uncovered costs.

If your employer insurance is the secondary payer, you may need to enroll in Medicare Part B before that insurance will pay. (The Part B premium for most Americans in 2013 is $104.90 per month.)

Here’s who pays first in various situations:

If you have retiree insurance (coverage from a former job), Medicare pays first.

If you’re 65 or older, have group coverage based on your or your spouse’s current employment, and the employer has 20 or more workers, your group plan pays first. (If the company has fewer than 20 employees, Medicare pays first.)

Your group plan also pays first when you’re under 65 and disabled, have group coverage based on your or a family member’s current employment, and the employer has 100 or more employees. (Medicare pays first if the company has fewer than 100 employees.)

If you have Medicare because of end‑stage renal disease (permanent kidney failure), your group plan pays first for the first 30 months after you become eligible for Medicare. Medicare pays first after this 30‑month period.

Medicare may pay second if you’re in an accident or have a workers’ compensation case in which other insurance covers your injury or you’re suing another entity for medical expenses.

In these situations you or your lawyer should tell Medicare as soon as possible. These types of insurance usually pay first for services related to each type:

* No-fault insurance (including automobile insurance);

* Liability (including automobile and self-insurance);

* Black lung benefits;

* Workers’ compensation.

Medicaid and TRICARE (the healthcare program for U.S. armed service members, retirees, and their families) never pay first for services that are covered by Medicare. They only pay after Medicare, employer plans, and/or Medicare Supplement Insurance (Medigap) have paid.

For more information, visit www.medicare.gov/publications and read the booklet “Medicare and Other Health Benefits: Your Guide to Who Pays First.”

You can also call 1-800-MEDICARE (1‑800‑633‑4227) and ask for a copy to be mailed to you. TTY users should call 1‑877‑486‑2048.

If you have questions about who pays first, or you need to update your other insurance information, call Medicare’s Coordination of Benefits Contractor at 1‑800‑999‑1118. TTY users should call 1-800-318-8782.

You can also contact your employer or union benefits administrator. You may need to give your Medicare number to your other insurers so your bills are paid correctly and on time.

David Sayen is Medicare’s regional administrator for California, Arizona, Nevada, Hawaii, and the Pacific Trust Territories. You can always get answers to your Medicare questions by calling 1-800-MEDICARE (1-800-633-4227).