What is coordination of benefits, and how does it work when someone is covered by two health insurance plans?

Coordination of benefits (COB) applies when you are covered by more than one health insurance plan — for example, through your own employer and your spouse's employer. The primary plan pays first according to normal benefit terms, and the secondary plan may then cover some or all of the remaining patient responsibility, including co-pays and coinsurance. COB rules prevent double-dipping (receiving more than 100% of the total cost), but when properly coordinated, dual coverage can dramatically reduce or eliminate out-of-pocket expenses. Understanding which plan is primary — usually determined by the birthday rule for dependents — is essential for filing claims correctly.