Senior citizens wrestle with a complex health issue: Medicare. What does the federal health plan for people 65 and older cover? What doesn't it cover? Is additional coverage necessary? If so, how much?

The Montgomery County Office of Consumer Affairs publishes a "Medicare Supplement Insurance Guide" that describes the health expenses not covered by the federal plan and provides tips on choosing a comprehensive supplemental plan to fill in the gaps. The guide also surveys 31 different Medicare supplement insurance plans in Maryland.

"Medicare does not cover everything when you turn 65," said Judy Doctor, manager of consumer education. "You need another plan so a tremendous medical bill won't wipe out your savings."

When choosing another insurance plan, Doctor suggests the following guidelines:

*Be wary of buying more than you need. "People think if you buy multiple policies, that each will pay, and that is not necessarily the case."

*Watch out for plans that provide fixed benefits. "You want a plan that would keep up with changes that occur because of inflation."

*Be aware that Medicare, as well as most supplement plans, do not cover custodial nursing home care (when a person needs help with the daily necessities of life).

*If you have a chronic problem requiring continual treatment and are thinking of changing insurance plans, realize that new plans are probably not going to cover preexisting conditions.

County residents can receive a copy of the guide by sending 39 cents in stamps to: Office of Consumer Affairs, 100 Maryland Ave., Rockville, Md. 20850. Non-residents should send the stamps plus $1. For more information call: 251-7373.