Federal workers and retirees who anticipate heavy medical bills next year need to be especially careful when they pick a health insurance plan.

The open season for chosing 1985 health plans ends Friday for most nonpostal federal workers. The U.S. health program covers nearly half the people in the Washington area.

Retirees who are making health insurance changes by mail should have their letters postmarked no later than Friday.

The U.S. Postal Service has extended the open enrollment period for its 600,000 workers -- but not for postal retirees -- until Dec. 21. It decided to grant the extra time because the recent major fire at headquarters disrupted its health insurance operations, and there also has been a shortage of change-of-insurance forms.

Employes who anticipate "catastrophic" medical bills (in which out-of-pocket costs for covered items exceed $2,000 a year) can buy good protection in the federal health program, if they know how to shop.

The Washington Consumers Checkbook recommends that persons seeking catastrophic coverage consider most of the health maintenence organizations (HMOs) available locally. Checkbook says most of them provide excellent coverage at relatively low cost.

Standard health insurance plans that Checkbook rates highly are: the Foreign Service Benefit Plan (limited to certain employes and retirees); Government Employees Hospital Association; Postal Supervisors; National Association of Letter Carriers; Special Agents Mutual Benefit Association (limited to certain groups), and the American Federation of Government Employees.

Also, Government Employees Benefit Association (for NSA personnel only); National Treasury Employees Union standard option and the National Association of Government Employees standard option plan.

Also rated highly for single persons seeking the best protection against catastrophic medical costs are the National Alliance of Postal and Federal Employees plans.

Bear in mind that none of the plans will pay all of the bills for catastrophic out-patient treatment of mental disorders or dental bills.

This is the final column in our series about rating health plans. On Nov. 5 we listed Checkbook's "best buys" for an average federal family. On Nov. 11 we covered the plans with the best dental coverage. On Nov. 18 we listed the best buys for retirees with and without Medicare coverage.

Copies of the Checkbook rating guide are still available at newsstands, or from the publication's office at 806 15th St. NW, Suite 925 (telephone 347-9612).