Many of the 169,000 federal workers injured on duty each year could be forced to pay 7 percent of their medical bills, which have been fully paid by the government, as a consequence of a change in disability rules that went into effect Saturday, unions representing the workers say.

Because of requirements of the new federal deficit reduction legislation, the Labor Department must trim 7.3 percent from all disability medical bills that are paid after March 1, even if the bills were submitted months ago.

That means the government will pay only about $92 of every $100 in medical charges when civil servants are treated for job-related injuries. Last year, those bills amounted to more than $170 million.

Federal unions think doctors will require employes to make up the 7.3 percent of the bills disallowed by the government. Doctors who are reimbursed directly by the government will face the cuts as well. And the unions contend there is nothing to stop them from raising fees to make up the difference.

Nobody knows how it's going to work, but somebody somewhere along the line is going to lose 7.3 percent.

Postal unions are particularly upset because most of the 85,000 injuries requiring time off and medical care sustained by employes each year occur in the U.S. Postal Service as the result of dog bites, wrecks or falls.

The rules change will affect thousands of federal workers in the Washington area, as well as the doctors and hospitals that treat them. About 14 percent of the federal work force is located here.

Every year about half the feds hurt on the job require outside medical care and lose time from work.

The Office of Management and Budget and the Congressional Budget Office ordered the change to comply with the Gramm-Rudman- Hollings deficit reduction requirements. That legislation directs most nondefense federal agencies to cut spending 4.3 percent during the remainder of the 1986 fiscal year that ends Sept. 30.

Federal employes injured on the job are paid full salaries for the first 45 days they have to stay home disabled.

But if they don't recover after that, they may be placed on disability status. That qualifies single employes for tax-free payments equal to 66 2/3 percent of salary. Workers with at least one dependent receive benefits equal to 75 percent of pay.

Those benefits will not be changed. Drug Test Protest

Many feds are unhappy with the recommendation of a presidential commission that civil servants be required to take tests to determine if they are drug users.

An Interior Department employe called to say that a colleague is encouraging fellow workers to send, uh, bottled specimens to the official of their choice to protest the plan. The folks who work in government mail rooms are probably hoping the movement doesn't catch on. Retirement Recommendations

The Post Office-Civil Service Committee says the House should ignore presidential proposals to raise the federal retirement age from 55 to 62 and raise employe pension contributions from 7 percent of salary to 9 percent.

The committee, which oversees the civil service, sent its recommendations to the House Budget Committee, which is working up an alternative to the budget recommended by the president.