THE JUSTICE Department's anti-trust policy drifts further and further from reality. The latest example is the letter the department sent to a health planning agency in Richmond, warning it that some of its work might violate the anti-trust laws, The department's lawyers seemed particularly offended by the health agency's attempts to enlist hospitals' cooperation in consolidating overbuilt and underused services.That looked to Justice like an illegal agreement to restrict competition.

This interpretation of the law aplies to all of the 200 or so health planning agencies set up throughout the country under the National Health Planning and Resources Development Act of 1974. These agencies are publicly financed, nonprofit groups established to bring some order to a rapid and recklessly costly expansion of hospitals and medical services. The further absurdity of the Justice Department's letter is that neither it nor anyone else is actually likely to sue any of these regional agencies for anti-trust damages. But the letter provides an ironclad excuse and defense to anyone who doesn't care to cooperate with the health planners.

Competition in the field of hospital care does not keep the bill down for the public but -- notoriously -- pushes it up as doctors and administrators vie with each other for expensive equipment. The Justice Department knows that perfectly well. But it can't seem to disentangle itself from the rules worked out years ago in dealing with oil trusts and conspiracies among light-bulb manufacturers. a

The department shook a warning finger at, specifically, a plan in central Virginia to consider reorganization obstetrical services in the hospitals there. Not long ago in Massachusetts, a lot a small neighborhood hospitals, with too few maternity cases to support adequate staffing and equipment, were persuaded to get out of the baby business altogether. A few hospitals were developed as regional center for difficult deliveries. Infants lived who otherwise would have died. The Justice Deparment says that if health planning agencies try anything like that in Virginia, they will be in violation of the anti-trust statues.

Those statutes, and the tradition-bound manner in which the Justice Department administers them, have become obsolete in many parts of the economy. They do not apply sensibly to large area of foreign trade. They produce strange results inhighly concentrated industries. But in planning for medical care, they are not only obsolete. They undercut the public's real interests.