Sgt. John Hart responded to hundreds of emergency calls as a Fairfax County paramedic and had an unblemished record, but the county decided a year and a half ago that he was physically unfit for duty. The reason: Hart has diabetes.

He was told he would be forced to retire on a disability pension of $8,000 a year. The only option would be to take a desk job, but none seemed available at that time. County officials contended that Hart's medical condition made him a potential danger to himself and to others.

Hart is still employed by the fire department, monitoring the department's radio and telephone equipment. "Everything's going great," he said last week. Then, the following day, Hart got word that the county had reclassified him as permanently disabled. This means he is ineligible for promotion to a lieutenant, even though there are some lieutenant desk jobs, including one he has already applied for.

Hart discovered in 1982 that he had insulin-dependent diabetes mellitus, a condition controllable with medication. He continued to pass the physical examinations required of firefighters, and he argued with the county over the nature of his condition.

Hart says he is not sure what he will do now. His application for the lieutenant job is still pending. If he is turned down for that promotion, as he thinks he will be, he will consider filing a grievance, he said.