The tuberculosis rate in the District, which has been among the highest in the country and an embarrassment to the city, dropped 30 percent last year to the lowest rate in 30 years, Mayor Marion Barry announced yesterday.

The mayor said that the number of new reported TB cases in the city dropped from 341 in 1980 to 239 in l981. But one physician active in the fight against the disease said that the statistics cited by the mayor may be misleading.

Dr. Alfred Munzer, chairman of the tuberculosis committee of the District Chapter of the American Lung Association, said the numbers are lower because city health officials adopted a new system for counting cases in 1981. Munzer says that the dimension of the decrease cited by Barry is "biologically impossible."

"It would be very sad if the mayor tried to make it appear that tuberculosis in the city was under control," Munzer said.

Munzer said that his assessment was based on the nature of the bacterium that causes the disease. It can remain latent in a person for years before it triggers an "active" case, Munzer said, meaning that any person harboring the bacterium in a latent state must have been treated within the year for any statistics detailing such a sharp decrease to be credible.

This is not the case, Munzer contended, under the new reporting system.

Dr. Martin Levy, head of the District's Disease Prevention Administration, conceded that a new reporting system is in effect, but denied that it distorts the incidence of tuberculosis in the city.

Levy explained that under the old system all suspected tuberculosis cases reported to the TB control program by doctors and health clinics were included in statistical reports unless tests specifically ruled out the disease in an individual case. But in 1981, only medically confirmed cases were included, Levy said, which contributed to the lower numbers cited by the mayor.

Dr. Hazel Swann, director of the city's TB Control Program, said the decrease in confirmed cases has reduced the rate of the disease from 53 per 100,000 people here to 37. She pointed out, however, that Washington still ranks in the top 10 cities in the number of tuberculosis cases. The national average is 19 cases per 100,000 people, she said.

Swann said that "we think the drop is because in 1981, everyone physicians and clinics agreed to use Isoniazid and Rifampin." This double-drug therapy is a relatively new treatment for tuberculosis.

Swann said the treatment alleviates the hacking cough that is characteristic of TB patients, and "once you stop the cough, you stop the spread of the disease." The net result is that fewer people are spreading the disease through coughing, she said.

Swann also said that, because of continuing staff and supply shortages in 1981, she decided to discontinue the program's free X-ray screening and concentrate primarily on keeping track of TB patients under treatment. TB outreach personnel and public health nurses went into homes if necessary to ensure that patients took their medicine. Patient compliance in is often difficult in TB cases because medication must be taken for periods up to 24 months.