PRACTITIONERS and friends know that The American Musical is a very sick patient. For diagnosis and treatment of TAM, a unique antiseptic laboratory is being created on the Kennedy Center's terrace level.

The space, which was devoted during the Bicentennial to a study of past performing arts, now contains test tubes for TAM's future. Enveloped in dark drapes, a grid of lights above, the working area consists of a few platforms and a stairway that can be rolled anywhere.Vari-colored coverings will protect the center's rich wood flooring and indicate performing areas.

Like church pews covered with red carpeting, rows of seats can fixed on metal poles fitted into holes of platforms and can be patterned anywhere it's decided an audience of 110 will sit.

This is the physical setting for a "Musical Theater Lab." Under director Edward Berkeley and with Patricia Birch as its first choreographer, ten professional musical stage performers have been rehearsing a new musical that will have free public performances for two weeks starting March 15.

There will be performances Tuesdays through Saturdays at 8 p.m. and Sunday afternoons at 2 p.m. on a first-come, first-served basis. No reviews of Lab productions will be printed or aired.

No one is assuming that the health of TAM automatically will improve, but it is a considered and not inexpensive treatment. The lab's benefactors are concerned producers willing to return past profits for the future health of this uniquely American creature.

The American Musical has been this country's contribution to world theater, the telling of a story through seamless fusion of words, songs, dances and an almost indefinable esprit. It's less formal than its major progenitors, European opera and opereta. Verdi and Puccini, Offenbach, the Strauses and the Strausses and Gilbert and Sullivan would recognize the offspring but note the variations.

But Romberg and Kern, Rodgers and Hart, Rodgers and Hammerstein, Adler and Herman, Styne and Sondheim, Lerner and Loewe, de Mille and Robbins, Kidd and Prince, Bernstein and Comden and Green injected a fresh verve into inherited traditions. Such works as "pal Joey," "Oklahoma," "South Pacific," "West Side Story," "Guys and Dolls," "Fiddler on the Rood," "Hello, Dolly!," "Gypsy" and "Grease" have been universally recognized as expressing our national buoyancy.

Lately economics and solemnity have threatened the TAM's existence.

The cost of moving a splendidly elaborate "My Fair Lady" revival in New York cancelled a national tour. Recent economic disasters are too close to call. Fine musical stages have no new works to show.

The passion for overt preaching - "relevance" - has dampened high spirits. If analyzed, the subject matter of many musicals can be seen as serious indeed, social preaching in "My Fair Lady" and "Hello, Dolly!" tolerance in "West Side Story" and "Fiddler," integration in "South Pacific" and "The King and I." But the American spirit of affirmation is seen by solemn pundits as "sugar-coating" to be scorned. Previously recognized as an attribute of sophistication, humor is now misapplied to cruelty or simple-mindedness.

Stuart Ostrow, whose "1776" and "Pippin" had their first enthusiastic receptions here, has plowed some of the profits from these shows into a foundation that created the lab in New York four years ago. Now the Kennedy Center's Roger L. Stevens, a vital backer of "West Side Story" and savior of "Pippin," has united the Kennedy Center and Ostrow's lab. Using the Center's space, a first-season program of four new works has been budgeted for $120,000 through summer. It's likely that next season will continue the series with eight new works.

Berkeley, the director, is a 31-year old musical enthusiast who sees the form as "emotional release through song."

Berkeley's first view of TAM was "Damn Yankees." He was 9 at the time, and his New Yorker parents took him to the theater regularly. He's so steeped in TAM that when I reached for the title of a show few saw or remembered, he came up with it, "A Family Affair." At 16, Berkeley also picked out in the chorus Linda Lavin., star of TV's "Alice." Those musical injections set his life course. He trained at Minnesota's Carleton College and the University of Iowa, where returns regularly to teach theater.

Berkeley recognizes that large casts, large orchestras and elaborate productions must yield to smaller numbers and simpler surroundings.

He made his name with Off-Broadway productions, as co-founder of Shade Company, as director for Joseph Papp productions an as associate director of summer's Williamstown (Mass.) Theather Festival. The Shade's "Dr. Hero," by Israel Horowitz, attracted attention. He had experimental ideas for "The Tempest," "Macbeth," "Pericles" and "A Midsummer Night's Dream" for Papp's stages. He made a modest splash with last season's "The Primary English Class," and his direction of a Lab production, "Saints" by Merie Kessler and Bill Penn, obviously impressed Ostrow, who gave him his present assignment.

First on his program is "Hot Grog," with book and lyrics by Jim Wann to music by Bland Simpson. The scheme is for a four-week rehearsal period to be followed by two weeks of performances. Future scripts aren't yet all decided.