Sensitivity of the T. Pallidum Immobilization (TPI) TestA Function of the Number of Spirochaetes in the Antigen

Abstract

The TPI test is most sensitive when there is only a single treponeme per high dry darkfield, and sensitivity decreases when 5 or 15 treponemes are present per high dry darkfield. This is evidenced both by a decrease in titer, and by rendering doubtful or negative results in tests which are positive when there is only a single treponeme per darkfield. Since the routine use of an antigen containing only one treponeme per darkfield is impractical, dilution of the antigen to 5 treponemes per darkfield is advised. If a doubtful reaction is obtained with this test, repeat the test with only a single treponeme per darkfield.