PPD from human tuberculin (from M. tuberculosis), bovine (M. bovis) and avian (M. avian) PPDs have also been made for testing against other species of Mycobacteria.

PPD consists of hundreds of different proteins from the tuberculosis bacterium and the bioassay method does not cover all the differences between batches. Products calibrated to the same "strength" on the same standard can still show slight differences in how they produce reactions in people receiving the test.It has also been argued that calibration results from single-point