A PPD (purified protein derivative) intermediate skin test consists of 0.1 ml (5 tuberculin units) of PPD intradermally, usually on the volar surface of the forearm and read at 48 to 72 hours, recording the size of induration, not redness.
This test is not 100% sensitive for mycobacterium tuberculosis infection. Positive reactions also occur in patients who have received BCG (Bacillus of Calmette and Guerin) vaccination in the past as is often seen in patients from countries with a high incidence of TB infections.
Infections with atypical Mycobacteria (e.g. Mycobacterium avium complex (MAC) often seen in AIDS and/or HIV positive patients) can also result in a reaction to PPD.
Immunosuppressed patients (anergic), on the other hand, may have a negative PPD even though they have active TB or were infected with TB in the past.
In addition 10 to 20% of non-immunosuppressed patients, particularly those who are poorly nourished and/or protein-depleted, may have a negative PPD, even though they have active TB or were infected with TB in the past.