The Mantoux test, usually referred to on the series as a PPD (for purified protein derivative) is a diagnostic test for tuberculosis. It is preferred to sputem cultures in many cases because it can also detect the disease when it is latent. However, it does have it's limitations. False negatives are also common. AIDS patients with very low T-cell counts will often test negative even when they have the disease, as do others who are immunocompromised. Patients who are using steroids, who have malnutrition or suffer from sarcoidosis as well usually have a false negative.