Tuberculin Skin test
z False positive tests occur because of previous BCG vaccine or exposure to non tuberculous mycobacteria.
z False negatives occur because of T cell mediated immune deficiency due to drugs or disease.
z Up to 30% of patients with active TB will have a negative skin test making it poor for diagnostic purposes.
z Administered intradermally and then read in 48-72 hours requiring the patient to return for a reading.
z Both administration and the reading itself requires skill to be performed correctly.