How is latent tuberculosis infection diagnosed?
For decades, evidence of skin test reactivity to tuberculous antigens has been used to determine if someone has latent tuberculosis infection (LTBI) (6). The Mantoux tuberculin skin test (TST) has been the standard method of identifying persons with LTBI. The Mantoux tuberculin skin test is performed by injecting 0.1 ml of purified protein derivative (PPD) intradermally (between layers of skin) into the volar (inner) surface of the forearm (Figure 10) (http://www2a.cdc.gov/podcasts/player.asp?f=3739). An individual who has been trained to read skin tests should read the test 48 to 72 hours later. The diameter of the indurated area should be measured across the forearm (perpendicular to the long axis). The amount of erythema (redness) is not measured. The appropriate cutoff for defining a positive TST depends on the individual’s risk factors for tuberculosis (6) (Table 1). For persons who are severely immunocompromised, have scarring on the chest x-ray, or have had recent contact with i
