BCG vaccination can cause a false positive reaction to a TB skin test (TST). The gold standard for a positive TST used to be a chest x-ray, then sputum cultures if indicated clinically. The drawback of using a CXR alone, is that the CXR cannot determine latent TB status. It will show evidence of active disease or damage from previous disease. Mycobacterium tuberculosis can remain dormant for a significant period before a person shows signs of an infection clinically. This is a latent TB infection (LTBI). In the last 5 years, two different blood tests have been developed that are being used to better determine a person's LTBI status. They have been shown to be more specific than a TST and can detect a LTBI. These two blood tests are Quantiferon TB Gold and the T-spot TB Test. A person who has a positive Quantiferon TB Gold or T-spot TB Test is considered to have a LTBI if there are no other clinical signs of TB present (including a negative CXR). Because of the specificity of the the two tests, a false positive is rare and is usually the result of improper draw technique, processing time, etc. Essentially, if you have a positive TST, but a negative Quantiferon or T-spot, it is safe to say that you are negative for TB. On college and university campuses where there is large scale TB testing for incoming international students, many have gone to using these blood tests, either exclusively or in conjunction with skin testing. However, I haven't seen as many healthcare facilities using it until recently. Before going back to the ER recently, I worked for the last 3 years in college health as the nurse manager for the student health center at a major university that did a significant amount of TB testing. We used both the skin test and the blood test in conjunction.