in the US testers are told to ignore the possibility of false positive due to the BCG vaccine, as the BCG is seen as having waning efficacy over time. Therefore, the CDC urges that individuals be treated based on risk stratification regardless of BCG vaccination history, and if an individual receives a negative and then a positive TST they will be assessed for full TB treatment beginning with X-ray to confirm TB is not active and proceeding from there. Conversely, the UK guidelines acknowledge the potential effect of the BCG vaccination, as it is mandatory and therefore a prevalent concern – though the UK shares the procedure of administering two tests, one week apart, and accepting the second one as the accurate result, they also assume that a second positive is indicative of an old infection (and therefore certainly LTBI) or the BCG itself. In the case of BCG vaccinations confusing the results, Interferon-γ (IFN-γ) tests may be used as they will not be affected by the BCG. Interpretation According to the U.S. guidelines, there are multiple size thresholds for declaring a positive result of latent tuberculosis from the Mantoux test: For testees from high-risk groups, such as those who are HIV positive, the cutoff is 5 mm of induration; for medium risk groups, 10 mm; for low-risk groups, 15 mm. The U.S. guidelines recommend that a history of previous BCG vaccination should be ignored. For details of tuberculin skin test interpretation, please refer to the CDC guidelines (reference given below). The UK guidelines are formulated according to the Heaf test: In patients who have had BCG previously, latent TB is diagnosed if the Heaf test is grade 3 or 4 and have no signs or symptoms of active TB; if the Heaf test is grade 0 or 1, then the test is repeated. In patients who have not had BCG previously, latent TB is diagnosed if the Heaf test is grade 2, 3 or 4, and have no signs or symptoms of active TB. Repeat Heaf testing is not done in patients who have had BCG (because of the phenomenon of boosting). For details of tuberculin skin test interpretation, please refer to the BTS guidelines (references given below). Given that the US recommendation is that prior BCG vaccination be ignored in the interpretation of tuberculin skin tests, false positives with the Mantoux test are possible as a result of: