the six puncture points coalesce to form a circle; Grade 3: induration of 5 mm; or more Grade 4: induration of 10 mm or more, or ulceration The results of both tests are roughly equivalent as follows: Heaf grade 0 & 1 ~ Mantoux less than 5 mm; Heaf grade 2 ~ Mantoux 5–14 mm; Heaf grade 3 & 4 ~ Mantoux 15 or greater Tuberculin conversion Tuberculin conversion is said to occur if a patient who has previously had a negative tuberculin skin test develops a positive tuberculin skin test at a later test. It indicates a change from negative to positive, and usually signifies a new infection. Boosting The phenomenon of boosting is one way of obtaining a false positive test result. Theoretically, a person's ability to develop a reaction to the TST may decrease over time – for example, a person is infected with latent TB as a child, and is administered a TST as an adult. Because there has been such a long time since the immune responses to TB has been necessary, that person might give a negative test result. If so, there is a fairly reasonable chance that the TST triggers a hypersensitivity in the person's immune system – in other words, the TST reminds the person's immune system about TB, and the body overreacts to what it perceives as a reinfection. In this case, when that subject is given the test again (as is standard procedure, see above) they may have a significantly greater reaction to the test, giving a very strong positive; this can be commonly misdiagnosed as Tuberculin Conversion. This can also be triggered by receiving the BCG vaccine, as opposed to a proper infection. Although boosting can occur in any age group, the likelihood of the reaction increases with age. Boosting is only likely to be relevant if an individual is beginning to undergo periodic TSTs (health care workers, for example). In this case the standard procedure is called two-step testing. The individual is given their first test and in the event of a negative, given a second test in 1 to 3 weeks. This is done to combat boosting in situations where, had that person waited up to a year to get their next TST, they might still have a boosted reaction, and be misdiagnosed as a new infection. Here there is a difference in US and UK guidelines;