Protection against false positives

It is often necessary to add preservatives to specimens to protect against postmortem changes. For example, the addition of sodium fluoride to a tissue specimen can prevent the production of bacterial ethanol (which can potentially yield a false positive result for the presence of ingested ethanol). 

There is also a vaccine for TB, the BCG vaccine, made from live attenuated M. bovis [301]. The WHO recommends vaccination of infants in parts of the world where TB is common. However, BCG does not protect adults very well against TB, and people who have received the vaccine may show a false positive reaction on the TB skin test. Because of these limitations, BCG is rarely used in the US [302]. 

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