A heaf test is a diagnostic skin test performed in order to determine whether or not a child has been exposed to tuberculosis. The heaf gun injects a purified protein derivative – a testing serum – into the skin making six small punctures. The patient is then re-examined between two and seven days later and the level of blistering at the puncture site determines whether the patient needs a vaccination or whether they have already experienced a mild form of the disease. As the box indicates, this apparatus could be used multiple times so long as it was boiled and disinfected before re-use. Today it is much more common to have a heaf gun with disposable, single-use heads.