A 1 ml syringe and size 26 needle are used to inject 0.1 ml of tuberculin (PPD) into the skin (intradermally) over the inner side of the left forearm. It is very important that the tuberculin is injected into the skin and not under the skin (subcutaneously). If the tuberculin is correctly injected into the skin a raised, pale weal of 5 to 10 mm is formed. If no weal is raised, the tuberculin has been injected too deep in error.
Incorrect injection under the skin may make the result difficult to interpret.