the red area is not what is being measured.
here's a little information for you from http://www.medicinenet.com/tuberculo...test/page1.htm
"the tuberculin skin test is based on the fact that infection with m. tuberculosis
produces a delayed-type hypersensitivity skin reaction to certain components of the bacterium. the components of the organism are contained in extracts of culture filtrates and are the core elements of the classic tuberculin ppd (also known as purified protein derivative). this ppd material is used for skin testing for tuberculosis. reaction in the skin to tuberculin ppd begins when specialized immune cells, called t cells
, which have been sensitized by prior infection, are recruited by the immune system to the skin site where they release chemical messengers called lymphokines. these lymphokines induce induration (a hard, raised area with clearly defined margins at and around the injection site) through local vasodilation (expansion of the diameter of blood vessels) leading to fluid deposition known as edema, fibrin deposition, and recruitment of other types of inflammatory cells to the area
"reading" the skin test means detecting a raised, thickened local area of skin reaction, referred to as induration. induration is the key item to detect, not redness or bruising.
skin tests should be read between 48 and 72 hours after the injection when the size of the induration is maximal. tests read after 72 hours tend to underestimate the size of the induration.
the area of induration (palpable, raised, hardened area) around the site of injection is the reaction to tuberculin. again, redness is not measured