How is reading of tb test?

➢ The reaction is read by measuring the diameter of induration (palpable raised, hardened area) across the forearm (perpendicular to the long axis) in millimeters. ➢ If there is no induration, the result should be recorded as "0 mm". ➢ Positive responses require the subject's exposure to the Ag at least 4-6 weeks prior to skin testing. ➢ The lack of a DTH response to a recall Ag is often regarded as an evidence of anergy. ➢ In the absence of underlying diseases, anergy may indicate primary or secondary T-cell immunodeficiency ➢ The same cell-mediated mechanisms are elicited by an actual infection with the living microbes, in which case the inflammatory response continues, and the ensuing tissue damage and granuloma formation can cause serious damage