A positive tuberculin skin test, or PPD test, is characterized by:

a. erythema and induration.
b. redness and rash.
c. swelling and erythema.
d. edema and rash.


A
The tuberculin skin test, or PPD test, is based on the premise that after being infected with M. tuberculosis, a patient will develop a delayed hypersensitivity cell-mediated immunity to certain antigenic components of the organism. To determine whether a person has been infected with M. tuberculosis, a culture extract of M. tuberculosis (i.e., PPD of tuberculin) is intracutaneously in-jected. After 48 to 72 hours, a person who has been infected will exhibit a delayed hypersensitiv-ity reaction to the PPD test. This reaction is characterized by erythema (redness) and, most im-portant, induration (i.e., firmness as a result of an influx of immune cells). The diameter of indu-ration is measured and then interpreted as to whether the patient has been infected with M. tu-berculosis.

Biology & Microbiology

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