While reading a tuberculin test, the most important criteria is?

#PULMONOLOGY 
While reading a tuberculin test, the most important criteria is?

A. Erythema
B. Induration 
C. Erythema + induration
D. None of the above

Figure 1 and 2 below

Ans: Option B✔

The TB skin test is also called the Mantoux tuberculin skin test (TST).
-The test is performed by injecting a small amount of fliud (called tuberculin) into the skin on the lower part of the arm (anergy testing is not recommended).
-A person given the tuberculin skin test must return within 48 to 72 hours to have a trained health care worker look for a reaction on the arm. 
- The area of induration (not erythema) is measured in millimeters in the axis perpendicular to the long axis of the arm (figure 1).
POSITIVE SKIN TEST : This means the person's body was infected with TB bacteria. Additional tests are needed to determine if the person has latent TB infection or TB disease. 
NEGATIVE SKIN TEST: This means the person's body did not react to the test, and that latent TB infection or TB disease is not likely. 
There is no problem in repeating a TB skin test using a different location on the body. 
The TB skin test is the preferred TB test for children under the age of 5.
NB: Degree of induration required for positive test depends on patient's population.