Positive Induration diameter ≥ 10 mm Questionable Induration diameter 5-9 mm Negative Induration diameter < 5 mm A false negative result may be caused by: i. Recent or overwhelming TB; ii. Persons under prolonged corticosteroid therapy; iii. Recent viral infections (rubella, mumps, influenza, measles, chicken pox) iv. Persons suffering from Hodgkin’s disease, or with active sarkoidosis. A false positive result may be caused by: Cross reactions resulting from infection with non-tuberculosis mycobacterium.

Table 2: Classification of tuberculin reaction.