A positive TB skin test is characterized by an induration, or raised area, at the injection site that measures 15 mm or greater and develops within 48 to 72 hours following injection, according to MedicineNet. The presence of blisters also indicates a positive reading.
In some people, an induration of 10 mm is a positive reading, notes MedicineNet. This includes IV drug users, recent immigrants to the United States from areas where TB is prevalent, and children under 4 years of age. An induration of 5 mm is a positive reading for recent organ-transplant recipients, individuals recently exposed to TB, and people who are HIV-positive or have weakened immune systems.