What is measured for a positive result in tuberculosis (TB) skin testing?

In tuberculosis (TB) skin testing, primarily known as the Mantoux test, a positive result is determined by measuring the raised area of induration that develops at the site of the injection after 48 to 72 hours. This induration, or swelling, is a reaction to the purified protein derivative (PPD) injected into the skin, which indicates that the immune system has responded to the TB antigens.

The size of the raised area is critical for interpretation; specific criteria apply depending on the individual's risk factors for TB. For example, a larger induration is considered positive for individuals who have a higher risk of latent TB infection, while smaller sizes may indicate a positive result in those at lower risk.

Other factors, such as the length of a scar, color of the skin, or duration of swelling, do not provide reliable information regarding TB exposure and are not used as criteria for a positive test result.