Tuberculosis is an infection caused by the Mycobacterium tuberculosis bacteria. TB mainly affects the lungs. Other organs can be affected, sometimes at the beginning but usually in long-term infections. TB is a contagious disease (contact with aerosols caused by sneezing and spitting). Only some individuals infected with the bacteria will develop symptoms of the disease (active tuberculosis). Infected individuals with no symptoms have latent TB and are not contagious. However, TB can reactivate if the immune system is weakened. There are TB screening tests such as the tuberculin skin test (TST) and QuantiFERON (blood test). The QuantiFERON test assesses the strength of the body’s immune response to TB by measuring the production of a defensive molecule (gamma interferon) in the blood by leukocytes exposed to TB antigens. QuantiFERON-TB Gold results are expressed in units per millilitre of blood (IU/mL) and a value greater than 0.34 IU/mL is consistent with a Mycobacterium tuberculosis infection. This screening test cannot be used to differentiate between latent and active TB. Further testing (bacterial identification) is required. Negative results are seen in approximately 15% to 20% of carriers of the bacteria. A negative result therefore does not completely rule out the possibility of a latent or active Mycobacterium tuberculosis infection.