Tuberculosis

Tuberculosis (TB) is caused by the bacterium Mycobacterium tuberculosis, and can affect any part of the body, but most commonly the lungs (pulmonary).

Only TB of the lung or of the throat can be infectious. This infection is generally transmitted by prolonged and/or frequent contact with an infected person. The bacteria enter the body through the lungs by breathing. It can however travel through the blood and infect other organs causing non-pulmonary forms of TB.

A few years ago TB was thought to be a disease of the past, particularly in the developed world. However, even in the United States the disease continues to be a significant challenge due to:
          • Immunosuppressive conditions (e.g. HIV, immunosuppressive therapy)
          • Immigration
          • Increasing non-compliance with testing and treatment protocols

In 2008 there were 12,898 new cases of active tuberculosis disease in the United States. Additionally, it is estimated that 10-15 million people are infected with latent TB in the US.¹

Tuberculosis Infection and Tuberculosis Disease

It is important to define the difference between tuberculosis infection (latent TB infection or LTBI) and tuberculosis disease (active disease):

Tuberculosis infection (latent TB infection, or LTBI) occurs when people carry small numbers of dormant M. tuberculosis bacilli in their body. Dormant bacteria are normally controlled by the immune system and do not cause disease or any TB symptom. People with latent TB infection are not infectious. Until recently the only in vitro test capable of identifying LTBI has been the tuberculin skin test (TST or PPD). Now, new blood tests, such as the
T-SPOT^®.TB test, are used to test for LTBI.

Tuberculosis disease (active TB disease) occurs when the bacterial load is increased and overcomes the body's immune defense. Tuberculosis disease presents with clinical signs and symptoms and people with the disease may be infectious. Active TB disease can often (but not always) be identified by culture, smear microscopy, NAAT or chest x-ray. Blood tests, such as the T-SPOT®.TBTB test can be used as an aid in the diagnosis of active TB.

To view the relationship between active disease and latent infection click on the thumbnail. 

_{¹ Centers for Disease Control and Prevention (CDC). Trends in tuberculosis-United States, 2008. MMWR Morb Mortal Wkly Rep. 2009;58:249-253.}