According to the Centers for Disease Control and Prevention, tuberculosis is a disease caused by a bacterium called mycobacterium tuberculosis that typically attacks the lungs, but can spread to other organ systems as well. Tuberculosis, or TB, is spread when someone with an active infection in the lungs coughs, sneezes or otherwise puts droplets into the air. When another person nearby inhales these droplets, that person may become infected. While TB can be a life-threatening infection, it is typically curable if detected and treated.
Significance
Testing for TB can detect evidence of tuberculosis infection in patients who are infected with TB bacteria and who are sick, as well as patients who are infected with TB and are asymptomatic. Patients with a TB infection who are asymptomatic are classified as having a latent TB infection. It is important to diagnose these patients, because they have the potential to become sick later in life if the bacteria become active and multiply. While the treatment of patients with active and latent TB is different, both infections are treated.
Types
According to the Mayo Clinic, two main types of tests exist to diagnose likely TB infection. The most common type is a skin test in which a small amount of substance with tuberculin protein is injected just below the skin. Within 2 to 3 days, the injection site is checked for a raised area that appears as a result of a reaction to the tuberculin protein. The size of the raised bump determines the likelihood that the patient has a TB infection. There are also blood tests known as interferon gamma release assays that measure the immune system's reaction to TB. These tests have fewer false positives.
Testing for Active TB
If one of the first tests is positive, further testing is performed to determine whether the patient has active or latent disease. The first test performed is a chest X-ray. If a patient has active TB, this test may demonstrate cavities in the lungs created by the bacteria. If evidence of TB in the lungs is found, a culture will be sent to determine whether TB bacteria are present in the mucus coughed up from the lungs.
Potential
As discussed by the CDC, diagnosing TB early results in earlier and more effective treatment and prevents the spread of TB. Certain populations of people are at higher risk for TB and should undergo testing. These include people who have spent time with someone with known TB or who have symptoms of TB, patients with underlying immune diseases such as HIV, and individuals from a country where active TB is prevalent, who have potential exposures in hospitals or nursing facilities or who inject illegal drugs.
Considerations
Unfortunately, there is not currently a test that can measure mycobacterium tuberculosis directly to determine whether a patient has a TB infection. The skin and blood tests that measure immune responses can also react to non-tuberculous mycobacterium, so a patient may have a false positive if he is infected with this other type of bacteria. If the patient has active TB, mycobacterium tuberculosis can sometimes be cultured from the affected area.
