To test for Mycobacterium tuberculosis infection, a skin test called PPD can be performed in a clinic or doctor's office. PPD stands for purified protein derivative. It contains pieces of the bacteria and is injected directly under the skin. If someone has been exposed to the bacteria in the past, the immune system will immediately recognize the PPD, resulting in a firm, relatively large bump at the site of the injection. If this reaction occurs, a person is said to have a positive PPD.
A positive PPD generally calls for additional testing. An X-ray of the chest is performed to look for signs of active disease. Blood tests, along with sputum (phlegm) samples, may also be sent to a lab for analysis. If the bacteria is found in these samples, it may be tested further to see if it is resistant to any of the drugs commonly used to treat tuberculosis.
If someone has a positive PPD but does not have any signs or symptoms of active disease, he or she is said to have latent TB infection. It is generally recommended that people with latent TB infection begin taking drug therapy to prevent the infection from developing further (discussed in next section). If someone has a positive PPD and has signs and symptoms of tuberculosis, he or she is said to have active TB. Treatment is necessary for someone with active TB, which usually involves a combination of antibiotics to treat the infection (also discussed in next section).
PPD testing in people with HIV can be problematic. As discussed above, PPD testing doesn't test for the presence of Mycobacterium tuberculosis, but instead looks for signs that the immune system is currently fighting the bacteria. In HIV-infected patients with compromised immune systems, there might not be enough immune activity to either fight the infection or respond to the PPD test. In other words, the bacteria might be present but is not being recognized by the immune system and, as a result, may not show up using PPD testing.
Because PPD testing may not be reliable in HIV-positive people with compromised immune systems, a diagnosis of TB might not be made until symptoms are reported and X-rays or blood tests are performed. For some HIV-positive people with compromised immune systems, it is better to be safe than sorry. For example, if an HIV-positive person lives in the same house or works with someone who has active TB and may be spreading Mycobacterium tuberculosis, it is generally recommended that the HIV-positive person be isolated from the person with active TB and to begin treatment.