Impact of HIV and AIDS on acquisition, activation, and outcome of TB:
HIV-infected patients who are immunocompromised have a 113 times greater likelihood of becoming infected with tubercle bacilli than an immunocompetent person. HIV infects helper T-cells, which leads to a decrease in cell-mediated immunity. This lack of immunity allows development of active TB on exposure to mycobacterium tuberculosis and/or reactivation of the disease in a previously infected patient.
About 37% of HIV-infected patients develop TB within 5 months of exposure, whereas this occurs in only 5% of exposed persons with an intact immune system. The lungs are most frequently affected (74 to 94%).
Extrapulmonary TB is much more common in TB patients who are HIV-infected as compared to patients who are not HIV-infected (42 to 72% versus 17.5%). Patients with AIDS have a much greater possibility of developing extrapulmonary TB than when compared to asymptomatic HIV-infected patients.
TB infections in HIV-infected patients are often very difficult to distinguish from other HIV-related pulmonary opportunistic infections [Pneumosystis juroveci, Mycobacterium avium complex (MAC)].
In any HIV-infected patient with a pulmonary opportunistic infection, TB must be ruled out as TB often precedes the opportunistic infection. A positive PPD intermediate skin test with 10 millimeters or more of induration will occur in only 30 to 50% of patients who are HIV-infected due to a compromised immune system.
Nevertheless, a PPD intermediate skin test should be placed on the forearm of an HIV-infected patient and an induration of 5 millimeters or more is considered positive in immunocomprimised patients.