Tuberculosis and HIV disease

November 2004     View PDF     En español

TB and HIV disease

For many people, TB is the first sign of immune dysfunction associated with HIV infection, and active TB is an AIDS-defining illness. One in ten people living with HIV will get active TB within a year of being diagnosed with HIV. It can occur early in HIV disease when CD4+ cell counts are relatively high, in the 300–400 range. In early HIV infection, TB usually infects and affects just the lungs. As CD4+ cell counts drop, however, TB is more likely to appear in other organs too.

When the immune system responds to TB it can cause HIV levels to increase, and HIV disease may then progress more quickly. This, in turn, increases the risk of other opportunistic infections. The good news is that TB treatment leads to lower HIV levels in people with both infections.

It’s very important for people with HIV to be screened regularly for TB. TB testing is recommended to begin when a person is first diagnosed with HIV, then yearly after that. Also, when starting anti-HIV therapy, a TB test is also recommended. Finally, for people living with HIV who come into contact with someone with active TB, a TB test is recommended.

Whether you have TB infection or active TB disease, it’s extremely important to get treated right away. If you are diagnosed with TB and HIV at the same time, you may not want to start treating both at the same time. It might be easier to stick to your regimens if you start the anti-TB treatment first and wait awhile before starting anti-HIV therapy. This might not always be possible, but it’s something to discuss with your doctor.

How does a doctor diagnose TB infection?

A PPD skin test is used to detect TB infection. A small amount of liquid (called PPD Tuberculin) is injected under the skin of the arm. After 48–72 hours, a nurse or doctor reads the test. A hard swelling at the site larger than 10mm means that the person is infected with TB. A smaller swelling of 5mm is used to detect TB infection in people with HIV. This is because people with HIV might not have a strong immune response resulting in the more pronounced swelling seen 7in people who are HIV-negative and otherwise healthy.

Unfortunately, the PPD test is not 100% accurate for three main reasons. First, the reaction is caused by an immune response, and this may take awhile to develop. There actually may be no reaction within the first 10–12 weeks after infection. Second, a person may react to the test if they’re infected with a related bacteria like MAC. Third, some people with weak immune systems do not react even if they’re infected with TB.

When a person is infected with TB but does not react to the PPD, he or she is said to be anergic. This is more common when CD4+ cell counts are below 200. A different test can be used to check for anergy, but the results are not very reliable in people living with HIV. Anergy testing is not routinely used, but it may be helpful in certain cases. So, a negative PPD test result does not always mean that a person is free of TB. For this reason, routine testing is very important.

How does a doctor diagnose TB disease?

Chest x-rays are used to detect active TB disease and to check for damage in the lungs. During or after active TB, x-rays will show lumps, holes or scars in the lungs. Chest x-rays may provide misleading or confusing results, however, because a chest x-ray may look unusual due to HIV or other HIV-related complications, including PCP and MAC. This is especially true for people with CD4+ cell counts below 200. This can make TB more difficult to diagnose in people living with HIV.

The best proof that a person has TB is to find the actual bacteria. In a TB smear test, a sample of sputum is studied under a microscope to see if it contains the bacteria. Unfortunately, it cannot tell the difference between TB and related bugs like MAC. If the smear test is positive, then the sputum can be grown in a lab to see if it contains active genetic TB material (called TB DNA). Tests that detect this genetic material have been developed, but are not routinely useful in all situations, such as diagnosing TB in places outside the lungs or definitely telling if someone is TB-negative. Also, growing bacteria from a sputum sample, in the lab, is needed to test for drug resistance.

Tests that are normally used to look for active TB in other organs include scans of the head, chest or abdomen; spinal tap; biopsy of lymph nodes or bone marrow; and urine culture.

Symptoms

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