Diagnosing latent tuberculosis

According to United States guidelines, people with HIV should be tested for tuberculosis when HIV is first diagnosed. This practice is less well supported by doctors in the United Kingdom.

The test most often used is a tuberculin or purified protein derivative (PPD) skin test. This works on the principle of injecting a small amount of purified tuberculosis protein into the skin. After three days, the injection site may show a reddening and hardening reaction, which means that there is an immune response to the protein. An immune response indicates previous or current infection with tuberculosis or vaccination. The larger the size of the reaction, the more likely it is that someone has been infected with M. tuberculosis in the past and has active disease or latent infection.

The absence of a reaction does not prove the absence of infection. HIV disease is one of a number of things that can reduce the immune response so a skin test is negative, despite M. tuberculosis being present in the body. This is termed 'anergy'. Successful treatment of HIV may restore this immune response, leading to a positive skin test without any new infection or exposure. In addition, vaccination sometimes makes interpretation of skin tests difficult. This is one reason why some doctors in the United Kingdom do not favour routine use of these tests in HIV-positive people.

Two new blood tests have been developed which provide more accurate identification of people with active or latent infection. The T-SPOT.TB and QuantiFERON-TB Gold tests look for interferon gamma that is produced by T-cells in response to the human form of M. tuberculosis.1 They can tell the difference between tuberculosis exposure and vaccination or exposure to other relatively harmless bacteria. They are also more rapid that the tuberculin skin test, giving results overnight.

A comparative study in 393 patients with suspected tuberculosis showed that the T-SPOT.TB test gave fewer indeterminate results than the QuantiFERON-TB Gold test, and that it was the only test to diagnose all individuals with extrapulmonary tuberculosis correctly. However, both tests are affected by treatments that affect the immune system, such as cancer chemotherapy or steroid treatment.2

References

  1. Ewer K et al. Comparison of T-cell-based assay with tuberculin skin test for diagnosis of Mycobacterium tuberculosis infection in a school tuberculosis outbreak. Lancet 361: 1168-1173, 2003
  2. Ferrara G et al. Use in routine clinical practice of two commercial blood tests for diagnosis of infection with Mycobacterium tuberculosis. Lancet 367: 1328-1334, 2006
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