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10.0 Tuberculin skin testing

In the pre-HIV era about 75% - 85% patients with newly diagnosed pulmonary tuberculosis had a positive response to 5 units [intermediate strength] PPD testing.

In HIV and TB co-infection there is a reduction in the proportion of those reacting to PPD as the CD4 count falls, from 50%-90% in those who have a CD4 count of ≥500 cells/uL down to 0% - 20%, in those patients who have AIDS or advanced HIV infection and a CD4 count of ≤200cells/uL.  This limits the usefulness of the tuberculin test as a diagnostic tool.

Specific non-reactivity to PPD is difficult to distinguish from the general poor immune responsiveness seen in HIV infected patients and anergy testing using a panel of antigens gives inconsistent and ambiguous results and is not a recommended strategy. 

HAART may improve immunological responses to tuberculosis but patients most likely to revert from a negative to a positive PPD are those with a rise in CD4 count of  >200 cells/uL from baseline. 81-89

10.1 Who should have tuberculin testing?

 A tuberculin test is performed in order to identify those patients who may have latent TB infection so treatment may be given in order to prevent reactivation. US guidelines recommend that all newly diagnosed HIV patients should have a tuberculin skin test and those with a positive test (>5mm induration) should be given isoniazid or other chemo preventative therapy. Whether this policy has any long-term public health impact on TB control in countries where tuberculosis has a relatively low prevalence is not known. 90-97

There are many factors that may affect the usefulness of such a broad strategy. These include the lower PPD positive rates in HIV positive patients, the effect of BCG immunization on PPD reactivity, the relative short term impact of chemo preventative therapy where there are high rates of exogenous infection and the effect of HAART in preventing tuberculosis reactivation and progression to infection. In newly diagnosed patients with CD4 counts <400 cells/uL the routine use of tuberculin testing is not recommended. [DII]