In February of this year the New York City Department of Health released a new policy paper indicating that they will no longer use the PPD as a screening tool for tuberculosis in their clinics.They have switched to the QuantiFERON-TB Gold, (QFT-G), a blood test. This test is an ELISA, which measures interferon-gamma secretion by t-lymphocytes in response to tuberculosis specific antigens. The test requires heparinized whole blood and must be processed within 12 hours of the blood draw.
The test exposes the patients t-lymphocytes to two proteins. The first is ESAT-6, or early secretory antigenic target-6. The second is CFP10, or culture filtrate protein 10. These two antigens are highly specific to m. tuberculosis. They are NOT found in BCG or in MAC. Therefore, the test has a much higher specificity for exposure to TB than PPD.
Like the PPD, it does not distinguish between latent or active infection. Unlike the PPD, it does differentiate BCG exposure. It also decreases the uncertainty we often find when a patient appears to have active TB but his/her PPD is negative. The CDC reports on three studies looking at sensitivity and specificity of the test. In composite, they describe the specificity between 67% in an immuno-compromised population to 98.1% in a healthy population. Sensitivity ranged 81 to 98% in these same populations. The only documented false positives are prior exposures to M. Kansasii, M. Marinum, or M. szulgai.
In contrast, the PPD has a documented specificity of 49% and sensitivity of 33%.
From a practical and economic point of view, the QFT-G is a huge advance beyond PPD. The test offers a positive or negative result and is not reader dependent. There is never a need for second, or booster, test. The patient does not have to return on a specific date to get the results, and the patient can learn the results over the phone. This will halve the number of necessary clinic appointments in the screened population. The only disappointing factor is the lack of specificity for the immunocompromised population. Still, the results are a vast improvement over those for PPD.
Please refer to the web-site below for the CDC guideline.
-Deborah Shapiro, MD, Attending Physician, Division of General Internal Medicine
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