Wednesday, November 22, 2006

Even the PPD may be replaced?

Nothing seems secure from progress or change or innovation.The Mantoux skin test has been around for over 60 years and is now being challenged by two commercially available tests that measure the blood levels of interferon-gamma release from sensitized T cells after stimulation by antigens fairly specific ( apparently there is some sharing of antigen with M. Kansasii) to Mycobacterium tuberculosis.

The QuantiFERON-TB gold Assay has been approved and in 2005 CDC recommended its use in all situations where PPD has been used. The results can be obtained quicker and the assay kits are said to have greater specificity and equivalent sensitivity.The blood must be received in the laboratory within 12 hours, which can be a drawback.The test is to be used just as the PPD is used.A good review of particulars is found here. CDC is recommending its use as a replacement not as an addition to the PPD. The in vitro assays should eliminate the problem of the booster phenomenon which in the past has lead to pseudo-mini-epidemics of TB in hospital personnel, described here, and the vagaries of interpretation of the PPD in a person who has received BCG.

The other assay is called T-Spot.TB.Here is a interesting comparison of how results from the two tests may differ.

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