NEW TB SCREENING TEST

On May 2, 2005, a new test received final approval from the U.S. Food and Drug Administration as an aid for diagnosing tuberculosis (TB). This new in vitro test, QuantiFERON - Gold (QFT-G), manufactured by Cellestis LTD, Carnegie, Victoria, Australia, received final approval as an aid in diagnosing both latent tuberculosis infection (LTBI) and tuberculosis (TB) disease. In December, 2005, CDC published its guidelines for use of this test as an alternative to the tuberculin skin test, creating the potential to move much of conventional tuberculin skin testing (TST) activities to a laboratory-based test and affect a shift in the approach to the diagnosis of TB and LTBI. Therefore, it's important to take a look at this new test, understand its function and application, and be prepared to consider its use in your laboratory.


QFT-G is an enzyme-linked immunosorbent assay (ELISA) test that detects the release of interferon-gamma (IFN-y) in fresh heparinized whole blood from sensitized persons. The test is performed by mixing patient blood with mixtures of synthetic peptides that simulate two proteins present in M. tuberculosis: early secretory antigenic target-6 (ESAT-6) and culture filtrate protein-10 (CFP-10). Both peptides are secreted by all M. tuberculosis and pathogenic M. bovis strains, but are absent from all Bacille Calmette-Guérin vaccine strains and most nontuberculous mycobacteria, except M. kansasii, M. szulgai, and M. marinum.

The testing method involves combining mixtures of peptides to stimulate cells in heparinized whole blood, and detection of the in vitro responses by ELISA. Aliquots of heparinized whole blood are incubated with the test antigens (ESAT-6 and CFP-10), a phytohemaglutinin (mitogen) positive control and a saline negative control. The blood is then incubated with the test antigens for 16 24 hours. After incubation the concentration of IFN-y released in the plasma is determined by ELISA and results are calculated by manufacturer-provided software. There are a number of advantages: it is a serologic test for detecting TB and LTBI; the ELISA testing format is familiar to many laboratories; results can be available <24 hours after testing without the need for a second patient visit; it is not subject to biases and errors of TST placement and reading; and it is not effected by prior BCG vaccination and does not trigger an anamnestic response (called "boosting").

There are also limitations to the test: testing must be initiated within 12 hours of specimen collection, accuracy may be decreased by errors in collecting and transporting specimens, additional studies to further define sensitivity and specificity are needed, and performance of the test in certain special populations, such as young children and immunocompromised individuals, has not been determined. The advantages of the test suggest that laboratorians initiate a dialogue with physicians, TB Control Programs and other health officials and those in the medical community to determine the potential uses of this new approach to detecting TB and LTBI. QFT-G can be used in all circumstances in which the TST is used and can be used in place of TST. No reason exits to follow a positive QFT-G result with a TST and a positive QFT-G should prompt the same public health and medical interventions as a positive tuberculin skin test result. For further information about this test and the CDC Guidelines for TB screening, consult the December 16, 2005 issue of MMWR Vol. 54/RR-15.

By: Nancy G. Warren, Ph.D., Director
Bureau of Laboratories
Pennsylvania Department of Health