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A New Test for TB

Monday June 5, 2006
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QuantiFERON-TB Gold is a modern alternative to the 114-year-old tuberculin skin test.

Tuberculosis Eliminated in the U.S.! Imagine that as a headline. It sounds farfetched, but the new diagnostic blood test QuantiFERON-TB Gold, developed by the Australian company Cellestis, has filled public health experts with the hope that such a headline soon will become reality.
The Food and Drug Administration has approved QuantiFERON-TB Gold, and the Centers for Disease Control and Prevention recognizes its potential as a replacement for the tuberculin skin test (TST). Its more accurate method of TB detection yields fewer false positive results.
Nurses should know about QuantiFERON-TB Gold because it may soon become the mainstream detection test for TB infection, potentially replacing the TST entirely in their clinical practices.
Understanding the new test
QuantiFERON-TB Gold is an in vitro blood test that uses a whole blood specimen to identify M. tuberculosis infection. Highly specific, it measures immune responses to peptides that simulate the M.tuberculosis proteins ESAT-6 and CFP-10 in heparinized whole blood. In QuantiFERON-TB Gold, the peptide combination mimics the TB proteins, triggering a response by lymphocytes contained in the blood specimen to generate interferon-gamma (IFN-gamma). The detection of IFN-gamma and its quantification identify TB infection in the patient.
If a patient tests positive with QuantiFERON-TB Gold, adjunct TB testing protocols like chest radiography, sputum culture, risk assessment, and medical evaluation are initiated to diagnose for active TB disease. It's important to note, however, that the test cannot distinguish between latent TB infection and active TB disease (see sidebar, "Latent TB Infection vs. Active TB Disease").
Test procedure to identify
TB infection
The QuantiFERON-TB Gold blood test requires venipuncture and collection of at least 4 mL of whole heparinized blood. It's recommended that the evacuated heparinized tube (5, 9, or 10 mL capacity) be filled completely. Heparin is the only anticoagulant compatible with the QuantiFERON-TB Gold test.
The specimen should be kept at room temperature, and within 12 hours of collection, the sample must be sent to a lab to initiate testing. Clear collection and transport requirements must be communicated to all staff responsible for administering the test. Results are usually available within 24 hours of specimen collection.
Comparison of QuantiFERON-TB Gold and TB skin tests
With QuantiFERON-TB Gold, the patient visits the clinical setting for blood specimen collection only one time. This is more convenient for patients than the two-visit process required by the TST technique.
The Mantoux is the standard TST used in most clinical practices today, having become preferred over the tine TST technique.
At the initial TST visit, a nurse injects tuberculin under the skin. Within 48 to 72 hours, the patient must return to the clinical setting so the characteristic bump, or induration, can be measured and test results determined. About 30% of individuals tested via the TST fail to return for the required reading.1
False positives have always been a major drawback for TST because only one individual measures the induration and interprets it to determine the test result. This is a subjective process and inherently variable with regard to test interpretation.
A patient's reaction to certain nontuberculosis bacteria or a patient's prior inoculation with the tuberculosis vaccine bacille Calmette-Guerin (BCG) - commonly used in other countries - also may contribute to false positive results with the TST because those who have had BCG will always have a skin reaction, which confuses the test interpretation. On the other hand, QuantiFERON-TB Gold does not identify BCG as TB infection, so this is not a factor in test interpretation.
Patients eligible for the test
QuantiFERON-TB Gold has been approved for use in adult patients. It's not approved for use in children or pregnant women, although scientific testing in these populations is in progress.
QuantiFERON-TB Gold also is not intended for use in persons with compromised immune systems resulting from HIV infection or AIDS. Patients receiving immunosuppressive drug regimens used for managing organ transplants or certain malignancies also are not candidates for this test.
Major pluses
TB is the largest microbial killer in the world, and its resurgence is linked primarily to the AIDS epidemic, substance abuse, homelessness, and immigration trends. The more accurate diagnosis given by QuantiFERON-TB Gold can eliminate unnecessary treatment and improve patient management.
Each QuantiFERON-TB Gold test kit costs about $15, plus an additional labor cost of $10. TST supplies are less expensive, but the additional labor cost makes the total TST cost equivalent to the other.
QuantiFERON-TB Gold is a reliable and cost-effective TB test that can be used in every community to better diagnose tuberculosis. For further information, go to www.cdc.gov/tb.