December 16, 2005
CDC said Thursday it is recommending a new, more accurate blood test for TB. The test is believed to yield fewer false positives and is more convenient for patients.
For the past 100 years, health care workers have used a skin test to detect TB, in which a small amount of protein-bearing testing fluid -- tuberculin -- is injected just under the skin on the underside of a patient's forearm. After two to three days, the patient must return so a health care worker can assess the swelling and determine if the test is positive or not.
The QuantiFERON-TB Gold test, made by the Australian firm Cellestis Limited, requires patients to come in only once to have blood drawn. The patient is then notified when the results are available, which can be as soon as the following day.
QFT Gold, which was approved by the Food and Drug Administration in May, also seems to address some of the problems found with the skin test. "I like to refer to the tuberculin skin test as the test we love to hate," said CDC's Dr. Kenneth Castro, director of the Division of Tuberculosis Elimination. Some types of non-TB bacteria can cause the skin test to produce a false positive result. People who have been inoculated with a certain type of TB vaccine used in other countries can also yield false positive results. In addition, immuno-compromised patients, such as those with AIDS, sometimes receive false negatives.
While QFT Gold has shown to be better at reducing false positive readings, CDC cautioned that more studies are needed, particularly in children and people with compromised immune systems. "We're saying this can be used instead of the tuberculin test," said Castro. "But we're not going as far as saying it should replace it."
The QFT Gold test costs around $15, plus an additional $10 to administer. While the skin test is less expensive, its labor costs bring it to around the same price, said Dr. Masae Kawamura, director of the TB control section at the San Francisco health department, which now conducts most of its TB screening using the QFT Gold test.