Study Spotlights More Accurate Way to Diagnose Tuberculosis

Tuberculosis (TB) remains a public health threat in the United States, with as many as 14 million Americans infected with the bacteria. At any given time, members from this large pool of infected people can develop full-blown, highly contagious forms of the disease. TB bacteria usually attack the lungs but can affect any part of the body such as the kidney, spine, and brain. If not treated properly, TB can be fatal.

TB bacteria is spread through the air when a person with TB disease of the lungs or throat coughs, sneezes, speaks, or sings, which may lead people in close proximity to become infected.

According to the World Health Organization, about one person dies of TB every 17 seconds, causing nearly 2 million deaths annually. The disease continues to be a contagious scourge in developing countries, and with the world shrinking rapidly due to global migration, it is a major public health threat in developed nations as well, including the United States. Each infected person represents a potential yet preventable future outbreak, WHO says. Testing for TB infection is necessary in order to quickly identify the appropriate persons for treatment and thereby prevent its spread.

New data from a meta-analysis of existing literature published this week in CHEST, the official journal of the American College of Chest Physicians, provide evidence supporting a new, scientifically-proven standard for detecting TB infection. The study demonstrates that Interferon Gamma Release Assays (IGRAs) are superior to the previous standard in diagnostics, the 100+-year-old tuberculin skin test (TST), for detecting confirmed active TB disease. This was especially true when the IGRAs -- both QuantiFERON®-TB Gold (QFT) and T-SPOT®-TB (T-Spot) -- were administered in developed countries.

"Tuberculosis remains a global public health threat and as such, should be regarded with a corresponding level of urgency," said lead study author Roland Diel, M.D., MPH, assistant professor in the Department of Pulmonary Medicine, Medical School (MHH) Hannover. "The study results support IGRAs as modern advances in diagnosing a very old disease that is just as significant today as it was in the 1700s."

The authors critically reviewed and analyzed 124 studies investigating the commercial QFT and T-Spot blood tests and TST for sensitivity (the percentage of infected persons who return a positive test result) in confirmed active TB cases and specificity (the percentage of uninfected persons who return a negative test result) in individuals with no risk factors for infection. Researchers found that the newer blood tests provide significant improvements in sensitivity over TST. The authors reviewed studies from developed countries and found the TST had only 71.5 percent sensitivity compared with QFT at 84.5 percent and the T-Spot at 88.5 percent.

In a country with a relatively low rate of TB disease, such as the United States, specificity is more important because poor specificity leads to greater numbers of false positives. QFT is highly specific at 99.2 percent versus 86.3 percent for T-Spot. By comparison, TST specificity may be as low as 59 percent in persons who are BCG-vaccinated and only 97 percent in non-vaccinated populations. To highlight the importance of specificity in low prevalence settings, in testing 1,000 people without TB, the TST would return between 30 and 410 false-positives; T-Spot would return 157 false-positive test results and QFT only eight.

"In addition to aiding the prevention of future outbreaks, improved tuberculosis diagnostics reduce the costs associated with TB control," said Tony Radford, CEO of Cellestis Limited, the biotechnology company that manufactures QFT. "With QFT virtually eliminating false-positive readings that are common with TST, TB control and treatment can now be more accurately targeted to those persons who truly are infected, minimizing unnecessary further testing and treatment.”

The CHEST analysis was conducted to establish performance benchmarks, and to this end, took into account only work based on accepted “gold standards” of diagnostic confirmation of active TB disease. The analysis highlights an added benefit that IGRAs offer over TST -- built-in control for measuring immune status of people tested, which reduces the risk of false-negative results in those with compromised immune systems. IGRAs flag such individuals with a result of "Indeterminate." The pooled rate for all populations of indeterminate results was low, 2.1 percent for the QFT and 3.8 percent for T-Spot.

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