False-Positive TB Skin Tests Prevalent in U.S. Army

BETHESDA, Md., May 30 -- Clusters of positive skin tests for latent tuberculosis infection in the U.S. Army appear to reflect false-positives on tuberculin skin tests rather than true disease, researchers found.

In a re-evaluation of eight TB pseudoepidemics around the world, 30% to 100% of positive tuberculin skin tests were found to be negative on retesting, James Mancuso, M.D., M.P.H., of the Uniformed Services University of the Health Sciences here, and colleagues reported in the June 1 issue of the American Journal of Respiratory and Critical Care Medicine.

The high false-positive rate was the result of error and variability in administering, reading, interpreting, and documenting the skin test, variability between tests, and cross-reactivity with nontuberculous mycobacteria, they said.

"Future efforts in TB prevention in the U.S. Army should include targeted testing for [latent TB infection], with increased emphasis on proper quality-control measures and follow-up procedures for those identified as high risk," they said.

They recommended exclusive use of Tubersol testing for troops, although they conceded that it, too, like the tuberculin skin tests or Aplisol, did not quality as a gold standard.

False-positive results could also be reduced, they added, by targeting high-risk individuals, reducing sources of error in test administration through proper training and oversight of medical personnel, better documenting manufacturer and lot information to facilitate better follow-up, and considering cross-reactivity to nontuberculous mycobacteria because of increased exposure to the organisms in military populations.

The researchers concluded that "close attention should be paid to the surveillance and control of infectious diseases among local nationals and detainees working or residing in contact with U.S. service members in deployed locations."

The true result of a tuberculin skin test often remains unclear because of variability in the responses to the test, they said.

In addition, the positive-predictive value of the test drops in populations with a low prevalence of latent TB.

Despite going into parts of the world with high rates of the disease -- such as Afghanistan and Iraq -- the military often has little contact with local populations and, therefore, has a low prevalence of latent TB, according to the researchers.

However, universal testing occurs in all U.S. troops before deployment and twice after deployment, making up more than 500,000 tuberculin skin tests performed by the Army each year,

Dr. Mancuso and colleagues re-investigated eight pseudoepidemics of positive skin tests from 1983 to 2005 in soldiers stationed in Fort Leavenworth, Kan. (two clusters), Fort Benning, Ga,, Afghanistan, Kosovo, Bosnia (two clusters), and Guantanamo Bay, Cuba from.

They obtained surveillance information from the U.S. Army Center for Health Promotion and Preventive Medicine, reviewed medical records, interviewed affected soldiers about possible exposure to TB, and repeated skin tests using Tubersol, which is more reliable than another test, Aplisol, according to the researchers.

Before re-evaluating the pseudoepidemics, the estimates of positive tests ranged from 1.3% in one of the Bosnian clusters to 15% in Afghanistan.

Following repeated tests, the revised estimates of positive skin tests ranged from 0.5% to 4.3% in the same clusters.

In the cluster located in Afghanistan, 81% of initially positive skin tests were found to be negative, and in Kosovo that number reached 95%.

Only one active TB case -- in Bosnia -- was identified.

In addition to problems with the test, product variability, and cross-reactivity, foreign birth was also associated with the outbreaks.

"Most of the pseudoepidemics reported here included several of these factors, emphasizing the difficulties with such a large, complex program in a deployed setting," the researchers said. "These pseudoepidemics cause confusion among medical providers, public health personnel, and in the Army and civilian communities at large."

The authors acknowledged that the study was limited by the lack of certainty regarding skin test results -- including the re-tests -- and obstacles resulting from having some medical records overseas.

"Despite these limitations," they said, "the consistent reporting of pseudoepidemics in Army populations with similar associated factors over a large time interval suggests that the findings are valid."

The authors reported no conflicts of interest.

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