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Blood Tests Recommended Over Skin Tests to Check for TB in Most Cases

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March 22, 2017

Recently, updated guidelines on screening for tuberculosis (TB) were published in the journal Clinical Infectious Diseases that now recommend blood tests to detect TB infection rather than the traditional skin test in most situations. These recommendations come from the U.S. Centers for Disease Control and Prevention (CDC), the Infectious Diseases Society of America (IDSA), and the American Thoracic Society (ATS). The recommendations are the first update in 17 years.

Tuberculosis is a bacterial infection most commonly found in the lungs. The infection can be latent—when TB bacteria are not growing in the body, there are no symptoms and no risk of infecting others—or active, when TB bacteria are growing in the body and there may be symptoms. People with active TB can spread the disease to others through droplets dispersed into the air when they speak, cough or sneeze. A latent case of TB can change to an active one when a person's immune system becomes weakened and less able to stop TB from growing in the body. Left untreated, the disease can spread to other parts of the body, including the kidneys, brain and spine.

In 2014, the disease caused more than a million and a half deaths worldwide. In the U.S., active TB is much less frequent. According to the CDC, there were fewer than 10,000 active cases in the United States last year and two-thirds were in foreign-born individuals. Health experts worry that because healthcare practitioners often don't see TB, they may overlook the disease when trying to make a diagnosis.

"Even though TB disease is not common in this country, it's important that healthcare practitioners remember it's still around, and that they should test patients when appropriate," says David Lewinsohn, MD, PhD, a professor of medicine at Oregon Health & Science University and the lead author of the new guidelines.

The guidelines recommend that healthcare practitioners consider testing people at highest risk for TB (e.g., anyone who lives with a person with active TB, laboratory personnel who work with TB, immigrants from countries with high risk of TB, and people who live or work in high-risk congregate settings such as homeless shelters and prisons). For individuals 5 years old or older who are at low or moderate risk of progressing to active TB, the TB blood test is recommended instead of the traditional TB skin test (TST). This is especially beneficial if the person has had the BCG (Bacille Calmette-Guérin) vaccine or is unlikely to return for a follow-up appointment.

The newer blood test, called an interferon-gamma release assay (IGRA), is recommended over the skin test in these cases because it is more specific. The IGRA is more often correctly negative when TB is not present. TB skin tests may be falsely positive when mycobacteria other than TB are present and in people who have received a BCG vaccine, often used in countries with a higher incidence of TB. Though IGRAs are not 100% accurate—no lab test is—and can sometimes give results that are not clearly positive or negative (indeterminate), the IGRA blood test is an improvement over the skin test in detecting TB.

Another advantage of the IGRA is that it requires only one healthcare visit to have a blood sample drawn. In contrast, the TB skin test requires two visits to a healthcare practitioner. During the first visit, a small amount of TB protein (purified protein derivative or PPD) solution is injected under the first layer of skin of the forearm. Then the person must return in two to three days so that the healthcare practitioner can evaluate the injection site to see if a local skin reaction has occurred. This presents a challenge as sometimes people do not return for the second visit within the required timeframe and the test cannot be completed. Nevertheless, the skin test is still recommended, according to the new guidelines, if the IGRA test is not available or difficult to do in a particular setting, or because it is more expensive than the skin test to perform.

IGRAs and TB skin tests are not diagnostic for an active TB infection. Positive results are followed by confirmatory diagnostic tests, such as AFB testing on sputum samples and a chest x-ray to see if TB can be detected in the lungs. Detection of TB while it is in the latent stage allows those patients to start antibiotics to help prevent TB from progressing to its active, infectious form. Patients with latent TB who are not treated have a 4 to 6 percent chance of developing active TB over their lifetimes.

There could be as many as thirteen million cases of latent TB in the U.S., according to the IDSA. "Without the application of improved [diagnostics] and effective treatment for [latent TB infection] new cases of TB will develop from within this group, which is therefore a major focus for the control and elimination of tuberculosis," according to the new guidelines.

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Lewinsohn D, et al. Official American Thoracic Society/Infectious Diseases Society of America/Centers for Disease Control and Prevention Clinical Practice Guidelines: Diagnosis of Tuberculosis in Adults and Children. Clin Infect Dis (2017) 64 (2): e1-e33. Available online at https://academic.oup.com/cid/article/doi/10.1093/cid/ciw694/2629583/Official-American-Thoracic-Society-Infectious. Accessed February 2017.

(May 6, 2016) Centers for Disease Control and Prevention. Tuberculosis (TB) Fact Sheet. Available online at https://www.cdc.gov/tb/default.htm. Accessed February 26, 2017.

(December 8, 2016) Infectious Diseases Society of America. News release: IDSA Guidelines Recommend Newer Tests to Diagnose Tuberculosis. Available online at http://www.idsociety.org/Templates/nonavigation.aspx?pageid=32212269113. Accessed February 26, 2017.