US Pharm. 2010;35(8):HS23-HS24.
Tuberculosis (TB) is an infectious disease caused by the transmission of Mycobacterium tuberculosis. This disease usually affects the lungs but is capable of attacking almost any part of the human body. TB is spread from person to person after being exhaled through the air by coughing, laughing, sneezing, singing, or even talking.1 However, it is not transmitted through daily contact, such as hugging or shaking hands.1,2 Signs and symptoms of active TB may consist of a persistent cough, constant fatigue, loss of appetite, weight loss, coughing up blood, fever, and night sweats.1,2
Persons who are at a greater risk for developing active TB include those infected with HIV, patients who have been infected with TB bacteria in the past 2 years, users of illicit IV drugs, and those who currently have other diseases that weaken the immune system. Other patients at risk may include infants, the elderly, and people who were not treated appropriately for TB in the past.3
Epidemiology
The World Health Organization (WHO) estimates that more than one-third of the world’s population is infected with M tuberculosis.2 Left untreated, each person with active disease will infect on average between 10 and 15 people every year.2 An estimated 1.3 million people died from TB in 2008.2 Accordingly, with the increasing number of new cases, early detection is vital.
Diagnosis
Tuberculin Skin Test: Traditionally, screening a person for a TB infection is performed with the tuberculin skin test (TST). TST screening is administered via the Mantoux method. This test involves the intracutaneous injection of tuberculin, which is also known as purified protein derivative, into the volar surface of the forearm; test results are read 48 to 72 hours postadministration.2,4 This interval of time is mandatory for the development of a delayed-type hypersensitivity reaction. If the patient does not return for the results within the specified time, the injection must be repeated. Results are interpreted by measuring the extent of induration at the site of injection.
Enzyme-Linked Immunosorbent Assay (ELISA): Another screening test that may be performed is the ELISA. Employed for the detection of interferon (IFN)-gamma secretions from lymphocytes stimulated with TB-specific antigens, the ELISA-based method measures the amount of IFN-gamma in the blood.5
T-SPOT.TB
Developed by Oxford Immunotec, the T-SPOT.TB test (FIGURE 1) is based on T-cell measurement technology.5-7 The test is intended for use as an aid in the diagnosis of an infection and in conjunction with risk assessment, radiography, and other medical and diagnostic evaluations. T-SPOT.TB uses enzyme-linked immunospot (ELISPOT) technology to specify M tuberculosis-sensitized T cells by capturing IFN-gamma from the secreting T cells and revealing it as spots.6,7 Device instructions are provided in TABLE 1, and a screening algorithm can be found in TABLE 2.7
Efficacy: Oxford Immunotec researchers studied a total of 2,355 subjects, using 11 study sites in the United States and one in Brazil.7 Of the participants enrolled, 492 samples did not meet the study criteria, leaving 1,863 subjects available for analysis. Results of this study included a sensitivity of 95.6% (175/183; 95% CI, 91.6%-98.1%) in subjects with culture-confirmed TB disease, using the prespecified cut-off of ³6 spots, as well as a specificity of 97.1% (297/306; 95% CI, 94.5%-98.7%) in subjects with a low risk of TB infection. This study also demonstrated T-SPOT.TB to be reliable in immunosuppressed patients; results were not affected by the Bacille Calmette-Guérin (BCG) vaccine, which is a live vaccine against TB derived from a strain of Mycobacterium bovis.7,8
The T-SPOT.TB test was shown to reduce the number of false-negative results associated with other TB screening tests. Additionally, it is beneficial in that it is a single blood test that does not require a return visit or follow-up. Other advantages include its ability to be used in all risk groups, including health care workers, the immunosuppressed, and those suspected to have TB. Finally, this product does not require special collection tubes, but instead utilizes standard blood collection tubes.6,7
Conclusion
T-SPOT.TB is a very accurate and precise test. It is not affected by BCG vaccination and does not cross-react with common environmental mycobacterium. Immunosuppression also has minimal effect on its accuracy. Moreover, it identifies infected subjects who may have been missed by TST screening. For more information, visit www.oxfordimmunotec.com or www.tbtestingservices.com.
REFERENCES
1. Tuberculosis. American Lung Association. www.lungusa.org/lung-disease/
2. World Health Organization. Tuberculosis. Fact sheet no. 104. March 2010. www.who.int/mediacentre/
3. Centers for Disease Control and Prevention fact sheet. Tuberculosis (TB). Basic TB facts. www.cdc.gov/tb/topic/basics/
4. Centers for Disease Control and Prevention fact sheet. Tuberculosis (TB). Tuberculin skin testing. www.cdc.gov/tb/publications/
5. Kim S, Choi S, Kim H, et al. Diagnostic usefulness of a T-cell based assay for extrapulmonary tuberculosis. Arch Intern Med. 2007;167:2255-2259.
6. T-SPOT.TB. Oxford Immunotec. www.oxfordimmunotec.com/T-
7. T-SPOT.TB package insert. Marlborough, MA: Oxford Immunotec, Inc; July 2008. www.oxfordimmunotec.com/
8. CDC. The role of BCG vaccine in the prevention and control of tuberculosis in the United States. A joint statement by the Advisory Council for the Elimination of Tuberculosis and the Advisory Committee on Immunization Practices. MMWR Recomm Rep. 1996;45(RR-4):1-18.
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