Tuberculosis is a leading cause of disease and death worldwide, though there were only 13,299 TB cases in the United States in 2007, according to Merck.com. A bacterium called Mycobacterium tuberculosis causes this disease. Most of the time, the disease affects the lungs, but it can also affect the brain, heart, bones, urinary system, liver and lymph nodes. There are three main tests for tuberculosis.
PPD Test
PPD, or purified protein derivative, is a skin test for tuberculosis where a lab technician injects PPD into the forearm. A positive result does not mean that there's an active case of tuberculosis, but rather that the person was exposed to tuberculosis at some time, and may have an active case with symptoms, or the disease could be dormant. A positive skin result makes a hardened area in the skin. Merck.com explains that a positive result depends on the size of that area and the circumstance of the person taking the PPD test. An area that is 5 mm in diameter is called positive for people who are in close contact with someone who has an active case of tuberculosis, those taking steroids, are organ transplant recipients, have HIV or have a chest X-ray that shows a past tuberculosis infection. An area of 10 mm is a positive result for people who have recently moved from a country where there are many tuberculosis cases, those who are homeless, have a long-term illness, are intravenous drug users, are prisoners or residents of a health care institution. For everybody else, a positive result is an area that measures 15 mm in diameter.
Chest X-Ray
Anyone who has a positive PPD result will then have a chest X-ray to help determine whether the tuberculosis is an active infection or a dormant one. A normal X-ray indicates a dormant tuberculosis infection, referred to as a latent infection; if the X-ray is abnormal, the next step is to have sputum acid-fast stain test.
Sputum Acid-Fast Stain
Sputum is the medical term for mucus from the lungs. Microbiologists classify bacteria by staining them. A particular dye is used to stain Mycobacterium tuberculosis, and a mixture of alcohol and acid cannot remove that stain, according to "Jawetz, Melnick, & Adelberg's Medical Microbiology." This is why microbiologists call Mycobacterium tuberculosis acid-fast. The sputum acid-fast stain test uses sputum from the person with an abnormal X-ray and stain to check for bacteria. If there are no bacteria in the sample, the person has a latent infection; if there are bacteria, it is an active infection.
Potential New Testing
"Jawetz, Melnick, & Adelberg's Medical Microbiology" mentions a new DNA test that is beginning to be used at the Centers for Disease Control and Prevention (CDC). With the DNA test, the CDC can track the transmission of tuberculosis from one person to another, determine if the infection is a new infection or if the person is sick because of the activation of a latent infection. They will also be able to use the DNA tests to analyze outbreaks.
Merck.com mentions a test that can sometimes be used along with acid-fast staining. When this test is used, the results only take one or two days instead of the usual two weeks.
References
- "Jawetz, Melnick, & Adelberg's Medical Microbiology"; George Brooks, M.D., Karen Carroll, M.D., Janet Butel, Ph.D., Stephen Morse, Ph.D.; 2007
- Merck.com: Tuberculosis (TB)
- WebPath: Lung, M. Tuberculosis, acid fast stain, high power microscopic
- WebPath: Lung, tuberculosis with granulomatous inflammation, gross
- University of Maryland Medical Center: Pulmonary tuberculosis -- Overview
