Most people have been tested for TB infection at some time—and perhaps many times—for school, work, and the like. These skin tests are part of the public health effort to control TB by detecting it early. Like any disease, diagnosing it early and beginning treatment are the keys to success. In the case of TB, it is best to catch the infection while it is latent. If the infection is not detected and treated early, it can turn into active disease, endangering the patient and putting others at risk of infection.
You should be tested for TB if:
Tell your doctor about any contact you have had with people who might have active TB disease and about any activities, such as injecting drugs, that might increase your risk of infection. If you have symptoms, your doctor will want to know when they began.
Part of controlling TB is the public health effort to restrict its spread. If you have active TB, people you come in contact with will need skin tests and chest X-rays in case you have infected them.
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Many schools and workplaces in the United States and abroad give TB skin tests. This is a preventive measure designed to limit the spread of active TB disease. Active TB can be spread through the air and can become a huge public health problem if not immediately identified and treated.
TB skin tests use tuberculin, a non-infectious protein made by the bacteria that cause TB. For the most common TB test, a healthcare worker will use a tiny needle to inject a small amount of tuberculin just under the surface of the skin, usually on the inside of your forearm. You must return 48 to 72 hours after the injection to have your skin test read.
If you are currently infected with TB or ever have been infected with TB, your immune system will mount a swift response against the tuberculin, and a bump will form at the injection site over the next day or two. The bump is from inflammation, one of the ways your body's immune system responds to germs that make you sick.
If you have not been infected with TB, your body's immune system will not recognize the tuberculin as being harmful and therefore will not mount an immune response against it. A healthcare professional will look at the bump on your skin and determine whether it is large enough to represent a positive result.
A positive skin test does not necessarily mean that you have active disease. It is much more likely that you have a latent TB infection, which means you have been infected but are not sick and cannot pass the disease on to others. A latent TB infection should be treated so it does not develop into active disease. A positive skin test means that you'll have to go through more tests. These additional tests are required to determine whether you have the latent (inactive) or the active form of the disease.
A negative skin test does not mean you definitely are not infected with the bacteria that cause TB. You may have been infected too recently for the test to be able to detect the infection. People with severely weakened immune systems sometimes are unable to mount the inflammatory reaction that causes a positive result, so they can have a negative skin test even if they are infected with the bacteria.
If you have had the BCG vaccination, you may have a positive skin test. If you have had the BCG vaccination but think you may have TB, you will need further testing.
In addition to the tuberculin skin test we now have two blood tests that can identify TB infection. Each tests involves collection of blood that is then stimulated with a group of antigens (proteins) found in the bacteria that causes TB. If your immune system has ever seen these antigens, your cells will produce interferon-gamma, a substance produced by our immune system, which can be measured in a laboratory. There are two tests available, the QuantiFERON-TB® Gold (QFT) Test and the T-Spot® TB Test. These tests appear to be more sensitive and specific than the tuberculin skin test. Importantly, unlike the tuberculin skin tests, they are not affected by previous BCG vaccination.
If a person has been infected with TB but active disease has not developed, the chest X-ray will often be normal. Most people with a positive tuberculin skin test have normal chest X-rays and continue to be healthy. For such people, preventive medication may be recommended.
However, if the germ has attacked and caused inflammation in the lungs, an abnormal shadow may be visible on the chest X-rays. The doctor may be able to see spots where immune cells are surrounding the TB bacteria or larger diseased areas indicating active TB disease. An X-ray of the lungs may show signs of pneumonia, cavities (holes in the lung), or scarring, all of which can happen in active TB disease. These people may need further diagnostic tests (sputum tests) and will need treatment.
For a chest X-ray, you will be asked to stand in front of a special panel. The technician aims the X-ray tube at you from about 6 feet away. You may be asked to stand in different positions to ensure a good view of your lungs. You will be asked to remove all jewelry and any other metal objects, and you may be asked to remove some of your clothing.
The radiation from an X-ray does slightly increase the risk of cancer, but this tiny increase in risk is outweighed by the benefits of looking at the lungs. Be sure to inform your physician and the technician if there is any chance you are pregnant. The technician can take special precautions to make sure your abdomen is shielded from radiation.
A computed tomography (CT or CAT) scan takes many X-ray pictures to build detailed images of the chest. The pictures are more detailed than a typical X-ray. During a CT scan of the chest, pictures are taken of cross sections or slices of the thoracic structures in your body. The thoracic structures include your lungs and heart and the bones around these areas.
Before the study, you will need to remove all clothing and jewelry from the waist up. You will be given a hospital gown to wear. Avoid having any barium studies done two to three days before the CT scan.
For the CT scan, you will lie on a special table that slides back and forth through a doughnut-shaped ring. The technologist will give you instructions during the test, asking you to raise your arms sometimes and to hold your breath for 10 to 12 seconds. While you hold your breath, the table will move through the ring as machinery in the ring takes X-ray images. It is important to lie still while the images are taken. You may be asked to lie on your stomach to have extra pictures taken.
A CT scan of the chest takes about 30 minutes. This includes 15 to 20 minutes for the test and 10 minutes for preparation.
Samples of sputum (phlegm) coughed up from the lungs, can be tested for the bacteria that cause TB. For this test, you will collect sputum from a deep cough first thing in the morning and bring it to the hospital or doctor's office. A lab technician will smear a sample of your phlegm onto a glass slide and add a special stain that will make any TB-causing bacteria show up under a microscope.
A sample of the sputum specimen will be added to culture media in order to grow the organism so that it can be correctly identified as TB and to determine which drugs will be most effective in treating the disease. Because the bacteria that cause TB grow slowly, this can take several weeks. The results will help your doctor treat your TB disease. (Many strains of the bacteria that cause TB are resistant to one or more drugs used to treat the disease.)
If you are unable to cough up sputum, it may be necessary to obtain a specimen by having you inhaled a strong saline solution to induce a cough. Alternatively it may be necessary to obtain a specimen with a bronchoscope or isolate the bacteria from the stomach or other locations. Children with active TB often do not produce sputum. For these children, it may be necessary to make treatment decisions based on collecting and growing bacteria from the person who infected the child.
Last reviewed on 12/8/09
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