With a fluorescent microscope the smear is illuminated with a quartz halogen or high pressure mercury vapour lamp, allowing a much larger area of the smear to be seen and resulting in more rapid examination of the specimen. The use of fluorescent microscopy is a way of making sputum tests more accurate. In some countries sputum smear microscopy is being phased out, and is being replaced by molecular tests. 9 In countries with a high prevalence of both pulmonary TB and HIV infection, the detection rate can be even lower, as many people with HIV and TB co-infection have very low levels of TB bacteria in their sputum, and are therefore recorded as sputum negative. The sensitivity though is only about 50-60%. In addition the results are available within hours. Sputum smear microscopy is inexpensive and simple, and people can be trained to do it relatively quickly and easily.
#Negative tb test series#
A series of special stains are then applied to the sample, and the stained slide is examined under a microscope for signs of the TB bacteria. To do the test a very thin layer of the sample is placed on a glass slide, and this is called a smear. 5 In 2012 it was suggested that two specimens can be collected on the same day without any loss of accuracy. Several samples of sputum will normally be collected. A sample of sputum is usually collected by the person coughing. Sputum is a thick fluid that is produced in the lungs and the airways leading to the lungs. Smear microscopy of sputum is often the first test to be used in countries with a high rate of TB infection. 4Ī test for TB, a sputum smear stained using fluorescent acid fast stain © CDC/R W Smithwick
#Negative tb test skin#
3 In low prevalence resource rich settings, IGRAs are beginning to be used in place of the TB skin test. It is also thought that the IGRAs may not be as accurate in people who have HIV. Disadvantages include the fact that the blood sample must be processed fairly quickly, laboratory facilities are required, and the test is only for latent TB. Results can be available within 24 hours, and prior BCG vaccination does not cause a false positive result. The advantages of an IGRA includes the fact that it only requires a single patient visit to carry out the TB test. Food and Drug Administration (FDA), and are commercially available in the U.S., are the QuantiFERON® TB Gold test, and the T-SPOT® TB test. Two IGRAs that have been approved by the U.S. In practice you carry out one of these TB tests by taking a blood sample and mixing it with special substances to identify if the cytokine is present. These TB tests work by detecting a cytokine called the interferon gamma cytokine. The immune system produces some special molecules called cytokines. IGRAs are blood tests that measure a person’s immune response to the bacteria that cause TB. False negative results particularly happen with children, older people and people with HIV. This vaccine is widely used in countries with high rates of TB infection. It can also happen because the person has been vaccinated with the BCG vaccine. False positive results happen with the skin test because the person has been infected with a different type of bacteria, rather than the one that causes TB. None of these tests though will guarantee a correct result. The Mantoux TB is the type of test most often used, although the Heaf and Tine tests are still used in some countries. 2 The skin test also cannot tell if the person has latent TB or active TB disease. But interpreting the skin test result, that is whether it is a positive result, may also involve considering the lifestyle factors of the person being tested for TB. The larger the size of the affected area the greater the likelihood that the person has been infected with TB bacteria at some time in the past. The test result depends on the size of the raised hard area or swelling. A health care worker measures the size of the reaction to the tuberculin skin test © CDC
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