Blood Test (e.g., The T-SPOT.TB Test)
Blood tests are a relatively new development in TB disease testing.  These tests are also known as Interferon Gamma Release Assays (IGRAs).  The principle of the T-SPOT.TB test is the detection of effector T cells that respond to stimulation by M. tuberculosis antigens (ESAT-6 and CFP 10) by capturing interferon gamma (IFN-γ) in the vicinity of T cells from which it was secreted. 

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Tuberculin Skin Test (TST) 
The tuberculin skin test (also known as PPD or Mantoux test) has been in existence for over 100 years.  The test works by injecting purified protein derivative (PPD) intradermally (between the layers of the skin).  PPD is an extract of inactive cultures of M. tuberculosis, the bacterium that causes TB disease. The basis of the test is to determine if the body has been exposed to TB infection.  If previously infected, the body will recognize the proteins of the PPD and will generate an immune response.  This response will take place in the form of an induration (lump, swelling or blister) at the site of injection.  An induration may mean the person is infected.  The injection site must be evaluated by a trained healthcare professional 48-72 hours after the PPD was administered. The sensitivity of the TST can vary and  it can be affected by a previous Bacille Calmette-Guerin (BCG) vaccination, a depressed immune system (immunocompromised) and by other illnesses or medical treatment.

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Chest Radiograph (X-ray)
Chest x-rays are used to check for lung abnormalities in people who have symptoms of pulmonary TB disease.  The results of chest x-rays may be suggestive of TB disease.  However, a chest x-ray alone cannot diagnose a tuberculosis infection in the lungs. Furthermore, scarring in the lungs remains after previous TB disease (even if the patient is completely cured) and therefore it is difficult to distinguish past cured TB from current TB disease.

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Sputum Smear Microscopy
The sputum smear is a simple laboratory test that examines sputum for the detection of bacteria, i.e. M. tuberculosis.  This test can also identify other types of bacteria that are not M. tuberculosis and so it cannot always distinguish between TB and other infections. The World Health Organization (WHO) estimates that the test only identifies 35% of patients with TB disease.  Despite this shortcoming, it is still a frontline tool for  TB disease diagnosis.  The sputum smear can quickly determine if a person is infected while the more definitive culture technique (see below) takes longer to obtain test results.

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Culture
Culture tests are seen as the gold standard for active TB disease as they are extremely specific.  M. tuberculosis can be cultured from a variety of specimens and can be used to detect pulmonary as well as extra-pulmonary disease.  By assessing the effect of antibiotics on the cultured bacteria, this test can also provide data on which antibiotics would be effective in treating the infection.  Several drawbacks of this test are the difficulty in obtaining specimens containing live organisms and the length of time to obtain a result, which can range from 2-6 weeks. 

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Nucleic Acid Amplification Tests (NAATs)
Nucleic acid amplification tests (NAATs), such as polymerase chain reaction (PCR), are a relatively new development in active TB testing. These tests detect the presence of genetic material in bacteria.  Samples for these tests  have to contain a certain number of TB bacteria for the detection to occur.  This is not always possible, particularly with non-pulmonary TB where sensitivity can be low.  These tests are relatively complicated to run in the laboratory and can be expensive. 

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