The picture here shows a chest x-ray, and the arrow points to a TB cavity in the right upper part of the lung (in an x-ray, the right and left side are reversed). On a chest x-ray from someone with TB you can often see the cavitation that the TB bacteria form in the lung tissue. This allows the health worker to examine the lungs of the person with suspected TB. The first step to detecting TB infection, after taking a medical history and doing a physical exam, is to do a chest x-ray. For example a TB diagnostic with a sensitivity of 95% and a specificity of 99% would correctly identify 95 out of 100 people who have TB, but it would also incorrectly identify one of out 100 people who do not have TB. Good tests should have a sensitivity and a specificity at least above 90%. Sensitivity and specificity are expressed in percentages. A good diagnostic test is one with a high sensitivity (detects people with TB) and also a high specificity (correctly identifies healthy people without TB). No diagnostic test is ever completely foolproof – if thousands and thousands of people are tested, mistakes happen. If of the 100 people tested for TB, 50 people are healthy, then a test with a good specificity would correctly identify the 50 healthy people and show 50 negative results. Specificity means that a test only detects people with a disease, and does not wrongly detect a disease in somebody healthy. In this case, health workers would speak of a test with a 'high sensitivity'. For example, if 100 people are being tested, and 50 of them have TB, then a good test should find all 50 people. Sensitivity means that a test needs to detect all people with a disease. The diagnosic tests for TB, as for all diseases, have to follow two important criteria of quality: sensitivity and specificity. Usually this is an infection of the lungs (pulmonary tuberculosis), it can be spread through droplets while coughing and sneezing. People can have TB even if the sputum culture results come back as negative.Īctive tuberculosis is an infection with Mycobacterium tuberculosis which can be transmitted to other people. The arrow shows the TB cavity in the upper part of the right lung.
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