Radiological diagnosis of TB: Chest X-Ray and CT scan (Part 14)
Welcome to the 14th part of the multi-part series on Tuberculosis, abbreviated as TB.
The diagnostic tests for TB, as for all diseases, have to follow two important criteria of quality: sensitivity and specificity. Sensitivity means that a test needs to detect all people with a disease. For example, if 100 people are being tested, and 50 of them have TB, then a good and reliable test should find all those 50 people affected by TB. Specificity means that a test only detects people with a disease, and does not wrongly detect disease in somebody healthy. For instance, if out of the 100 people tested for TB, 50 people are healthy, then a test with good specificity would correctly identify the 50 healthy people and 50 people that are affected by TB.
No diagnostic test is ever completely foolproof — if thousands and thousands of people are tested, mistakes happen. A good diagnostic test is one that has high sensitivity and high specificity. Sensitivity and specificity are expressed in percentages. Good tests should have a sensitivity and specificity of at least above 90%. For example, a TB diagnostic test 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 person out of 100 people who does not have TB.
Chest radiography
The first step in detecting TB infection is to do a chest x-ray. This allows the healthcare provider to examine the lungs of the person with suspected TB. It is important to know the principle behind X-ray studies because of which they are able to visualize the internal structures of a patient. X-rays are a form of electromagnetic radiation produced by an X-ray tube. These electromagnetic radiations are absorbed by parts of our body and the radiations that are not absorbed hit the photographic ‘plate’ to form a ‘negative image’ or X-ray. Different tissues in the body vary in their absorption of X-rays: for instance, dense bones absorb more radiation, while soft tissues allow more radiation to pass through. This variance produces contrasts within the image to give a two-dimensional representation of the three-dimensional structures. Thus imaging the chest is done to assess lung and intrathoracic pathologies. And any abnormalities in the lung can be easily identified by a chest x-ray.
From a chest X-ray, it can be determined, whether a person is suffering from active TB or latent TB infection. For instance, the chest x-ray of a person suffering from active TB may show infiltration, cavities, or nodular opacities on the lungs of a patient. Additionally, lesions may also appear on the lungs of an affected person. On the other hand, calcified nodular lesions in the form of white spots on chest X-rays may indicate that a person is suffering from latent TB. Nodules and fibrotic scars, seen on chest x-ray, may contain slowly multiplying TB bacteria, with the potential, for future progression to active tuberculosis. But the diagnosis of TB cannot be made by a chest x-ray alone.
Chest X-ray has a high sensitivity for diagnosing lungs because it can easily identify any abnormality in the lung. However, a chest X-ray has poor specificity; although some abnormalities detected by chest x-ray are rather specific for pulmonary TB for example, cavities, but a variety of other health conditions or illnesses may produce abnormalities whose appearance on a chest x-ray resembles TB. Therefore relying only on a chest X-ray for TB diagnosis sometimes leads to overdiagnosis, and sometimes may lead to underdiagnosis. Thus the diagnosis of TB cannot be made by a chest x-ray alone.
And the chest x-rays are always required to be followed by other diagnostic tests that aim at finding the TB bacteria e.g. sputum-smear microscopy, culture, or a molecular test.
CT scan
Another medical imaging technique CT scan can also be done to detect TB. CT scan stands for computed tomography. In this technique, a narrow X-ray beam is used, which circles around one part of the body. This provides a series of X-ray images from many different angles. A computer stacks these scans one on top of the other to form clear and detailed images of the organs inside the chest. Therefore, a CT scan can detect more subtle signs that might indicate tuberculosis.
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