5 The gold standard test for an ankle fracture is an X-ray but because so few ankle injuries are fractures it is considered inappropriate to X-ray everyone. Ankle injuries are very common, but fractures are only present in approximately 15% of cases. Imagine that you have 100 patients in your emergency department (ED) waiting room who have all presented with an acute ankle injury. 3 The emotional and economic costs of this have led to the development of decision-aids to help women make an informed decision about undergoing screening. Mammography, the radiographic detection of potential breast tumours, is thought to have an over-detection rate of between 7% and 32%. Over-detection is the identification of an abnormality that causes concern but if left untreated is unlikely to cause harm. This phenomenon is currently a concern in medicine, discussed as over-detection, over-diagnosis and over-treatment-together these could be described as over-medicalisation. This will cause anxiety and unnecessary follow-up for well people. The screening test in figure 2 will capture all those who have the disease but also many who do not. There is a risk that a test with high specificity will capture some people who do not have Disease D ( figure 3). 2 However, low sensitivity can be compensated for by frequent screening, which is why most cervical screening policies rely on women attending every three tofive years. This is a poor performing test and has led to a suggestion that we add in or switch instead to screening for high-risk variants of the human papilloma virus, which has a higher sensitivity. 1 This means that up to 70% of women who have cervical abnormality will not be detected by this screening test. Meta-analysis suggests that the cervical smear or pap test has a sensitivity of between 30%–87% and a specificity of 86%–100%. A good example of this is screening for cervical cell changes that might indicate a high likelihood of cancer. These are good numbers when we compare with some screening tests for which there are high stakes outcomes. The test will correctly identify 82% who do not have the disease, but it will also identify 18% of people as having the disease when they do not. This test will correctly identify 60% of the people who have Disease D, but it will also fail to identify 40%. So, in our example, the sensitivity is 60% and the specificity is 82%. We can then discuss sensitivity and specificity as percentages. See box 1 for definitions of common terms used when describing sensitivity and specificity.īox 2 Calculation of sensitivity and specificity from figure 2 test resultīecause percentages are easy to understand we multiply sensitivity and specificity figures by 100. That group of 20% will be identified as having the disease when they do not, this is known as a false positive. A test that has an 80% specificity can correctly identify 80% of people in a group that do not have a disease, but it will misidentify 20% of people. This smaller group of people have the disease, but the test failed to detect them-this is known as a false negative. When a test has a sensitivity of 0.8 or 80% it can correctly identify 80% of people who have the disease, but it misses 20%. The gold standard test, when compared with other options, is most likely to correctly identify people with the disease (it is specific), and correctly identify those who do not have the disease (it is sensitive). There is often a ‘gold-standard’ screening test-one that is considered the best to use because it is the most accurate. In the clinical setting, screening is used to decide which patients are more likely to have a condition. We often think of sensitivity and specificity as being ways to indicate the accuracy of the test or measure. Whenever we create a test to screen for a disease, to detect an abnormality or to measure a physiological parameter such as blood pressure (BP), we must determine how valid that test is-does it measure what it sets out to measure accurately? There are lots of factors that combine to describe how valid a test is: sensitivity and specificity are two such factors.
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