Font size. Font family A A. Content Preview Arcu felis bibendum ut tristique et egestas quis: Ut enim ad minim veniam, quis nostrud exercitation ullamco laboris Duis aute irure dolor in reprehenderit in voluptate Excepteur sint occaecat cupidatat non proident. Lorem ipsum dolor sit amet, consectetur adipisicing elit. Odit molestiae mollitia laudantium assumenda nam eaque, excepturi, soluta, perspiciatis cupiditate sapiente, adipisci quaerat odio voluptates consectetur nulla eveniet iure vitae quibusdam?
Excepturi aliquam in iure, repellat, fugiat illum voluptate repellendus blanditiis veritatis ducimus ad ipsa quisquam, commodi vel necessitatibus, harum quos a dignissimos. Close Save changes. Help F1 or? Hypothetical Example 1 - Screening Test A people are tested for disease.
Try it! Under what circumstance would you really want to minimize the false positives? When would you want to minimize the false negatives? Save changes Close. Because sensitivity seems often to be confused with PPV, and specificity seems often to be confused with NPV, unambiguous definitions for each pair are necessary. These are provided below. Each of these definitions is incontestably accurate, but they can all be easily misinterpreted because none of them sufficiently emphasizes an important distinction between two essentially different contexts.
In the first context, only those people who obtain positive results on the reference standard are assessed in terms of whether they obtained positive or negative results on the screening test. In the second context, the focus changes from people who tested positive on the reference standard to people who tested positive on the screening test.
Here, an attempt is made to establish whether people who tested positive on the screening test do or do not actually have the condition of interest. Expressed differently, the first context is the screening test being assessed on the basis of its performance relative to a reference standard, which focuses on whether the foundations of the screening test are satisfactory; the second context is people being assessed on the basis of a screening test, which focuses on the practical usefulness of the test in clinical practice.
By way of further explanation, sensitivity is based solely on the cells labeled a and c in Figure 1 and, therefore, requires that all people in the analysis are diagnosed according to the reference standard as definitely having the target condition. The determination of sensitivity does not take into account any people who, according to the reference standard, do not have the condition of interest who are in cells b and d. Those cells do not include any people who, according to results from the screening test, do not have the condition who are in cells c and d.
Less elaborated, but perhaps also less helpfully explicit, definitions are possible, for example, that sensitivity is the proportion of people with a condition who are correctly identified by a screening test as indeed having that condition.
Expressed differently and more economically, PPV is the probability that people with a positive screening test result indeed do have the condition of interest. Inspection of Figure 1 supports the above definitions and those that are provided within the next subsection.
As with the definitions often offered for sensitivity, these definitions are accurate but can easily be misinterpreted because they do not sufficiently indicate the distinction between two different contexts that parallel those identified for sensitivity. Specificity is based on the cells labeled b and d in Figure 1 and, therefore, requires that all the people in the analysis are diagnosed, according to a reference standard, as not having the target condition. Specificity does not take into account any people who, according to the reference standard, do have the condition as pointed out above, those people, in the cells labeled a and c, were taken into account when determining sensitivity.
Those cells do not include any people who, according to the screening test, do have the condition who are located in cells a and b. Less elaborated, but perhaps also less helpfully explicit, definitions are possible, for example, that specificity is the proportion of people without a condition who are correctly identified by a screening test as indeed not having the condition. Expressed differently and more economically, NPV is the probability that people with a negative screening test result indeed do not have the condition of interest.
Sensitivity and specificity are concerned with the accuracy of a screening test relative to a reference standard. Here, the screening test is being assessed. There are two main questions of relevance in that second situation. Second, if the screening test yields a negative result, what is the probability that the person does not have the condition NPV?
More precisely, sensitivity and specificity indicate the concordance of a test with respect to a chosen referent, while PPV and NPV, respectively, indicate the likelihood that a test can successfully identify whether people do or do not have a target condition, based on their test results.
The two contexts i. Of particular importance, although it is desirable to have tests with high sensitivity and specificity, the values for those two metrics should not be relied on when making decisions about individual people in screening situations.
The lack of correspondence between sensitivity, specificity, and predictive values is illustrated by the inconsistent pattern of entries in Table 1 and should become more obvious in the next section. Because the pairs of categories into which people are placed when sensitivity and specificity values are calculated are not the same as the pairs of categories that pertain in a screening context, there are not only important distinctions between sensitivity and PPV, and between specificity and NPV, but there are also distinct limitations on sensitivity and specificity for screening purposes.
Akobeng [ 9 , p. Sensitivity does not provide the basis for informed decisions following positive screening test results because those positive test results could contain many false positive outcomes that appear in the cell labeled b in Figure 1.
Those outcomes are ignored in determining sensitivity cells a and c are used for determining sensitivity. Therefore, of itself a positive result on a screening test, even if that test has high sensitivity, is not at all useful for definitely regarding a condition as being present in a particular person.
Conversely, specificity does not provide an accurate indication about a negative screening test result because negative outcomes from a screening test could contain many false negative results that appear in the cell labeled c, which are ignored in determining specificity cells b and d are used for determining specificity. Therefore, of itself , a negative result on a screening test with high specificity is not at all useful for definitely ruling out disease in a particular person.
Failing to appreciate the above major constraints on sensitivity and specificity arises from what is known in formal logic as confusion of the inverse An example of this with regard to sensitivity, consciously chosen in a form that makes the problem clear, would be converting the logical proposition This animal is a dog; therefore it is likely to have four legs into the illogical proposition This animal has four legs; therefore it is likely to be a dog. A parallel confusion of the inverse can occur with specificity.
An example of this would be converting the logical proposition This person is not a young adult; therefore this person is not likely to be a university undergraduate into the illogical proposition This person is not a university undergraduate; therefore this person is not likely to be a young adult. These examples demonstrate the flaws in believing that a positive result on a highly sensitive test indicates the presence of a condition and that a negative result on a highly specific test indicates the absence of a condition.
Instead, it should be emphasized that a highly sensitive test, when yielding a positive result, by no means indicates that a condition is present many animals with four legs are not dogs , and a highly specific test, when yielding a negative result, by no means indicates that a condition is absent many young people are not university undergraduates.
Despite the above reservations concerning sensitivity and specificity in a screening situation, sensitivity and specificity can be useful in two circumstances but only if they are extremely high.
First, because a highly sensitive screening test is unlikely to produce false negative outcomes there will be few entries in cell c of Figure 1 , people who test negative on that kind of screening test i. Expressed differently, high sensitivity permits people to be confidently regarded as not having a condition if their screening test yields a negative result. Second, because a highly specific screening test is unlikely to produce false positive results there will be few entries in cell b in Figure 1 , people are very unlikely to be categorized as having a condition if they indeed do not have it.
Expressed differently, high specificity permits people to be confidently regarded as having a condition if their diagnostic test yields a positive result. The mnemonics snout and spin , it must be emphasized, pertain only when sensitivity and specificity are high. Their pliability, therefore, has some strong limitations. Furthermore, these mnemonics are applied in a way that might seem counterintuitive.
In addition, Pewsner et al. As a consequence, both sensitivity and specificity remain unhelpful for making decisions about individual people in most screening contexts, and PPV and NPV should be retained as the metrics of choice in those contexts. Considerations might also include over- versus under-application of diagnostic procedures as well as the possibility of premature versus inappropriately delayed application of diagnostic procedures.
Input from clinicians and policymakers is likely to be particularly informative in any deliberations. Decisions about desirable PPVs and NPVs can be approached from two related and complementary, but different, directions. One approach involves the extent to which true positive and true negative results are desirable on a screening test.
The other approach involves the extent to which false positive and false negative results are tolerable or even acceptable. A high PPV is desirable, implying that false positive outcomes are minimized, under a variety of circumstances.
If the trait is not present, the disease is unlikely to be present and can be ruled out. The specificity of a test is the proportion of people who test negative among all those who actually do not have that disease. A specific test helps rule a disease in when positive e.
If a disease UTI has a trait nitrites in urine that is rare in other diseases, a test for that trait can be thought of as being highly specific because the trait is specific to that disease. However, a positive result would not mean they definitely have a UTI because a highly specific test does not factor in how common the disease is prevalence.
Positive predictive value PPV and negative predictive value NPV are directly related to prevalence and allow you to clinically say how likely it is a patient has a specific disease. The positive predictive value is the probability that following a positive test result , that individual will truly have that specific disease.
The negative predictive value is the probability that following a negative test result , that individual will truly not have that specific disease. For any given test i. Therefore, as prevalence decreases , the NPV increases because there will be more true negatives for every false negative. This is because a false negative would mean that a person actually has the disease, which is unlikely because the disease is rare low prevalence. The examples given should allow you to see how and why these vary as different factors change.
Clinical Examination. An Introduction to the Arclight. Eye Drops Overview. Prescribing in Renal Impairment. Interpreting Hepatitis B Serology. Medicine Flashcard Collection. A collection of surgery revision notes covering key surgical topics. Surgery Flashcard Collection. Septic Arthritis. Compartment Syndrome.
Anatomical Planes.
0コメント