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Lack of goals—not knowing what is important to do, or when it should be doneCHAPTER 36—EPIDEMIOLOGY IN THE CLINICAL LABORATORYTopic SummaryI.Introduction: EpidemiologyIllness and disease outcomes are not chance occurrences in people, but assumed to be due to a wide variety of determinants, including behavioral and biological characteristics. Because the presence or absence of these factors and the levels to which that are present may differ in population subgroups, patterns of disease frequency and distribution may vary within a population. The science that investigates this variability in disease occurrence is epidemiology and, as defined by Last (1995), it is the study of determinants and distribution of disease in populations groups. Epidemiology, in essence then, investigates, the who, what, where, and when
of disease outcomes. Disease outcomes may be considered as any altered health state, such as injury, illness, or death.Epidemiology provides information useful for disease prevention and control efforts for establishing the causes of disease. For example, epidemiology is routinely used by public health scientists and healthcare providers for planning strategies, for program evaluation, as well as for disease surveillance. Other researchers use epidemiology methods to establish the etiologic agents of disease. Clinical laboratories integrate these activities because they provide essential screening and diagnostic test services used to assess the health status of the public. Further, epidemiology methods are routinely applied in the clinical laboratory to assess the quality and accuracy of these tests as well as to monitor mandatory disease reporting to state agencies.II.The purpose of this chapter is to illustrate the following epidemiology principles and methods used in the clinical laboratory:1.Evaluation of screening test validitya.Sensitivityb.Specificityc.Predictive value positived.Predictive value negative2.Evaluation of interrater reliability-kappa3.Surveillance compliance
III.Evaluation of Screening Test ValidityTest validitymeasures how closely the test value is to the true measure—in other words, accuracy (Last, 1995). In the laboratory, a screening test is valid when the result correctly identifies a patient with the disease a positive and a patient without the condition as negative. The screening test must then be validated using a highly accurate diagnostic test or procedure, generally known as a “gold standard” (the most accurate measure currently available, Last 1995). Examples of gold standards include the process of collecting bone marrow aspirations, clinically diagnosed cases of myocardial infarction, and a positive streptococcal B result from culture.Table 36.1, (p. 218) illustrates the relationship between a screening test and a gold standard. As shown, test validity is measured by sensitivityand specificity. The sensitivity of a screening test, its ability to detect positives among the truly diseased, is calculated as: a/(a + c), and is measured as a proportion (0.0 – 1.0).