Unformatted text preview: Chapter 11 Chapter 11 Screening for Disease in the Community Screening for Disease Screening for Disease Screeningthe presumptive identification of unrecognized disease or defects by the application of tests, examinations, or other procedures that can be applied rapidly. Positive screening results are followed by diagnostic tests to confirm actual disease. Multiphasic Screening Multiphasic Screening Defined as the use of two or more screening tests together among large groups of people. Information obtained on risk factor status, history of illness, and health measurements. Commonly used by employers and health maintenance organizations. Mass Screening and Selective Screening Mass Screening and Selective Screening
Mass screeningscreening on a large scale of total population groups regardless of risk status. Selective screeningscreens subsets of the population at high risk for disease. More economical, and likely to yield more true cases. Example: Screening highrisk persons for TaySachs disease. Mass Health Examinations Mass Health Examinations Population or epidemiologic surveyspurpose is to gain knowledge regarding the distribution and determinants of diseases in selected populations. No benefit to the participant is implied. Mass Health Examinations (cont’d) Mass Health Examinations Epidemiologic surveillanceaims at the protection of community health through case detection and intervention. Case finding the utilization of screening tests for detection of conditions unrelated to the patient’s chief complaint. Appropriate Situations for Appropriate Situations for Screening Tests and Programs Social Scientific Ethical Social Social The health problem should be important for the individual and the community. Diagnostic followup and intervention should be available to all who require them. There should be a favorable costbenefit ratio. Public acceptance must be high. Scientific Scientific Natural history of the condition should be adequately understood. This knowledge permits identification of early stages of disease and appropriate biologic markers of progression. Prevalence of the disease or condition is high. Ethical Ethical The program can alter the natural history of the condition in a significant proportion of those screened. Suitable, acceptable tests for screening and diagnosis of the condition as well as acceptable, effective methods of prevention are available. Characteristics of a Good Screening Test Characteristics of a Good Screening Test Simpleeasy to learn and perform. Rapidquick to administer; results available rapidly. Inexpensivegood costbenefit ratio. Safeno harm to participants. Acceptableto target group. Evaluation of Screening Tests Evaluation of Screening Tests Reliability types Repeated measurements Internal consistency Interjudge Validity types Content Criterionreferenced
Predictive Concurrent Construct Reliability (Precision) Reliability (Precision) The ability of a measuring instrument to give consistent results on repeated trials. Repeated measurement reliabilitythe degree of consistency among repeated measurements of the same individual on more than one occasion. Reliability (cont’d) Reliability Internal consistency reliabilityevaluates the degree of agreement or homogeneity within a questionnaire measure of an attitude, personal characteristic, or psychologic attribute. Interjudge reliabilityreliability assessments derived from agreement among trained experts. Validity (Accuracy) Validity (Accuracy) The ability of a measuring instrument to give a true measure. Can be evaluated only if an accepted and independent method for confirming the test measurement exists. Validity (cont’d) Validity Content validitythe degree to which a measure covers the range of meanings included within the concept. Criterionreferenced validityfound by correlating a measure with an external criterion of the entity being assessed. Validity (cont’d) Validity Two types of criterionreferenced validity: Predictive validitydenotes the ability of a measure to predict some attribute or characteristic in the future. Concurrent validityobtained by correlating a measure with an alternative measure of the same phenomenon taken at the same point in time. Validity (cont’d) Validity Construct Validitydegree to which the measurement agrees with the theoretical concept being investigated. Representation of Reliability Representation of Reliability and Validity Sources of Unreliability and Invalidity Sources of Unreliability and Invalidity Measurement biasconstant errors that are introduced by a faulty measuring device and tend to reduce the reliability of measurements. Example: A miscalibrated blood pressure manometer. Sources of Unreliability and Invalidity Sources of Unreliability and Invalidity
(cont’d) Halo effectbias that affects the validity of questionnaire measurements. Social desirability effects Example: All items of a checklist evaluation of an employee may be filled out in the same general direction based on the supervisor’s opinion of the individual. Respondent answers questions in a manner that agrees with desirable social norms. Fourfold Table Fourfold Table Gold Standard Present Absent Test Result Positive Negative Total a c a + c b d b + d Total a + b c + d Measures of the Validity of Screening Measures of the Validity of Screening Tests Sensitivitythe ability of the test to identify correctly all screened individuals who actually have the disease (a/a+c). Specificitythe ability of the test to identify only nondiseased individuals who actually do not have the disease (d/b+d). Measures of the Validity of Measures of the Validity of Screening Tests (cont’d) Predictive value (+)the proportion of individuals screened positive by the test who actually have the disease (a/a+b). Predictive value ()the proportion of individuals screened negative by the test who do not have the disease (d/c+d). Other Measures from the 2 by 2 Table Other Measures from the 2 by 2 Table Accuracy of a screening testdetermined by the formula: (a+d)/(a+b+c+d). Prevalencedetermined by the formula: (a+c)/(a+b+c+d) Sample Calculation Sample Calculation Effects of Disease Prevalence on the Effects of Disease Prevalence on the Predictive Value of a Screening Test When the prevalence of a disease falls, the predictive value (+) falls, and the predictive value () rises. Relationship Between Sensitivity Relationship Between Sensitivity and Specificity To improve sensitivity, the cut point used to classify individuals as diseased should be moved farther in the range of the nondiseased (normals). To improve specificity, the cut point should be moved farther in the range typically associated with the disease. Relationship (cont’d) Relationship Procedures to Improve Sensitivity Procedures to Improve Sensitivity and Specificity Retrain screenersreduces the amount of misclassification in tests that require human assessment. Recalibrate screening instrumentreduces the amount of imprecision. Utilize a different test. Utilize more than one test. Evaluation of Screening Programs Evaluation of Screening Programs Randomized control trials Subjects receive either the new screening test or usual care. Compare geographic regions with screening programs to those without. Ecologic time trend studies Casecontrol studies Casesfatal cases of the disease. Controlsnonfatal cases. Exposurescreening program. Sources of Screening Evaluation Bias Sources of Screening Evaluation Bias Lead time bias The perception that the screendetected case has longer survival because the disease was identified early. Length bias Particularly relevant to cancer screening. Tumors identified by screening are slower growing and have a better prognosis. Selection bias Motivated participants have a different probability of disease than do those who refuse to participate. Natural History of Disease Natural History of Disease Issues in the Classification of Issues in the Classification of Morbidity and Mortality The nomenclature and classification of disease are central to the reliable measurement of the outcome variable in epidemiologic research. Nomenclaturea highly specific set of terms for describing and recording clinical or pathologic diagnoses to classify ill persons into groups. Issues in the Classification of Morbidity and Issues in the Classification of Morbidity and Mortality (cont’d) Classificationthe statistical compilation of groups of cases of disease by arranging disease entities into categories that share similar features. Two types of criteria used for the classification of ill persons: Causal Manifestational ...
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