Chapter 11Screening for Disease in the Community
Learning ObjectivesDefine and discuss reliability and validityDefine the term screeningDefine and discuss sensitivity a...
Screening for DiseaseScreening--the presumptive identification of unrecognized disease or defects by the application of t...
Multiphasic ScreeningDefined   as the use of two or more screening tests together among large groups of peopleInformatio...
Mass Screening and Selective ScreeningMass  screening--screening on a large scale of total population groups regardless o...
Mass Health ExaminationsPopulation  or epidemiologic surveys--purpose is to gain knowledge regarding the distribution and...
Mass Health Examinations (cont’d)Epidemiologic  surveillance--aims at the protection of community health through case det...
Appropriate Situations forScreening Tests and ProgramsSocialScientificEthical
SocialThe   health problem should be important for the individual and the communityDiagnostic follow-up and intervention...
ScientificNatural  history of the condition should  be adequately understood  ◦   This knowledge permits identification o...
EthicalThe  program can alter the natural history of the condition in a significant proportion of those screenedSuitable...
Characteristics of a GoodScreening TestSimpleRapidInexpensiveSafeAcceptable
Evaluation of Screening TestsReliability    types       Validity      types ◦   Repeated                ◦   Content     ...
Reliability (Precision)The  ability of a measuring instrument to give consistent results on repeated trialsRepeated meas...
Reliability (cont’d)Internal consistency reliability--evaluates the degree of agreement or homogeneity within a questionn...
Validity (Accuracy)The  ability of a measuring instrument to give a true measureCan be evaluated only if an accepted and...
Validity (cont’d)Content  validity--the degree to which a measure covers the range of meanings included within the concep...
Validity (cont’d)Two    types of criterion-referenced validity: ◦   Predictive validity--denotes the ability of a     mea...
Validity (cont’d)ConstructValidity--degree to which the measurement agrees with the theoretical concept being investigated
Interrelationships BetweenReliability and ValidityIt is possible for a measure to be highly reliable but invalidIt is no...
Representation of Reliability andValidity
Sources of Unreliability andInvalidityMeasurement  bias--constant errors that are introduced by a faulty measuring device...
Sources of Unreliability andInvalidity (cont’d)Halo effect--bias that affects the validity of questionnaire measurements
Example of HaloAll items of a checklist evaluation of an employee may be filled out in the same general direction based o...
Sources of Unreliability andInvalidity (cont’d)Social   desirability effects  ◦   Respondent answers questions in a manne...
Contingency (2 by 2)Table                   Disease                   Present Absent   TotalScreen Positive       a       ...
Sensitivity and SpecificitySensitivity = the probability that a person  with the disease will test positiveP(+ | D)Speci...
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Predictive ValuesPositive Predictive value = the probability that a person who tests positive has the diseaseNegative Pred...
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Sample Calculation
Effects of Disease Prevalence onthe Predictive Value of aScreening TestWhen   the prevalence of a disease falls, the pred...
Relationship Between Sensitivityand SpecificityTo  improve sensitivity, the cut point used to classify individuals as dis...
Relationship Between Sensitivity andSpecificity (cont’d)
Procedures to Improve Sensitivityand SpecificityRetrain  screeners--reduces the amount of misclassification in tests that...
Evaluation of Screening ProgramsRandomized      control trials ◦   Subjects receive either the new screening     test or ...
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Screenings in the community

  1. 1. Chapter 11Screening for Disease in the Community
  2. 2. Learning ObjectivesDefine and discuss reliability and validityDefine the term screeningDefine and discuss sensitivity and specificity
  3. 3. Screening for DiseaseScreening--the 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
  4. 4. 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
  5. 5. Mass Screening and Selective ScreeningMass screening--screening on a large scale of total population groups regardless of risk statusSelective screening--screens subsets of the population at high risk for disease ◦ More economical, and likely to yield more true cases.
  6. 6. Mass Health ExaminationsPopulation or epidemiologic surveys--purpose is to gain knowledge regarding the distribution and determinants of diseases in selected populations
  7. 7. Mass Health Examinations (cont’d)Epidemiologic surveillance--aims at the protection of community health through case detection and interventionCase finding (opportunistic screening)--the utilization of screening tests for detection of conditions unrelated to the patient’s chief complaint
  8. 8. Appropriate Situations forScreening Tests and ProgramsSocialScientificEthical
  9. 9. SocialThe health problem should be important for the individual and the communityDiagnostic follow-up and intervention should be available to all who require themThere should be a favorable cost-benefit ratioPublic acceptance must be high
  10. 10. 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
  11. 11. 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
  12. 12. Characteristics of a GoodScreening TestSimpleRapidInexpensiveSafeAcceptable
  13. 13. Evaluation of Screening TestsReliability types Validity types ◦ Repeated ◦ Content measurements ◦ Criterion- ◦ Internal consistency referenced ◦ Interrater  Predictive  Concurrent ◦ Construct
  14. 14. Reliability (Precision)The ability of a measuring instrument to give consistent results on repeated trialsRepeated measurement reliability--the degree of consistency among repeated measurements of the same individual on more than one occasion
  15. 15. Reliability (cont’d)Internal consistency reliability--evaluates the degree of agreement or homogeneity within a questionnaire measure of an attitude, personal characteristic, or psychological attribute.Interrater reliability--reliability assessments derived from agreement among trained experts
  16. 16. 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
  17. 17. Validity (cont’d)Content validity--the degree to which a measure covers the range of meanings included within the conceptCriterion-referenced validity--found by correlating a measure with an external criterion of the entity being assessed
  18. 18. Validity (cont’d)Two types of criterion-referenced validity: ◦ Predictive validity--denotes the ability of a measure to predict some attribute or characteristic in the future ◦ Concurrent validity--obtained by correlating a measure with an alternative measure of the same phenomenon taken at the same point in time
  19. 19. Validity (cont’d)ConstructValidity--degree to which the measurement agrees with the theoretical concept being investigated
  20. 20. Interrelationships BetweenReliability and ValidityIt is possible for a measure to be highly reliable but invalidIt is not possible for a measure to be valid but unreliable
  21. 21. Representation of Reliability andValidity
  22. 22. Sources of Unreliability andInvalidityMeasurement bias--constant errors that are introduced by a faulty measuring device and tend to reduce the reliability of measurements
  23. 23. Sources of Unreliability andInvalidity (cont’d)Halo effect--bias that affects the validity of questionnaire measurements
  24. 24. Example of Halo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
  25. 25. Sources of Unreliability andInvalidity (cont’d)Social desirability effects ◦ Respondent answers questions in a manner that agrees with desirable social norms
  26. 26. Contingency (2 by 2)Table Disease Present Absent TotalScreen Positive a b a+bResult Negative c d c+d Total a+c b+d
  27. 27. Sensitivity and SpecificitySensitivity = the probability that a person with the disease will test positiveP(+ | D)Specificity = the probability that a person who does not have the disease will test negative 27
  28. 28. 28
  29. 29. Predictive ValuesPositive Predictive value = the probability that a person who tests positive has the diseaseNegative Predictive value = The probability that a person who tests negative does not have the disease 29
  30. 30. 30
  31. 31. 31
  32. 32. Sample Calculation
  33. 33. Effects of Disease Prevalence onthe Predictive Value of aScreening TestWhen the prevalence of a disease falls, the predictive value (+) falls, and the predictive value (-) rises.
  34. 34. Relationship Between Sensitivityand 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
  35. 35. Relationship Between Sensitivity andSpecificity (cont’d)
  36. 36. Procedures to Improve Sensitivityand SpecificityRetrain screeners--reduces the amount of misclassification in tests that require human assessmentRecalibrate screening instrument-- reduces the amount of imprecisionUtilize a different testUtilize more than one test
  37. 37. Evaluation of Screening ProgramsRandomized control trials ◦ Subjects receive either the new screening test or usual careCase-control studies ◦ Cases--fatal cases of the disease ◦ Controls--nonfatal cases ◦ Exposure--screening program

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