Chapter 11Chapter 11Screening for Disease in the Community

Learning ObjectivesLearning ObjectivesDefine and discuss reliability and

validityDefine the term screeningDefine and discuss sensitivity and

specificity

Screening for DiseaseScreening 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

Multiphasic ScreeningMultiphasic 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 Mass Screening and Selective ScreeningSelective 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.

Mass Health Mass Health ExaminationsExaminations

Population or epidemiologic surveys--purpose is to gain knowledge regarding the distribution and determinants of diseases in selected populations

Mass Health Examinations Mass Health Examinations (cont’d)(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

Appropriate Situations for Appropriate Situations for Screening Tests and Screening Tests and ProgramsPrograms

SocialScientificEthical

SocialSocial

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

ScientificScientific

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

EthicalEthical

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 Characteristics of a Good Screening TestScreening Test

SimpleRapidInexpensiveSafeAcceptable

Evaluation of Screening Evaluation of Screening TestsTests

Reliability types◦ Repeated measurements

◦ Internal consistency

◦ Interrater

Validity types◦ Content◦ Criterion-referenced Predictive Concurrent

◦ Construct

Reliability (Precision)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

Reliability (cont’d)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

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 (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

Validity (cont’d)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

Validity (cont’d)Validity (cont’d)

Construct Validity--degree to which the measurement agrees with the theoretical concept being investigated

Interrelationships Between Interrelationships Between Reliability and ValidityReliability 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

Representation of Representation of Reliability and ValidityReliability and Validity

Sources of Unreliability and Sources of Unreliability and InvalidityInvalidity

Measurement bias--constant errors that are introduced by a faulty measuring device and tend to reduce the reliability of measurements

Sources of Unreliability and Sources of Unreliability and Invalidity (cont’d)Invalidity (cont’d)

Halo effect--bias that affects the validity of questionnaire measurements

Example of HaloExample of HaloAll 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

Sources of Unreliability and Sources of Unreliability and Invalidity (cont’d)Invalidity (cont’d)

Social desirability effects◦ Respondent answers questions in a

manner that agrees with desirable social norms

Contingency (2 by Contingency (2 by 2)Table2)Table

Disease

Present Absent Total

ScreenResult

Positive a b a + b

Negative c d c + d

Total a + c b + d

Sensitivity and SpecificitySensitivity and SpecificitySensitivity = the probability that

a person with the disease will test positive

P(+ | D) Specificity = the probability that

a person who does not have the disease will test negative

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Predictive ValuesPredictive Values

Positive Predictive value = the probability that a person who tests positive has the disease

Negative Predictive value = The probability that a person who tests negative does not have the disease

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Sample CalculationSample Calculation

Effects of Disease Effects of Disease Prevalence on the Prevalence on the Predictive Value of a Predictive Value of a Screening TestScreening Test

When the prevalence of a disease falls, the predictive value (+) falls, and the predictive value (-) rises.

Relationship Between Relationship Between Sensitivity and SpecificitySensitivity 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 Between Relationship Between Sensitivity and Specificity Sensitivity and Specificity (cont’d)(cont’d)

Procedures to Improve Procedures to Improve Sensitivity and SpecificitySensitivity and 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 testUtilize more than one test

Evaluation of Screening Evaluation of Screening ProgramsPrograms

Randomized control trials◦ Subjects receive either the new

screening test or usual careCase-control studies

◦ Cases--fatal cases of the disease◦ Controls--nonfatal cases◦ Exposure--screening program