Managerial Epidemiology: Week 5

Chapter 11

Screening for Disease in the

Community

Learning Objectives

• Define and discuss reliability and

validity, giving differentiating

characteristics and interrelationships

• Identify sources of unreliability and

invalidity of measurement

• Define the term screening and list

desirable qualities of screening tests

Learning Objectives

(Cont’d)

• Define and discuss sensitivity and

specificity, giving appropriate

formulas and calculations for a

sample problem

• Identify a classification system for a

disease

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.

– Example: phenylalanine loading test in

children positive on PKU 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 physiologic and

health measurements.

• Commonly used by employers and health

maintenance organizations.

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.

– Example: Screening high-risk persons for Tay-Sachs disease.

Mass Health Examinations

• Population or epidemiologic surveys--

purpose 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)

• Epidemiologic surveillance--aims at the

protection of community health through case

detection and intervention (e.g., tuberculosis

control).

• Case finding (opportunistic screening)--the

utilization of screening tests for detection of

conditions unrelated to the patient’s chief complaint.

Appropriate Situations for

Screening Tests and Programs

• Social

• Scientific

• Ethical

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.

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.

• A knowledge base exists for the efficacy

of prevention and the occurrence of side

effects.

• Prevalence of the disease or condition is

high.

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

• Simple--easy to learn and perform.

• Rapid--quick to administer; results

available rapidly.

• Inexpensive--good cost-benefit ratio.

• Safe--no harm to participants.

• Acceptable--to target group.

Evaluation of Screening Tests

• Reliability types

– Repeated

measurements

– Internal

consistency

– Interjudge

• Validity types

– Content

– Criterion-

referenced

• Predictive

• Concurrent

– Construct

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)

• Internal consistency reliability-- evaluates the degree of agreement or homogeneity within a questionnaire measure of an attitude, personal characteristic, or psychological attribute.

• Interjudge reliability--reliability assessments derived from agreement among trained experts.

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)

• Content validity--the degree to which

the measurement incorporates the

domain of the phenomenon under

study.

• Criterion-referenced validity--found by

correlating a measure with an

external criterion of the entity being

assessed.

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)

• Construct Validity--degree to which

the measurement agrees with the

theoretical concept being

investigated.

Interrelationships Between

Reliability 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 Reliability

and Validity

Sources of Unreliability and

Invalidity

• Measurement bias--constant 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 (cont’d)

• Halo effect—the influence upon an observation of the observer’s perception of the characteristics of the individual observed. The influence of the observer’s recollection or knowledge of findings on a previous occasion.

– Example: a health care provider’s tendency to rate a patient’s sexual behavior use in a particular manner, based on a general opinion about a patient’s characteristics without obtaining specific information about past sexual behavior.

Sources of Unreliability and

Invalidity (cont’d)

• Social desirability effects - - Respondent answers questions in a manner that agrees with desirable social norms.

– Example: Teenage boys might respond to a screening interview about sexual behavior by exaggerating their frequency of sexual activities because these behaviors might be perceived as socially desirable among some male peer groups.

Fourfold (2 by 2)Table

Measures of the Validity of

Screening Tests

• Sensitivity--the ability of the test to identify correctly all screened individuals who actually have the disease (a/a+c).

• Specificity--the ability of the test to identify only nondiseased individuals who actually do not have the disease (d/b+d).

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

Fourfold (2 by 2) Table

• Accuracy of a screening test

– determined by the following formula:

(a+d)/(a+b+c+d).

• Prevalence

– determined by the formula:

(a+c)/(a+b+c+d)

Sample Calculation

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.

Exhibit 11-4

• Illustrates the importance of positive

predictive value in the prostate cancer

screening controversy.

• PSA routine screening was widespread in

the U.S. by 1991.

• The U.S. Preventive Services Task Force

calculated that the harms of PSA

screening outweigh the benefits.

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 Between Sensitivity

and Specificity (cont’d)

Procedures to Improve

Sensitivity 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 test.

• Utilize more than one test.

Evaluation of Screening

Programs

• Randomized control trials – Subjects randomly receive either the new

screening test or usual care. • Ecologic time trend studies

– Compare geographic regions with screening programs to those without.

• Case-control studies – Cases--fatal cases of the disease. – Controls--nonfatal cases. – Exposure--screening program.

Sources of Bias in Screening • Lead time bias

– The perception that the screen-detected 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

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.

• Nomenclature--a 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 Mortality (cont’d)

• Classification--the 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 (e.g., tuberculosis or syphilis)

– Manifestational (e.g., affected anatomic site:

hepatitis or breast cancer)