Managerial Epidemiology
Helpm31990
51589_PPTx_CH11.pptxChapter 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)
