11: Screening for Disease Flashcards
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 Screening
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
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.
Mass Health Examinations
- Population or Epidemiologic Surveys
- Epidemiologic Surveillance
- Case Finding (Opportunistic Screening)
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.
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 Social Situations for Screening Tests and Programs
- Health problem should be important for individual and community
- Diagnostic follow-up and intervention available.
- Favorable cost-benefit ratio.
- High public acceptance.
Appropriate Scientific Situations for Screening Tests and Programs
Early detection efforts are most likely successful when:
- natural history of condition is understood
- high prevalence of disease or condition
Appropriate Ethical Situations for Screening Tests and Programs
It is ideal if the screening program is implement so that:
- it can alter natural history in significant proportion of those screened.
- suitable, acceptable tests for screening and diagnosis, as well as acceptable, effective methods of prevention are available.
Characteristics of a Good Screening Test
- Simple
- Rapid
- Inexpensive
- Safe
- Acceptable
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.
Internal Consistency Reliability
Evaluates the degree of agreement or homogeneity within a questionnaire measure of an attitude, personal characteristic, or psychologic attribute. Ex. Questionnaires with many “similar” questions.
Interjudge Reliability
Reliability assessments derived from agreement among trained experts. % of agreement among judges.
Validity
ACCURACY. The ability of a measuring instrument to give a true measure. Can be evaluate only if an accepted and independent method for confirming the test measurement exists.
Content Validity
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. Predictive vs. Concurrent.
Predictive Validity
Denotes the ability of a measure to predict some attribute or characteristic in the future. EX. blood pressure reading and heart disease. Measure correlations of readings to future incidence.
Concurrent Validity
Obtained by correlating a measure with an alternative measure of the same phenomenon taken at the same point in time. EX. Self administered depression evaluation compared to psychiatric diagnosis.
Construct Validity
Degree to which the measurement agrees with the theoretical concept being investigated. (new test development)
Reliability vs. Validity
- Possible to be reliable but invalid
- Impossible to be valid but unreliable
Sources of Unreliability and Invalidity
- Measurement Bias
- Halo Effect
- Social Desirability Effects
Measurement Bias
Constant errors that are introduced by a faulty measuring device and tend to reduce reliability of measurements.
Halo Effect
Bias that affects the validity of questionnaire measurements (preconceived ideas will alter recorded data)
Social Desirability Effects
Respondent answers questions in a manner that agrees with desirable social norms.
Sensitivity
Ability of test to identify correctly all screened individuals who actually have the disease. A/A+C
Specificity
Ability of test to identify only non-diseased individuals who actually do not have the disease. D/B+D
Predictive Value (+)
Proportion of individuals screened positive by the test who actually have the disease. A/A+B
Predictive Value (-)
Proportion of individuals screened negative by the test who do not have the disease. D/C+D
Accuracy of Screening Test
(A+D)/(A+B+C+D)
Prevalence
(A+C)/(A+B+C+D)
Disease Prevalence vs. Predictive Value
- ^ Prevalence, ^ PP+, v PP-
- v Prevalence, v PP+, ^ PP-
To improve Sensitivity:
Cut point used to classify individuals as diseased should be moved farther in the range of the non-diseased. Include more values.
To improve Specificity:
Cut point should be moved farther in the range typically associated with the disease. Exclude more values.
Procedures to Improve Sensitivity and Specificity
- Retrain Screeners: reduces misclassification in tests that require human assessment
- Recalibrate Screening Instrument: reduces imprecision
- Utilize Different Test: replace low reliability and validity with improved test
- Utilize More Than One Test: multiple readings
Evaluation of Screening Programs
Potentially identify BIAS
- Randomized Control Trials
- Ecologic Time Trend Studies
- Case-Control Studies
Sources of Bias in Screening
- Lead Time Bias
- Length Bias
- Selection Bias
Lead Time Bias
Perception that screen-detected case has longer survival because disease was identified early.
Length Bias
Particularly relevant to cancer screening. Tumors identified by screening are slower growing and have better prognosis.
Selection Bias
Motivated participants have a different probability of disease than do those who refuse to participate.
Issues in Classification of Morbidity and Mortality
Nomenclature and Classification of disease are central to the reliable measurement of the outcome variable in epidemiologic research.
Nomenclature
Highly specific set of terms for describing and recording clinical or pathologic diagnoses to classify ill persons into groups.
Classification
Statistical compilation of groups of cases of disease by arranging disease entities into categories that share similar features. Causal vs. Manifestational
