1. Seminar 7: Applied Epidemiology
Chapters 11-12
Kaplan University
School of Health Sciences

2. Chapter 11: Screening for Disease in the Community
Introduction
Screening for Disease
Appropriate Situation for Screening Tests
Characteristics of a Good Screening Test
Sources of Unreliability and Invalidity
Measures of the Validity of Screening Tests
More ..

3. Introduction
Screening programs are very important for the secondary prevention of morbidity and mortality.
Efforts to control diseases by early detection through screening have led to a basic change in medical practice: from an exclusive focus upon a small number of ill persons to a targeting of large numbers of asymptomatic persons.

4. Screening for Disease
Primary prevention of disease is the best approach.
Then the next best strategy is early detection of disease in asymptomatic, apparently healthy individuals.
Screening is defined as the presumptive identification of unrecognized disease or defects by application of tests, examination, or other procedures that can be applied rapidly.

5. Screening for Disease
The qualifier presumptive is included in the definition to emphasize the preliminary nature of screening: diagnostic confirmation is required.
It should be noted that screening differs from diagnosis, which is the process of confirmation and actual case of a disease.

6. Multphasic Screening
A more cost-effective approach is to screen for more than one disease.
Multiphasic screening is defined as the use of two or more screening tests together among large groups of people.
It can be administered as a pre-employment physical.

7. Mass Screening and Selective Screening
Mass Screening (also known as population screening) refers to screening of total population groups on a large scale, regardless of any a prior information as to whether the individuals are members of a high-risk subject of the population.
Selective screening, sometimes referred to as targeted screening, is applied to subsets of the population at high risk for disease or certain conditions.

8. Mass Health Examinations
Population or epidemiologic surveys
Epidemiologic surveillance
Case finding (also referred to as opportunistic screening)
Textbook page 414

9. Appropriate Situations for Screening Tests and Programs
Social:
Scientific
Ethical

10. Characteristics of a Good Screening Test
Simple
Rapid
Inexpensive
Safe
Acceptable

11. Evaluation of Screening Tests
We introduce a few more concepts in order to evaluate a screening test’s performance.
Reliability (or precision) is the ability of a measuring instrument to give consistent results on repeated trials.
Repeated measurement reliability refers to the degree of consistency between or among repeated measurements of the same individual on more than one occasion.

12. Evaluation of Screening Tests
Internal consistency reliability evaluates the degree of agreement or homogeneity within a questionnaire measure of an attitude, personal characteristic, or psychologic attribute.
Two reliability measures, the KuderRichardson reliability coefficient, and Chronbach’s α coefficient, are particularly appplicable to epidemiologic research.

13. Evaluation of Screening Tests
Interjudge reliability refers to reliability assessments derived from agreement among trained experts.

14. Evaluation of Screening Tests
Validity (accuracy) is the ability of a measuring instrument to give a true measure.
Validity can be evaluated only if an accepted and independent method exists for confirming the test measurement.
Validity is an important component of epidemiologic research.

15. Evaluation of Screening Tests
Content validity is often used to measure the validity of survey instruments or paper-and-pencil measures.
It refers to the degree to which a measure covers the range of meanings included wihthin the concept.
Criterion-referenced validity generally refers to validity that is found by correlating a measure with an external criterion of the entity being assessed.

16. Evaluation of Screening Tests
The external standard used to assess validity is called the validity criterion.
The two types of criterion-referenced validity are predictive validity and concurrent validity (page 420).
Construct validity refers to the degree to which the measurement agrees with the theoretical concept being investigated.

17. Interrelationships Between Reliability and Validity
Figure 11-1 is designed to assist the reader in differentiating between reliability and validity and in understanding how the two terms are interrelated.

18. Sources of Unreliability and Invalidity
Measurement bias refers to constant errors that are introduced by a faulty measuring device and tend to reduce the reliability of measurements.

19. Measures of the Validity of Screening Tests
In the context of screening, there are four measures of validity that must be considered: sensitivity, specificity, positive predictive value, and negative predictive value.
Figure 11-2 shows the numbers of people by diagnostic test status and disease status.

20. Measures of the Validity of Screening Tests
Sensitivity: the ability of the test to identify correctly all screened individuals who actually have the disease.
Specificity: the ability of the test to identify only nondiseased individuals who actually do not have the disease.
Positive predictive value: the proportion of individuals screened positively by the test who actually have the disease.

21. Measures of the Validity of Screening Tests
Negative predictive value: an analogous measure for those screened negative by the test.
The accuracy of a screening test is found by the proportion of patients correctly identified.

22. Effects of Prevalence of Disease on Screening Test Results
Sensitivity and specificity are stable properties of screening tests and are unaffected by the prevalence of a disease.
Predictive value is very much affected by the prevalence of the condition being screened, as shown in Table 11-1.

23. Relationship Between Sensitivity and Specificity
Figure 11-3 illustrates the relationship between sensitivity and specificity.
When the screening test result is a continuous or ordered variable with several levels, then the choice for a cut-point that discriminates optimally between suspected diseased and normal individuals is a trade-off.

24. Evaluation of Screening Programs
It is imperative that screening programs be evaluated with the same rigor used to identify risk factors in the pathogenesis of disease.
Evaluation of screening programs is subject to several types of bias that have not been described fully in this chapter.
Lead time bias, Length bias, Selection bias (page )

25. Issues in the Classification of Morbidity and Mortality
Schemes for the nomenclature and classification of disease are central to the reliable measurement of the outcome variables in epidemiologic research.
Page 431

26. Epidemiology of Infectious Disease
Introduction
Agents of Infectious Disease
Characteristics of Infectious Disease Agents
Host
The Enviroment
Means of Transmission
Measures of Diease Outbreaks
Procedures Used in the Investigation of Infectious Disease Outbreaks.

27. Introduction
Although there have been many advances in the prevention and treatment of infectious diseases, they remain significant causes of morbidity and mortality.
The epidemiologic triangle model recognizes three major factors-agent, environment, and host, in the pathogenesis of disesae.

28. Agents of Infectious Disease
Bateria
Viruses and rickettsia
Fungi
Protozoa
Helminths
Anthopods

29. Characteristics of Infectious Disease Agents
Please read the textbook (page ) to see if you have any questions on:
Infectivity
Pathogenicity
Virulence
Toxigenicity
Resistance
Antigenicity

30. Host
The host, after exposure to an infectious agent, may progress through a chain of events leading from subclinical (in apparent) infection to an active case of the disease. The end result may be complete recovery, permanent disability, disfiguration, or death.

31. Host: Defense Mechanism
Nonspecific Defense Mechanism
Disease-specific Defense Mechanism
Page 444

32. The Environment
The environment refers to the domain in which the disease-causing agent may exist, survive, or originate.
Page 445

33. Means of Transmission Direct transmission Page 447-450
Inapparent Infection
Incubation period
Herd Immunity
Generation time
Colonization and Infestation
Iceberg Concept of Infection
More ..
Page

34. Measures of Disease Outbreaks
Attach Rate: is frequently used to describe the occurrence of foodborne illness, infectious disease, and other acute epidemics.
Secondary Attach Rate
Case Fatality Rate

35. Procdures Used in the Investigation of Infectious Disease Outbreaks
Define the problem
Appraise existing data
Formulate a hypothesis
Test the hypothesis
Draw conclusion and formulate practical applications.

36. Epidemiologically Significant Infectious Diseases in the Community
Please read the textbook page 457 to the end of this chapter to see if you have any questions.