Screenings in the community

1. Chapter 11
Screening for Disease in the Community

2. Learning Objectives
Define and discuss reliability and validity
Define the term screening
Define and discuss sensitivity and specificity

3. 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

4. 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 health
measurements
Commonly used by employers and
health maintenance organizations

5. 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.

6. Mass Health Examinations
Population or epidemiologic surveys--purpose
is to gain knowledge regarding the
distribution and determinants of diseases
in selected populations

7. Mass Health Examinations (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

8. Appropriate Situations for
Screening Tests and Programs
Social
Scientific
Ethical

9. 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

10. 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
Prevalence of the disease or condition is
high

11. 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

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

13. Evaluation of Screening Tests
Reliability types Validity types
◦ Repeated ◦ Content
measurements ◦ Criterion-
◦ Internal consistency referenced
◦ Interrater  Predictive
 Concurrent
◦ Construct

14. 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

15. 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

16. 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

17. 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

18. 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

19. Validity (cont’d)
ConstructValidity--degree to which the
measurement agrees with the theoretical
concept being investigated

20. 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

21. Representation of Reliability and
Validity

22. 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

23. Sources of Unreliability and
Invalidity (cont’d)
Halo effect--bias that affects the validity of
questionnaire measurements

24. Example of Halo
All 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

25. Sources of Unreliability and
Invalidity (cont’d)
Social desirability effects
◦ Respondent answers questions in a manner
that agrees with desirable social norms

26. Contingency (2 by 2)Table
Disease
Present Absent Total
Screen Positive a b a+b
Result
Negative c d c+d
Total a+c b+d

27. Sensitivity and Specificity
Sensitivity = 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
27

28. 28

29. Predictive 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
29

30. 30

31. 31

32. Sample Calculation

33. 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.

34. 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

35. Relationship Between Sensitivity and
Specificity (cont’d)

36. 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

37. Evaluation of Screening Programs
Randomized control trials
◦ Subjects receive either the new screening
test or usual care
Case-control studies
◦ Cases--fatal cases of the disease
◦ Controls--nonfatal cases
◦ Exposure--screening program