This document discusses the validity and reliability of analytical tests used for screening and diagnosis. It defines key terms like sensitivity, specificity, predictive value and discusses how changing cutoff levels can impact false positives and negatives. Screening tests are used to separate populations into those with and without a disease, while considering a test's accuracy. Continuous variable tests may require an artificial cutoff versus dichotomous screening tests. The document also examines how prevalence impacts predictive value and how using multiple screening tests can improve accuracy.
3. ā¢Two Important Objectives
To distinguish between people in the
population who have the diseases and
those who do not
To determine how good the test is in
separating populations of people with
and without the disease in question?
6. Epidemiological
Surveillance
ā¢ Definition - ongoing & systematic
collection, analysis & interpretation of data
related to health, disease & conditions
ā¢ Two types
ļ¼Passive Surveillance ā uses available data or
reporting from health care provider or
regional health officer
ļ¼Active Surveillance ā periodic field visits to
health care facilities to identify new cases
ā¢ The present approach is the survey
7. Epidemiological
Surveillance
ā¢ Why do it?
ļ¼Can help discover and control the
transmission of infectious diseases
ļ¼Prevention and control programs can be
planned and implemented
8. Screening
ā¢ Definition - use of quick and simple
testing procedures to identify and
separate persons:
ļ¼who have a disease from those that do not
OR
ļ¼who are apparently (appear to be) well, but
who may be at risk of a disease, from
those who probably donāt have the
disease.
9. Terms Related to
Screening Tests
ā¢ Validity - relates to accuracy (correctness)
ā¢ Reliability - repeatability
ā¢ Yield - the # of tests that can be done in a
time period
10. Terms Related to Screening
Tests (contād)
ā¢ Sensitivity - ability of a test to
identify those who have disease
ā¢ Specificity - ability of a test to
exclude those who donāt have
disease
11. Terms Related to Screening
Tests (contād)
ā¢ Tests with dichotomous results ā tests
that give either positive or negative
results
ā¢ Tests of continuous variables ā tests that
do not yield obvious āpositiveā or
ānegativeā results, but require a cutoff
level to be established as criteria for
distinguishing between āpositiveā and
ānegativeā groups
12. An important public health
consideration, particularly in screening
free-living populations, is:
How good is the test at
identifying people
with the disease
and without the disease?
In other words:
If we screen a population, what
proportion of people who have the
disease will be correctly identified?
13. POPULATION
Test Results With Disease
Without
Disease
Positive
True Positive
(TP)
False Positive
(FP)
Negative
False Negative
(FN)
True Negative
(TN)
14. Sensitivity = = X 100
True positives
True positives +
false negatives
True positives
All persons with
the disease
= TP
TP + FN
15. Specificity = = X 100
True negatives
True negatives+
false positives
True negatives
All persons
without the
disease
= TN
TN + FP
16. Percent false negatives = % of people with the disease who
were not detected by the test
FN
FN + TP X 100
17. Percent false positives = % of people without the disease
who were incorrectly labeled by the
test as having the disease
FP
FP + TN X 100
18. In the clinical setting, a more important
question is:
If the test results are positive (or
negative) in a given patient, what is
the probability that this patient has (or
does not have) the disease?
In other words:
What proportion of patients who test
positive (or negative) actually have (or do
not have) the disease in question?
19. Predictive Value
Pos. PV = X 100 = %True Positives
TP + FP
Neg. PV = X 100 = %True Negatives
TN + FN
21. Distribution of Tuberculin Reactions
Bimodal Distribution
Easy to distinguish
between exposed group
and those not exposed.
22. Distribution of
Systolic Blood Pressure
Unimodal Distribution
With continuous variables, a
cutoff level must be established
to separate the hypertensive
group. Could choose based on
statistics, but better to base on
biologic considerations.