Transcript Unit 15: Screening Unit 15 Learning Objectives: 1. Understand the role of screening in the secondary prevention of disease. 2.

Unit 15:
Screening
Unit 15 Learning Objectives:
1. Understand the role of screening in the
secondary prevention of disease.
2. Recognize the characteristics of diseases
appropriate for screening.
3. Understand the impact of implementing
screening on prevalence and incidence of
disease.
4. Calculate and interpret measures of the
validity of a screening test:
--- Sensitivity
--- Specificity
Unit 15 Learning Objectives (cont.):
5. Understand the relationship between
sensitivity and specificity.
6. Calculate and interpret measures of the
performance (yield) of a screening test:
--- Predictive value positive (PV+)
--- Predictive value negative (PV-)
7. Understand factors that influence PV+ and
PV8. Recognize issues and sources of bias in
evaluating screening programs.
Epidemiology in
Disease Control:
Screening
Screening for Disease Control

Screening: The application of a diseasedetection test to people who are as yet
asymptomatic.

Purpose: To classify individuals with
respect to their likelihood of having a
particular disease.

Screening procedure itself does NOT
formally diagnose illness.
Screening for Disease Control

Examination of asymptomatic people
likely

Classification as
unlikely
….. to have a disease
Screening for Disease Control

“Unlikely”
referred to next
screening cycle

“Likely”
further testing for
diagnosis
yes
treatment
no
referred to next
screening cycle
Screening for Disease Control

Screening Objective: To lower morbidity
and mortality of the disease in a
population (control, rather than
elimination of disease).

Screening provides access to the
medical care system which is not an
actual goal of screening, but is a benefit.
Screening for Disease Control
Screening is important because:
1) Diagnostic and therapeutic advances
are often slow, but screening may be a
“direct solution” to modify history of a
disease in a population.
2) It provides a model for studying disease
mechanisms and the natural history of a
disease.
Screening for Disease Control
Primary requirements for screening:
1) Early detection of disease leads to a
more favorable prognosis due to early
treatment, as compared to delayed
treatment.
2) Pre-clinical disease left untreated
typically progresses to clinicallyevident disease (e.g. no spontaneous
regression).
Screening for Disease Control
Primary requirements for screening:
3) The disease should be serious (relates
to cost effectiveness, ethics, and
prognosis).
4) Prevalence of pre-clinical disease
should be relatively high among those
screened.
Diseases for which screening
has been recommended

Cervical cancer

Breast cancer

Prostate cancer

Colon cancer

Diabetes

Hypertension
Screening for Disease Control
“PRICES” OF SCREENING:
1) Financial - may be very costly if screening is
spread out over an entire population.
2) Anxiety - Individuals may have to be screened
more often.
3) Some morbidity occurs - both in terms of the
initial screening procedure, and
subsequent procedures.
4) Creation of “lead time” morbidity.
Natural History of Disease
Age of Individual
20
30
40
45
50
Birth Exposure
Cells
Screened
Neoplasia Exfoliate Diagnosis
55
Symptom
Diagnosis
60
Death
Natural History of Disease
Age of Individual
20
30
40
45
50
Birth Exposure
Cells
Screened
Neoplasia Exfoliate Diagnosis
55
Symptom
Diagnosis
60
Death
Total Pre-Clinical Phase (TPCP)
TPCP: Begins at the initiation of disease; ends when the
disease is clinically manifested (25 years in this example)
Natural History of Disease
Age of Individual
20
30
40
45
50
Birth Exposure
Cells
Screened
Neoplasia Exfoliate Diagnosis
55
Symptom
Diagnosis
60
Death
Detectable Pre-Clinical Phase (DPCP)
DPCP: Begins when screening test is able to detect
disease; Ends when disease is clinically evident (10 years)
Impact of Screening
on Epi Measures
Prevalence of
clinical disease
(found by
either symptoms
or screening)
Steady state
Screening
Time
Impact of Screening
on Epi Measures
Incidence of
clinical disease
Note incidence
rises, and then
drops sharply
because the
“pool at risk” is
temporarily
depleted
Steady state
Screening
Time
Evaluating Screening Tests
Characteristics of a screening test:
• Validity – the extent to which the test
distinguishes between persons with and
without the disease: High validity requires:
•
High Sensitivity
•
High Specificity
• Reliability (High)
• Low cost, invasiveness, and discomfort
• Performance (Yield)
Validity of Screening Tests
True Disease Status
+
-
+
a
b
-
c
d
a = true positive
b = false positive
c = false negative
d = true negative
Validity of Screening Tests
How good is the screening test compared
with the confirmatory diagnostic test?

The test will actually classify a diseased
person as likely to have the condition
(“sensitivity”).

The test will actually classify a nondiseased person as unlikely to have the
condition (“specificity”).
Validity of Screening Tests
True Disease Status
+
-
+
a
b
-
c
d
Sensitivity: The probability of testing
positive if the disease is truly present
Sensitivity = a / (a + c)
Validity of Screening Tests
True Disease Status
+
-
+
a
b
-
c
d
Specificity: The probability of screening
negative if the disease is truly absent
Specificity = d / (b + d)
Validity of Screening Tests
Breast Cancer
Physical Exam
and Mammography
+
-
+
-
132
983
45
63650
Sensitivity: a / (a + c)
Sensitivity =
Specificity: d / (b + d)
Specificity =
Validity of Screening Tests
Breast Cancer
Physical Exam
and Mammography
+
-
+
-
132
983
45
63650
Sensitivity: a / (a + c)
Sensitivity = 132 / (132 + 45) = 74.6%
Specificity: d / (b + d)
Specificity = 63650 / (983 + 63650) = 98.5%
Validity of Screening Tests
Sensitivity:
a / (a + c)
Sensitivity = 132 / (132 + 45) = 74.6%
Specificity: d / (b + d)
Specificity = 63650 / (983 + 63650) = 98.5%
Sensitivity: Screening by physical exam and
mammography will identify 75% of all true breast
cancer cases.
Specificity: Screening by physical exam and
mammography will correctly classify 98.5% of all
non-breast cancer patients as being disease free.
Validity of Screening Tests
SETTING THE CRITERION FOR POSITIVITY
Population
Bi-modal
distribution
Blood Sugar
Question: What is the best cutpoint?
(Depends on the price for a negative outcome)
Validity of Screening Tests
RELATIONSHIP BETWEEN SENSITIVITY
& SPECIFICITY:
1.
Lowering the criterion of positivity
results in increased sensitivity, but at
the expense of decreased specificity.
2.
Making the criterion of positivity more
stringent increases the specificity, but at
the expense of decreased sensitivity.
Validity of Screening Tests
RELATIONSHIP BETWEEN SENSITIVITY
& SPECIFICITY:
3.
The goal is to have both high sensitivity
and high specificity, but this is often not
possible or feasible.
4.
The decision for the cutpoint involves
weighing the consequences of leaving
cases undetected (false negatives)
against erroneously classifying healthy
persons as diseased (false positives).
Validity of Screening Tests
RELATIONSHIP BETWEEN SENSITIVITY &
SPECIFICITY:
5. In general, specificity must be at least 98%to
be effective --- because misclassifying 2% of
the population will create as many false
positives as the sensitivity of the test will
actually detect.
Validity of Screening Tests
RELATIONSHIP BETWEEN SENSITIVITY &
SPECIFICITY:
6. Sensitivity should be increased when the
penalty associated with missing a case
is high (e.g. minimize false negatives)
--- when the disease can be spread
--- when subsequent diagnostic
evaluations are are associated with
minimal cost and risk
Validity of Screening Tests
RELATIONSHIP BETWEEN SENSITIVITY &
SPECIFICITY:
7. Specificity should be increased when the
costs or risks associated with further
diagnostic techniques are substantial
(minimize false positives – e.g. positive screen
requires that a biopsy be performed).