2. Lesson objective
• At the end of this lesson the student will be able
to :
– List the key characteristics of diseases appropriate for
screening.
– Describe the important features of a screening test.
– Define and calculate sensitivity, specificity, predictive
value of a screening test
2
3. Screening
• Application of a test or procedure to asymptomatic,
apparently well individuals, in order to separate
those with a relatively high probability of having a
given disease from those with a relatively low
probability of having the disease
3
4. Main Aim of Screening
• To reverse, halt, or slow the progression of
disease more effectively than would
probably normally happen.
• To alter the natural course of disease for a
better outcome for individuals affected.
• Reduce morbidity and mortality through
early detection and treatment.
4
5. Aim of screening…
• Protect society from contagious disease
• Rational allocation of resources
• Selection of healthy individuals: employment,
military…
• Research: study on natural history of
disease…
5
6. What characteristics of a disease make it
appropriate for screening?
1. Important public health problem – frequent or
serious
2. Reasonably long, recognizable pre
symptomatic stage
3. Effective treatment exists and is available, or
effective ways of preventing spread
4. Treatment (or measures take to prevent
spread) should be more effective if initiated in
the pre-symptomatic stage than when
initiated in symptomatic patients
6
7. Cont…
5. A suitable screening test or
procedure should be available and
be:
1. Reliable
2. Sensitive and specific
3. Acceptable to the population screened
4. Reasonably inexpensive and safe
7
8. WHO Criteria for Screening
• Is it a health problem?
• Is there treatment?
• Are there facilities in place?
• Is it detectable pre-clinically?
• Is there a suitable screening test?
• Is the screening test acceptable to people?
• Is the natural history of disease
understood?
• Are the costs acceptable?
8
9. • How do you decide to implement a new
screening test ?
9
10. Measures of Validity
• Validity: is the ability of a test to indicate which
individuals have the disease and which do not
• The two Measures of validity
• Sensitivity: The ability of the test to identify
correctly those who have the disease
• Specificity: The ability of the test to identify
correctly those who do not have the disease
10
11. • How do we judge the validity of a
screening test?
We compare the screening test against
some “gold standard”
11
13. Example – Prostate cancer
• Suppose that 100,000 men were screened for prostate cancer for
the first time. Of these, 4000 men had a positive result on the
screening blood test; of those who tested positive, 800 had a
biopsy indicating a diagnosis of prostate cancer. Among the
remaining 96,000 men who screened negative, 100 developed
prostate cancer within the following year and were assumed to be
false negatives to the screen.
A. Set up the two-by-two table for these data.
B. What is the prevalence of prostate cancer in this population?
C. Calculate and interpret the sensitivity of this screening test.
D. Calculate and interpret the specificity of this screening test.
13
14. Solution
Test
result
Prostate Cancer Total
Present Absent
Positive 800 3200 4000
Negative 100 95,900 96,000
Total 900 99,100 100,000
• Prevalence = 900/100,000 =
0.9%
• Sensitivity = 800/900 = 88.8 %
it means that 88.8 % of men
who have prostate Ca tested
positive on the screening test.
• Specificity = 95,900/99,100 =
96.8% It means that 96.8% of
the men who did not have
prostate Ca tested negative on
the screening Test.
14
15. Predictive Value
• How well does our screening test predict
who is diseased and who is not?
15
16. Predictive Value
• Among those who screen positive, what
proportion actually has the disease? Positive
Predictive Value (PPV)
• Among those who screen negative, what
proportion actually do not have the disease?
Negative Predictive Value (NPV)
16
18. Example – Prostate Cancer
• PPV= 800/(4000)= 0.2= 20%
i.e: one fifth of the men who screen
positive on test have prostate cancer,
leaving about 80% of men screening
positive falsely.
NPV = 95,900/96,000
• =0.998 = 99.9%
i.e. the test has perfect negative
predictive value. Among those who
are negative on the screening test, we
can be perfectly confident that none
of those individuals actually have the
disease.
18
19. Review questions
• Discuss the factors that affects sensitivity,
specificity, and predictive value of a
screening test
• Bias in screening test
• How is screening program evaluated?
19
21. • A new test for chlamydial infections of the cervix is
introduced for 400 women. Half of the women who are
tested have a positive test. Compared with the gold
standard of careful cultures, 50% of those with a positive
test are infected with chlamydia, and 75% of those with a
negative test are free of the infection.
1. Set up the two-by-two table for these data.
2. What is the prevalence of chlamydial infection in this
population?
3. Calculate and interpret the sensitivity of this screening test.
4. Calculate and interpret the specificity of this screening test
5. Calculate and interpret the PPV of this screening test.
6. Calculate and interpret the NPV of this screening test
22. Quiz
• A new test for chlamydial infections of the
cervix is introduced for 400 women. Half
of the women who are tested have a
positive test. Compared with the gold
standard of careful cultures, 50% of those
with a positive test are infected with
chlamydia, and 75% of those with a
negative test are free of the infection.
1. Set up the two-by-two table for these data.
2. What is the prevalence of chlamydial
infection in this population?
3. Calculate and interpret the sensitivity of
this screening test.
4. Calculate and interpret the specificity of
this screening test
5. Calculate and interpret the PPV of this
screening test.
6. Calculate and interpret the NPV of this
screening test
case Non case Total
tve 100 100 200
-ve 50 150 200
Total 150 250 400
