2. SCREENING
2
Definition
Is identification of presumed/
unrecognized disease
Is identification of asymptomatic
people in order to classify them as
likely or unlikely to have the disease
3. WHY SCREENING?
3
To change the natural course of the
disease
Decrease morbidity and mortality
(secondary prevention)
To protect the general public
To protect an institution
To make preliminary identification of
cases in epidemiological studies
5. 5
How?
Different types of screening,
1. Mass screening:
It involves the screening of the whole
population.
2. Multiple or multi-phase screening:
It involves the use of a variety of screening
tests on the same occasion.
3. Case finding or opportunistic screening;
It is restricted to patients who consult a health
practitioner for some other purposes.
6. CRITERIA
6
For introducing screening program:
1. The condition
- Importance (prevalent? Serious?)
- Has early stage?
2. The test
- Simple, precise, validated?
- Acceptable to the population?
- Agreed diagnostic test?
7. 7
3. The treatment
- Effective?
- Evidence based policy on whom to
treat?
4. The program
- Acceptable to public and professional?
- Benefits outweigh risks?
- Facilities and staff available?
- Overall costs assessed?
8. EVALUATION OF SCREENING PROGRAM
8
What proportion of screened test are positive?
What type of disease is being detected?
What is the predictive value of the test?
9. Bias in evaluation of screening program
Patient self-selection (volunteer) bias- sampling
error
Lead time bias(early diagnosis)-the systematic error
of apparent increased survival from detecting
disease in an early stage
Length bias(chronicity and disease progression)-the
systematic error from detecting disease with a long
latency or pre-clinical period
Each of these source of bias may result in an
erroneous appraisal of the mortality experience of
screened as compared with symptom diagnosed
10. Patient self-selection (volunteer) bias
Volunteers are different in number ways from non
volunteers that survival
Those screened volunterly would have lower
mortality rate regard less of any effect of screening
Lead time bias(early diagnosis)
It represents the amount of time by which the
diagnosis has been advanced as a result of
screening(days, months or years)
Here the disease starts in 1985, is diagnosed in 1992
and the person dies of that disease in 1995. How long
is his survival? Three years.
12. Length bias( chronicity and disease
progression)
Cases picked up by screening may be less
severe and slow progressive compared with
others
A screening initiative is more likely to detect slow-
growing disease. There is not much that a
researcher can do about this type of bias other
than to realize it is likely to occur
13. CHARACTERISTICS OF SCREENING TEST
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Validity-does it truly measure what it sets to
measure?
reference test/gold test
Screening test + - total
+
-
a (true positive) b (false positive) a +b
c (false negative) d (true negative) c+ d
a +c b + d a+b+c+d
14. 15
Measures of validity: sensitivity and specificity
a = True positive, TP – people with the disease who
test positive
b = False positive, FP – people without the disease
who test positive
c = False negative, FN – people with the disease
who test negative
d = True negative, TN – people without the disease
who test negative
15. 16
Sensitivity : the proportion of people with a
disease who have a positive test for the
disease
-Sensitivity (accuracy in classification of cases
)
-Sensitivity= a/a + c
Specificity:- is the proportion of people
without the disease who have a negative test
-Specificity (accuracy in classification of non
cases )
-Specificity= d/d + b
16. 17
Positive predictive value: is the
probability of the presence of the
disease in a person with a positive
test result.
PPV= a/a + b
Negative predictive value:- is the
probability of not having the disease
when the test result is negative .
NPV= d/c + d
17. 18
Prevalence :- the proportion of diseased
case from total observed people
-Prevalence – a+c / a+b+c+d
Yield is the capacity of identifying the
proportion of disease with positive result
Yield := a/a+b+c+d
100 100 100 100
18. Example1
19
Test on filarial on
patients attending
OPD
reference test
+ -
Answer
prevalence= 67%
SN=79%
SP=97%
PPV=98%
NPV=70%
50 1 51
13 30 43
63 31 94
Screening+
-
19. Example2
20
Test on filarial on
patients attending
OPD
reference test
+ -
Answer
prevalence = 43%
SN=44.2%,
SP=96.7%,
PPV=91.1%,
NPV=70%
1030 100 1130
1300 3000 4300
2330 3100 5430
Screening
+
-
20. Sensitivity and specificity depend on:
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- Degree of skill
- Equipment, reagent
- Severity, range of manifestation of
disease
- Presence of other disease
