screening is defined as the presumptive identification of unrecognized disease or defect by the application of tests, examinations, or other procedures which can be applied rapidly to sort out those who probably have a disease from those who probably do not. screening does not diagnose a disease but it is done to separate persons who has high probability of developing diseases during the study from apparently well person Specificity is the ability of a test to give a negative finding when the tested person is truly free of the disease under study. i.e true negative Sensitivity is the ability of a test to give a positive finding when the tested person truly has the disease under the study. i.e true positive Diseased individuals with positive screening test are True positive TP Healthy individual with positive screening test are False positive FP Diseased individuals with negative findings are False negative FN Healthy individual with negative screening test are True negative TN An ideally screening test have few false positives and false negatives as possible Mass screening: This involves screening of a whole population Multiple or multiphasic screening: Involves the use of a variety of tests on the same occasion for the same condition Targeted screening: Involves screening of groups with specific exposures Case-finding or opportunistic screening: Screening of patients visiting a health care delivery point for some other purpose Ideally we need this test to identify correctly those with the disease under investigation and to exclude this from all non diseased The test should give

1. SCREENING
Kuya Kabalo (BScN,RN,MPH*)

2. Screening
• Screening is defined as the presumptive identification of unrecognized
disease or defect by the application of tests, examinations, or other
procedures which can be applied rapidly to sort out those who
probably have a disease from those who probably do not.

3. Cont.…
• The screening procedure itself does not diagnose illness
• Its main purpose is to separate person who have high probability of
developing a disease under the study from an apparently well
population

4. Cont.….
• Then if diagnosed will brought to treatment
• screening is carried out with the assumption that early detection of
disease before development of symptoms will lead to a more favorable
prognosis because treatment began before the disease becomes
clinically manifest will be more effective than later treatment.

5. Types of screening
• Mass screening: This involves screening of a whole population
• Multiple or multiphasic screening: Involves the use of a variety of
tests on the same occasion for the same condition
• Targeted screening: Involves screening of groups with specific
exposures
• Case-finding or opportunistic screening: Screening of patients visiting
a health care delivery point for some other purpose.

6. Measurement characteristics of screening
• Ideally we need this test to identify correctly those with the disease
under investigation and to exclude this from all non diseased
The test should give
 true measurement of the attribute under investigation (accuracy)
consistent results in repeated trials (precision sometimes called
reproducibility)
 should have high sensitivity and specificity for the disease in
question (also called validity properties of a screening test

7. Cont.…
• Accuracy may be defined as a measurement process yielding values
that are equal on average to the true underlying value for the
diagnostic variable being measured
• Precision may be defined as the degree to which a series of
measurements fluctuates around a central measurement
• High sensitivity and specificity

8. the results of a screening test and disease status,
can be examined conveniently by use of the four
fold contingency table
Screening test Positive Negative Total
Test positive True positive (TP) a False positive (FP) b Total test positivr (T+)
a+b
Test negative False negative (TN) c True negative (TN) d Total test negative (T-)
c+d
Total disease positive
(D+) (a+c)
Total disease ngative
(D_)(b+d)
Total population( N)
(a+b+c+d)
DISEASE STATUS

9. Cont.…
• From the table D+ have the disease
• D- Have no disease
• (D+/D- )denotes the prevalence of the disease in this population
• T+ are positive on screening test and T-are negative on screening test

10. Cont.…
• Diseased individuals with positive screening test are True positive TP
• Healthy individual with positive screening test are False positive FP
• Diseased individuals with negative findings are False negative FN
• Healthy individual with negative screening test are True negative TN
• An ideally screening test have few false positives and false negatives
as possible

11. cont.…
• the capacity of a screening test to identify correctly the diseased
against the non-diseased is expressed by its SENSITIVITY and
SPECIFICITY which are a measure of the test's validity
• Sensitivity is the ability of a test to give a positive finding when the
tested person truly has the disease under the study. i.e true positive

12. Cont.…
• In other words it is the probability that the test will be reactive in a
diseased individual.
• A test with a high sensitivity will detect a high percentage of diseased
individuals.

13. Cont.…
T P
• Sensitivity = ---------X 100 %
D+

14. • Specificity is the ability of a test to give a negative finding when the
tested person is truly free of the disease under study. i.e true negative
• In other words it is the probability that a test result will be non-
reactive in an individual who is not diseased.
• specificity = (TN/D-)X100%TN
• .

15. Cont.…
• In practice, sensitivity of a test is usually determined on a group of
proven cases of a given disease,
• while its specificity is determined on a group of healthy people, or
people with diseases other than the one under investigation

16. Cont.…
• Predictive value of positive test =(a/a+b)x100%
• Predictive value of negative test =(d/a+c)x100%
• Percentage of false negative =(c/a+c)x100%
• Percentage of false positive =(b/b+d)x100%

17. Criteria for screening
• It is based on two considerations which are DISEASE to be screened
and TEST to be applied
DISEASE
Should fulfil the followings criteria before considered for screening
 The condition thought should be an important health problem(having
high prevalence)
 There should be recognizable latent or early asymptomatic stage
 The natural history of the condition should be adequately known

18. Cont.…
• There is a test that can detect the disease prior to onset of signs and
symptoms
• Facilities for confirmation of diagnosis should be present
• There is an effective treatment

19. Cant…
• There should be an agreed policy to whom to term as patient
• Evidence that early detection and treatment reduces morbidity and
mortality
• Expected benefits i.e. number of lives saved on early detection exceed
the risk and cost

20. Cont.…
• Screening test
• The test must satisfy the criteria of Acceptability , repeatability and
validity beside others such as yield ,simplicity ,safety , rapidity easy of
administration and cost
• Test likely to fulfil one condition may however be least likely to fulfil
another e.g. test with greater accuracy maybe more expensive and
time consuming
• The choice of the test must therefore often be based on compromise

21. Cont.…
• Acceptability
• Repeatability
• Validity refers to what extent the test accurately measure which it
purports to measure
• Accuracy refers to the closeness with which measured value agree to
the true value
• Validity components are sensitivity and specificity which must be
considered when assessing the accuracy of the diagnostic test

22. Cont.…
• Predictive accuracy , it depends on sensitivity , specificity and disease
prevalence
• The performance of screening test is measured by its predictive value
which reflect the diagnostic power of the test
• The positive predictive test indicates the probability that a patient with
positive test results in fact has a disease in question
• The positive predictive value is directly proportional to the prevalence

23. Cont.…
• Yield is the amount of previously unrecognized diseases that is
recognized as a result of screening effort .

24. Thank you