1. SCREENING CHP400: Community Health Program-lI Mohamed M. B. Alnoor
Muna M H Diab

2. SCREENING Suitable Screening Test: Acceptable Safe Rapid Inexpensive
Easy to do
Valid
Reliable
Valid
Reliable

3. SCREENING Suitable Screening Test: Validity:(Sensitivity, Specificity)
Acceptable
Safe
Rapid
Inexpensive
Easy to do
Validity:(Sensitivity, Specificity)
Reliability:(repeatability/precision)
Yield (performance): Predictive values of the test.

4. SCREENING Validity of Screening Test:
How good is the screening test compared
with the confirmatory diagnostic test
(Gold Standard test)?
The test will correctly classify a diseased person as likely to have the condition (“sensitivity”).
The test will correctly classify a non- diseased person as unlikely to have the condition (“specificity”).

5. SCREENING PS (FP) (TP) NS (TN) (FN) GT TH TD
Validity of Screening Test:
Screening test compared to gold standard
Total
Gold standard
Screening test
Negative
Positive PS
(FP)
(TP) NS
(TN)
(FN) GT TH TD
a b
c d
TP: True positive; FP: False positive; FN: False negative; TN: True negative; PS: positive screening; NS: negative screening TD: Total disease; TH: Total healthy; GT: Grand total

6. SCREENING Validity of Screening Test: - + a + b - c d
True Disease Status + -
Results of
Screening
Test + a b - c d
Sensitivity: The probability of testing positive if the disease is truly present
Sensitivity = a / (a + c)

7. SCREENING Validity of Screening Test: - + a + b - c d
True Disease Status + -
Results of
Screening
Test + a b - c d
Specificity: The probability of screening negative if the disease is truly absent
Specificity = d / (b + d)

8. SCREENING Adverse effects of screening:
Stress and anxiety caused by a false positive screening results.
Unnecessary investigation and treatment of false positive results
Prolonging knowledge of an illness if nothing can be done about it.
A false sense of security caused by false negatives, which may delay final diagnosis.
Overuse/waste of medical resources.

9. SCREENING Validity of Screening Test: Disease Test D No D 90 5 95 10
105
100
200
a b
c d
Sensitivity: a / (a + c) = 90/100 =90%
Specificity: d / (b + d) = 95/100 =95%
Prevalence of disease =(a+c)/(a+b+c+d)
=100/200 =50%

10. SCREENING Reliability of Screening Test:
RELIABILITY (Reproducibility) Precision:
The extent to which the screening test will produce the same (or very similar) results each time it is administered (repeated).
--- A test must be reliable before it can be valid.

11. SCREENING Yield(Performance) of Screening Test:
Yield is the amount of previously unrecognized disease that is diagnosed and brought to treatment as a result of screening.
It is measured by:
Predictive Value Positive (PV+)
Predictive Value Negative (PV-)

12. SCREENING Yield(Performance) of Screening Test: a d c b
True Disease Status + -
Results of
Screening
Test
Predictive value positive (PV+): The probability that a
person actually has the disease given that he or she
tests positive. i.e. The ability to predict the presence of disease from test results.
PV+ = a / (a + b)

13. SCREENING Yield(Performance) of Screening Test: a d c b
True Disease Status + -
Results of
Screening
Test
Predictive value negative (PV-): The probability that a
person is truly disease free given that he or she
tests negative. i.e. The ability to predict the absence
of disease from test results.
PV = d / (c + d)

14. SCREENING Yield(Performance) of Screening Test: Calculate:
Disease
Test D
No D 19 99
118 1
1881
1882 20
1980
2000
a b
c d
Calculate:
PV+ =19/118=16%
PV-= 1881/1882=99.95%

15. SCREENING Yield(Performance) of Screening Test: Calculate:
Disease
Test D
No D 57 2 59 3 38 41 60 40
100
a b
c d
Calculate:
PV+ =57/59=96.6%
PV-= 38/41=93%

16. SCREENING Factors affecting Yield:
Prevalence (%) Sensitivity Specificity PV+
% % %
% % %
% % %
% % %

17. SCREENING
SCREENING
Validity &Reliability of Screening Test:

18. SCREENING
SCREENING
Validity &Reliability of Screening Test: