Unit 9-Screeining in diseases control.ppt

health care epidemology

1. Unit-Eleven
Screening in disease control
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2. LEARNING OBJECTIVES/OUTCOMES
When you complete this unit, you will be able to:
1.Define Disease Screening
2.calculate and differentiate between sensitivity,
specificity, positive and negative predictive values of a
diagnostic test
3.Recognize considerations in the Establishment of
Screening Recommendations and Programs
4.Recognize how to evaluate a screening program
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3. Screening
Definition:
Screening refers to the presumptive identification
of a disease/defect by application of tests,
examinations or other procedures in apparently
healthy people.
Screening is an initial examination
Screening is not intended to be diagnostic
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4. Aims of screening program
•Changing disease progression efficiently
•Altering natural course of disease
•Protecting society from contagious disease
•Allocating resources rationally
•Selection of healthy people for job
•Studying the natural history of disease
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5. Criteria for selecting diseases for screening
Severity- The disease should be serious
Treatment- Early treatment should be more beneficial
Prevalence- Pre-clinical Prevalence should be high
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6. Criteria for establishing screening program
1.The problem should have public health importance
2.There should be accepted treatment for positives
3.Diagnostic & treatment facilities should be available
4.Recognized latent stage in the time course
5.Test is acceptable, reliable & valid
6.Natural history of the disease well understood
7.Case-finding is economical and continuous
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7. Screening tests
The performance of a screening test is evaluated
against a diagnostic test in 2X2 table
Diagnostic test
D
+
D
-
Screening testT
+
a ba+b
T
-
c dc+d
a+cb+dn
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8. Results of Screening Testing
Screenig
Test result
PositiveNegative Total
Positive TP (A)FP(B) TP + FP
Negative FN(C) TN(D) FN + TN
Total TP + FN FP +TN TP + FP +FN +TN
8
Gold standard
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9. Definitions of cells
True positives (a): Diseased identified by test as diseased
False positives (b): Disease free falsely labelled as disease
False negatives (c): Diseased falsely labelled as disease free
True negatives (d): Disease free identified as free
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10. Definition of totals
D
+
(a+c): total subjects with a disease
D
-
(b+d): total subjects without disease
T
+
(a+b): total test positives
T
-
(c+d): total test negatves
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11. Test performance
Validity of a test
The ability of a test to differentiate correctly those
who have the disease and those who don’t
•It is a function of sensitivity and specificity
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12. Test performance
A.Sensitivity of a test
•The ability of a test to correctly identify those
who have the disease
•The probability that a diseased individual will
have a positive test result
•The proportion of people with a disease who
have a positive test result
•True positive rate (TPR)
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13. Test performance
A.Sensitivity of a test (TPR)
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)ca(
a


False negative rate (FNR) = P(T
-
|D
+
)
Sensitivity = 1- FNR



)ca(
c
TPFN
FN




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14. Test performance
B. Specificity of a test
•The ability of a test to correctly identify those who
don’t have the disease
•The probability that a disease-free individual will
have a negative test result
•The proportion of people without the disease who
have a negative test result
•True negative rate (TNR)
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15. Test performance
B. Specificity of a test
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= FP
FP + TN
)db(
b


False positive rate (FPR)=P(T
+
|D
-
)

Specificity= 1- FPR
= TN
TN + FP
)bd(
d


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16. Test performance
Predictive value of a test
The ability of a test to predict the presence or
absence of disease
Two type: Positive & negative predictive values
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17. Test performance
A.Predictive value positive (PVP)
√The ability of a test to predict the presence of disease
among who test positive
√The probability that a person with a positive test
result has a disease
√The proportion of diseased individuals in a
population with a positive test result
√Prior/post-test probability
PVP=P(D
+
| T
+
)
= a/(a+b)
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18. Test performance
A.Predictive value negative (PVN)
The ability of a test to predict the absence of disease
among who test negative
The probability that a person with a negative test
result is disease-free
The proportion of disease-free individuals in a
population with a negative test result
Prior/post-test probability
PVN=P(D
-
| T
-
)
= d/(c+d)
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19. Test performance
Prevalence of a disease
The proportion of individuals with a disease
Prior/pre-test probability of a disease
Prevalence = P (D
+
)
= (a+c)/n
Yield of a test
Proportion of cases detected by the screening program
Yield = a/n
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20. Multiple testing
Parallel testing:
–Tests are given concurrently
–At least one positive indicates disease
–Results in
•Greater sensitivity
•Increased PVN
•Decreased specificity
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21. Multiple testing
Serial testing:
–Tests are administered sequentially
–All positive indicates disease
–Results in
•Lower sensitivity
•Increased specificity
•Increased PVP
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22. Reliability of a test
•Ability of a test to give consistent results up on
repeated measurements

•Two major factors affect reliability
–Method variation
–Observer variation
–Inter-observer variation
–Intra-observer variation
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23. Reliability of a test
Reliability can be classified as:
–Internal reliability
•Internal consistency reliability
–External reliability
•Alternate test reliability
•Test-retest reliability
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24. Evaluation of a screening program
Evaluation of a screening program involves
consideration of two issues
1.Feasibility: Determined by acceptability of the
screening program
2. Effectiveness: Determined by the outcome of the
screening program
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25. In general
In general, a screening test should be
–Reliable & valid
–Sensitive & specific
–Simple & acceptable
–Effective & efficient
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26. Exercise
1.What is the sensitivity of the test?
2.What is the specificity of the test?
3.What is the percent false positive?
4.What is the percent false negative?
5.What is the PVP of the test?
6.What is the NVP of the test?
7.What is the Yield of the test?
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Reference
test
Screening
test
  positivenegativetotal
+ve1555988 2543
-ve514 1394 1908
Total20692382 4451
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27. Thank you!
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