5 Sensitivity Sensitivity True positives Affected persons The sensitivity of a test in the ability of the test to identify correctly affected individuals Proportion of persons testing positive among affected individuals ID: 917698
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Slide1
Diagnostic Tests
Slide2Slide3Slide4Slide5Results of a Screening Test
5
Slide6Sensitivity
Sensitivity = True positives / Affected persons
The sensitivity of a test in the ability of the test to identify correctly affected individuals
Proportion of persons testing positive among affected individuals
Slide7Measures of Test Validity
7
Slide8Example
Patients with
GN
RBC cast
True positive
148
False negative
2
150
Sensitivity = 148 / (150) = 98%
Slide9Specificity
Specificity = True negatives / Non-affected persons
The specificity of a test is the ability of the test to identify correctly non-affected individuals
Proportion of person testing negative among non affected individuals
Slide10Example
Individuals without ADPKD
Renal US
False positive
12
True negative
188
200
Specificity = 188 / 200 = 94%
Slide11Identifying the cut-off to use with a test on the basis of panel analysis: Ideal case
Cut-off
0
5
10
15
20
25
1
2
3
4
5
6
7
8
9
10
11
12
13
14
Possible values of the test
Number of tests
Sick
Well
Slide12Identifying the cut-off to use with a test on the basis of panel analysis: Real case
Cut-off
0
5
10
15
20
25
1
2
3
4
5
6
7
8
9
10
11
12
13
14
Possible values of the test
Number of tests
Sick
Well
True negatives
False negatives
True positives
False positives
Slide13Predictive value of a positive test =
True positives / Persons testing positive
The predictive value of a positive test is the probability that an individual testing positive is truly affected
Proportion of affected persons among those testing positive
Slide14How is the test doing in a real population?
Status of persons
Affected
Non-affected
Test
Positive
True +
False +
A+B
Negative
False -
True -
C+D
A + C
B+D
A+C+B+D
The test is now used in a real population
This population is made of
Affected individuals
Non-affected individuals
The proportion of affected individuals is the prevalence
Slide15Results of a Screening Test
15
Slide16PVP = A / (A+B)
Slide17Predictive value of a negative test =
True negatives / Persons testing negative
The predictive value of a negative test is the probability that an individual testing negative is truly non-affected
Proportion of non-affected persons among those testing negative
Slide18Evaluation of Screening Program
18
Slide19Accuracy = (TP+TN)/ Total result
Tells how often you’re right
Best test: has high level of sensitivity and specificity
Slide20Suitable Test (cont’d)
Validity (Accuracy):
The ability of a test to give a true measure.
Can be evaluated only if an accepted and independent method for confirming the test measurement exists.
Valid Test: Correctly classifies people with disease as positive and people without disease as negative.
20
Slide21Suitable Test (cont’d)
Reliability (Precision):
The ability of a measuring instrument to give
consistent
results on repeated trials.
Repeated measurement reliability
-
-
the degree of consistency among repeated measurements of the same individual on more than one occasion.
21
Slide22Interrelationships Between Reliability and Validity
It is possible for a measure to be highly reliable but invalid.
It is
not
possible for a measure to be valid but unreliable.
22
Slide23Representation of Reliability and Validity
23
Slide24Slide25ADPKD screening of pts with positive family
hx
of ADPKD (Not Real Data)
Patients assigned to screening or usual care. Screening consisted of yearly renal US and physical exam. Five years of follow‑up produced these results:
ADPKD
Screening Test Result
25
Slide26Suitable Test
Sensitivity = 132/177 = 74.6%
Specificity = 63,650/64,633 = 98.5%
Interpretation: The screening was very good at picking out the patients who did not have ADPKD (see specificity) but it missed 25% of the patients who did have ADPKD (see sensitivity).
To measure sensitivity and specificity you can wait for disease to develop (as above) or you can measure the results of the screening test against the outcome of another screening or diagnostic test (the Gold Standard).
26
Slide27Evaluation of Screening Program:
Feasibility Measures
Acceptability, cost, predictive value of a positive test (PV+), predictive value of a negative test (PV-)
Yes
No
Total
Positive
a
b
a + b
Negative
c
d
c + d
Total
a + c
b + d
a + b + c + d
Screening Test
Result
Disease Status
27
Slide28Evaluation of Screening Program
28
Slide29ADPKD screening of pts with positive family
hx
of ADPKD (Not Real Data)
29
Slide30PV+ = 132/1115 = 11.8%
PV- = 63,650/63,695 = 99.9%
ADPKD confirmed
ADPKD not confirmed
Total
Positive
132
983
1,115
Screening
Test
Negative
45
63,650
63,695
Total
177
64,633
64,810
Slide31Evaluation of Screening Program
PV will increase when sensitivity, specificity, and disease prevalence increases.
For example, PV+ will increase if you perform ADPKD screening on higher risk population (i.e. people with a family history of ADPKD)
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Slide32Slide33END!?