likelihood ratio

2.  Sensitivity- ability of a test to correctly identify all those
who have the disease.
 Specificity- ability of a test to correctly identify those who
do not have the disease.

3. Sensitivity and specificity
 a- true positive
 b- false positive
 c -false negative.
d-true negative .

7.  An ideal screening test should be 100% sensitive and 100 %
specific
However, when sensitivity increases specificity of the test decreases; that is
sensitivity is inversely proportional to specificity and vice versa
In general
 Screening test should have high sensitivity
 Diagnostic test should have high specificity

8. Positive predictive value
 Proportion of individuals with a positive test result who
have the disease.
 Proportion of individuals with a negative test who do not
have the disease
Negative predictive value

10. PPV NPV characteristics
 Predictive value reflects diagnostic power of the test
 It is determined by the prevalence of a disease in the population
 Prevalence  PPV NPV
 Prevalence  PPV NPV

12. LIKELIHOOD RATIO
 To judge diagnostic tests.
 Two forms- positive LR and negative LR.
Positive likelihood ratio
 This ratio divides the probability that a patient with the disease
will test positive by the probability that a patient without the
disease will test positive.

14. Positive likelihood ratio
 The LR of a positive test result (LR+) is sensitivity/(1 − specificity).
 For instance, the positive LR of (CT) scan in the diagnosis of acute
appendicitis (sensitivity 94% and specificity 95%)
 + LR= 94/1- 95= +18.8.
 This indicates that an individual is about 19 times as likely to have
CT scan finding positive when they do have acute appendicitis than
when they do not have acute appendicitis

15.  Thus, a positive CT scan in a patient presenting with clinical
signs and symptoms suggestive of acute appendicitis indicates
a very strong likelihood that the diagnosis is correct.
 The higher the positive likelihood ratio, the better the test (a
perfect test has a positive likelihood ratio equal to infinity)

17. Negative likelihood ratio/ LR -
 - LR = 1- sensitivity/specificity.
 In the same test –sensitivity 94 % and specificity 95 %
 = 1-94/95 = 0.06.
 It indicates that when the test is neg there is less chance of the
disease.
 The lesser the value of neg LR, more accurate the test .
 The lower the negative likelihood ratio, the better the test (a
perfect test has a negative likelihood ratio of 0).

20.  200 patients enrolled in a study of new elisa test for influenza
100 patients had influenza according to the gold std test.
Out of which 80 had positive elisa and 20 had negative test.
Out of 100 who didn’t have influenza ,95 had neg elisa and 5 had
positive elisa.
What is the positive and negative likely hood ratio for elisa test ?

21. Positive likelihood ratio
Disease
positive test
result
Yes
80
a
No
5
b
Negative test
result
Yes
20 c
No
95 d
LR for positive test is
a/a+c
b/b+d
80/100
5/100 =
0.8/0.005 = 16

22. Negative likelihood ratio
Disease
Positive test Yes
80 a
No
5 b
Negative test Yes
20 c
No
95 d
Total 100 a + c 100 b + d
NEG LR = c/a+c
d/b+d
= 20/100
95/100 = 0.21
= 0.2/0.95