Computing
likelihood ratios from sensitivity and specificity
 Previous
Section  Main Menu  Next
Section 
Often, the literature reports diagnostic test characteristics as sensitivity
and specificity rather than likelihood ratios. You could compute them by
constructing a two by two table as we did above.

SLE 
No SLE 

Positive
ANA 
2822 
6798 

Negative
ANA 
58 
90322 


2880 
97120 
100000 
Going back to the original definition of likelihood ratio, we can compute
the probability of a positive ANA test in patients with SLE: (2822 / 2880)
or 0.98. We can also compute the probability of a positive ANA test in
patients without SLE: (6798 / 97120) or 0.07. The likelihood ratio for
a positive ANA is then 0.98 / 0.07 or 14. Using an analagous approach,
you should be able to compute the likelihood ratio for a negative ANA (0.02).
In more general terms:
LR^{+}
= Sensitivity / (1  Specificity)
LR^{} = (1  Sensitivity) / (Specificity)
 Previous
Section  Main Menu  Next
Section 