How much more likely is a positive test to be found in a patient with a pathological condition than in a patient without it? How much more likely is a negative test to be found in a patient without a pathological condition than in a patient with it? To give an answer to these questions, it is necessary to consider the positive and negative likelihood ratio. The Fagan nomogram is a simple method which can be used to easily obtain estimates of the post-test probability of disease for any given combination of the pre-test probability of disease and likelihood ratio.
