Fagan's Nomogram

Fagan's nomogram is a graphical tool used to calculate the probability of a patient having a disease after a diagnostic test, given the pre-test probability and the test's likelihood ratio. It simplifies the application of Bayes' theorem by visually representing the relationship between pre-test probability, likelihood ratio, and post-test probability

• 1. Likelihood Ratio:

  This value (positive or negative) reflects how much a test result (positive or negative) changes the probability of the condition. It's calculated from the test's sensitivity and specificity. 

• 2. Post-test Probability:

  This is the updated probability of the condition after considering the test result and its likelihood ratio. Pre-test Probability:

• 3. This represents your initial estimate of the patient's probability of having the condition before any testing is done.