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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. 

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When using this tool. Do not use 0 or 100%, as these values will create an error. Use 0.1 or 99.8% and the code should produce an error!!

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