### Probability Calculator

[ See also: Statistical Explanation of the Probability Calculator ]

#### What is a probability calculator?

The probability calculator is a simple program that computes the post-test probability of Lyme disease in a patient based on serologic test results and an accurate pre-test probability. The program (below) is an electronic version of a graphic nomogram of Bayes' Theorem, originally published by Terrence J. Fagan, MD, in the *New England Journal of Medicine* (V. 293:257, 1975) and included in the the ACP Initiative on Lyme Disease's *Guide to Clinical Practice*.

#### What is the purpose of the probability calculator?

The calculator measures the impact of an ELISA test result on the probability of Lyme disease in a patient. For example, the likelihood ratio of a positive ELISA test result **in a patient with non-specific myalgias** is 5.0 (five), which means that the likelihood of getting a positive ELISA test is 5 times greater in patients with Lyme disease than in patients without Lyme disease.

With a likelihood ratio of 5, the probability of having Lyme disease will only be meaningfully influenced by serologic testing when the pre-test probability is neither low nor high. For example, in a patient with a 50% pre-test probability of Lyme disease and a positive test, the calculator would report a post-test probability of 83% whereas a 1% pre-test probability translates into only a 5% post-test probability.

#### How useful is the probability calculator?

Of course, the calculator can only provide an accurate post-test probability if a reliable pre-test probability is entered in the first place. Since it is difficult to accurately assess a pre-test probability in many cases, it can be argued that the calculator is not very useful for clinical practice. However, the probability calculator may be useful in dissuading a sole reliance on serologic testing to confirm a diagnosis of Lyme disease. By using the calculator, the ACP Initiative on Lyme Disease hopes to discourage undue reliance on serologic testing and emphasize patient history and clinical judgment when evaluating patients with possible symptoms of Lyme disease. To this end, we hope that you find this calculator helpful to you in your practice.

**Probability Calculator**

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