#### II. Background

1. Paper based nomogram in slide-rule fashion calculates Post-Test Probability given Pre-Test Probability (Prevalence) and Likelihood Ratio

#### III. Technique

1. Start with a blank Fagan Nomogram
2. Mark the Pre-Test Probability (diseases Prevalence) on the axis in the left hand column
3. Mark the Likelihood Ratio (positive or negative) for the given test on the axis in the middle column
4. Draw a line from the Pre-Test Probability point, through the Likelihood Ratio and intersect the third column axis (Post-Test Probability)
5. Read the Post-Test Probability where the line intersects the axis on the third column
1. If Positive Likelihood Ratio (LR+) is used, the Post-Test Probability will reflect how likely it is for a positive test to be a true positive for disease
2. If Negative Likelihood Ratio (LR-) is used, the Post-Test Probability will reflect how likely it is for a negative test to be a true negative for disease