II. Background
- Paper based nomogram in slide-rule fashion calculates Post-Test Probability given Pre-Test Probability (Prevalence) and Likelihood Ratio
III. Technique
- Start with a blank Fagan Nomogram
- Mark the Pre-Test Probability (diseases Prevalence) on the axis in the left hand column
- Mark the Likelihood Ratio (positive or negative) for the given test on the axis in the middle column
- Draw a line from the Pre-Test Probability point, through the Likelihood Ratio and intersect the third column axis (Post-Test Probability)
- Read the Post-Test Probability where the line intersects the axis on the third column
- 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
- 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
IV. Resources
- Diagnostic Test Calculator