Drop the Likelihood Ratio. A Novel Non-electronic Tool for Interpreting Diagnostic Test Results.

Objective: To present a geometric ruler to determine predictive values of binary diagnostic test results from prevalence, sensitivity and specificity without calculation. Methods: On logarithmic scale the relationships between pre-/post-test odds and the likelihood ratio as well as sensitivity, specificity and the likelihood ratio appear as simple linear (additive) equations. Results: Each of these additive equations can be solved geometrically in form of a ruler with three scales. By amalgamation we devise a novel non-electronic tool which omits the intermediate step of likelihood ratio determination. Conclusions: We propose a simple geometric method to aid in interpretation of diagnostic test results for both practical and educational purposes.