Likelihood Ratio

1. Indications
   1. Determine whether a test offers value in a patient's evaluation for a particular condition
2. Definition
   1. Positive Likelihood Ratio (LR+): Rule-In Condition
      1. Extent to which a positive test increases the likelihood that a patient has that disease
      2. Calculation 1: LR+ = (pTruePos / pFalsePos)
         1. pTruePos = Probability that a person with the condition has a positive test
         2. pFalsePos = Probablity that a person without the condition has a positive test
      3. Calculation 2: LR+ = (Test Sensitivity) / (1 - Test Specificity)
   2. Negative Likelihood Ratio (LR-): Rule-Out Condition
      1. Extent to which a negative test decreases the likelihood that a patient has that disease
      2. Calculation 1: LR- = (pFalseNeg / pTrueNeg)
         1. pFalseNeg = Probability that a person with the condition has a negative test
         2. pTrueNeg = Probablity that a person without the condition has a negative test
      3. Calculation 2: LR+ = (1 - Test Sensitivity) / (Test Specificity)
3. Interpretation: Positive Likelihood Ratio (LR+)
   1. LR+ over 5 - 10: Significantly increases likelihood of the disease
   2. LR+ between 0.2 to 5 (esp if close to 1): Does not modify the likelihood of the disease
   3. LR+ below 0.1 - 0.2: Significantly decreases the likelihood of the disease