Calculating likelihood ratios for chest pain (+ and -)

The likelihood ratio is a measure of how much a test changes the likelihood of having a condition. It is often split into two quantities: the likelihood ratio of a positive test (LR+), and the likelihood ratio of a negative test (LR-).

The likelihood ratio for MI given a positive chest pain result is given by the following formulas:

The likelihood ratio for MI given a negative chest pain result would be given by the following formulas:

Since the patient is positive for the presence of chest pain, only LR+ applies in this case. To get LR+, we use the appropriate numbers:

LR+ = (TP/(TP + FN)) / (FP/(FP + TN)) 
    = (15/(15 + 5)) / (100/(100 + 280)) 
    = 0.750 / 0.263 
    = 2.85