Hey Sabio, I think the formula you quoted for [LR-] has an error,The positive predictive value is not the same as positive likelihood ratio though they are closely related.
The positive predictive value is the probability the disease is actually present when you have a positive result and it's calculated (a/a+c).
The positive likelihood ratio (LR+) reflects the degree to which a positive test result increases the likelihood that the disease in question is present.
It's calculated by divding true positive rates over false positive rates or in other words sensitivity divided by 1-specificity.
LR- on the other hand is the rate of true negatives divided by false negatives (specificity divided by 1-sensitivity).
Nowadays LR+ is being used to derive the positive predictive value from it in a somewhat complex calculation steps. Negative predicitve value is derived from LR-.
LR+/LR- are more seen in European journals than in US medical journals and it's extremely unlikely to be seen in USMLE exam and you should not worry about it. However, you may be asked to calculate the positive predictive value.
[LR-] = (1-Sensitivity)/Specificity