Although sensitivity and specificity do not change, we know intuitively that sending troponins in all patients presenting to the ED will produce many more false positives. This is where conditional probability comes into play. Positive predictive value (PPV) is the probability of disease given a positive test, and negative predictive value (NPV) is the probability of no disease given a negative test.
PPV = P(disease | positive test)
PPV = TP / (TP+FP)
NPV = P(no disease | negative test)
NPV = NP / (TN+FN)
These two values correlate either positively (PPV) or negatively (NPV) with the prevalence of the population you are testing. Try out the first two case examples below.