In relation to the disorder I'm studying Screen A is reported as having a sensitivity of 90% and a specificity of 89%. Screen B is reported as having a AUC of .79 with no other data provided. Could someone please explain the relative difference between these screens? I assume screen A is more useful but but by how much?
It is not possible to conclude much from these very limited data. For screen A you only have a point estimate at a given sensitivity/specificity combination. For screen B you have a summary measure for a ROC-curve of many Spec/Sens combinations. These measures are incomparable.
It also depends on what you see as the most "useful" screen, e.g. how many false positives/false negatives you are willing to tolerate. The two screens could be desirable under different requirements.