How can I calculate the sensitivity and specificity (or analogous measures) of a continuous diagnostic test in predicting a continuous outcome (e.g., blood pressure) without dichotomizing the outcome? Any ideas?

It appears that researchers have done this using mixed effects modeling (see link below), but I'm not familiar with their use of the technique: http://www.ncbi.nlm.nih.gov/pmc/articles/PMC3026390/

By the way, I'm most familiar with R, so it would be ideal for the implementation you suggest to be accompanied with an R function (but it's okay if not).

Thank you in advance for any suggestions!