Continuous monitoring of clinical biomarkers offers the exciting possibility of 
new therapies that use biomarker levels to guide treatment in real time. This 
review explores recent progress toward this goal. We initially consider 
measurements in body fluids by a range of analytical methods. We then discuss 
direct tissue measurements performed by implanted sensors; sampling techniques, 
including microdialysis and ultrafiltration; and noninvasive methods. A future 
directions section considers analytical methods at the cusp of clinical use.