This Veterans Administration (VA) study analyzed individuals treated for high blood pressure at VA centers and found that those whose cases were complicated by other medical conditions were more likely to receive quality care.  The findings, based on both medical records and patient-reported quality, confound expectations that complicated cases will receive lower quality care under pay-for-performance systems because doctors will not want to treat cases that may lower their ratings.  The study authors note that VA facilities may not be analogous to other U.S. health care facilities  

Peterson, L. A., Woodward, L. D., Henderson, L. M. et al. (2009). Will hypertension performance measures used for pay-for-performance programs penalize those who care for medically complex patients? Circulation, 119: 2978-2985. DOI: 10.1161/CIRCULATIONAHA.108.836544 http://circ.ahajournals.org/cgi/content/short/119/23/2978 

Authors: Laura A. Petersen MD, LeChauncy D. Woodard, Louise M. Henderson, Tracy H. Urech, Kenneth Pietz