Surrogate markers and microbiologic end points.

For treatment studies of an infectious disease, such as pneumonia, microbiologic eradication is the logical primary end point. Several problems for pneumonia in general and several more specific to VAP preclude the use of microbiologic eradication as a primary end point. These problems include no positive culture result at baseline, difficulty distinguishing colonization from infection on baseline cultures, no specimen available for testing when determining cure, and induction of colonization by antibiotic treatment. These problems have led to interest in serial quantitative cultures and biomarkers to determine the microbiologic outcome. Although promising, especially for open-label studies focused on multidrug-resistant pathogens, further research is needed before serial quantitative cultures can be used to define microbiologic failure. Of the biomarkers, procalcitonin level may be a valuable adjunct to clinical evaluation but cannot be a primary end point alone. A decreasing or low procalcitonin level after initiation of antibiotic treatment correlates well with bacterial eradication.