Chapter 7 : Molecular Methods for Detection of Pathogens Directly from Blood Specimens

Abstract:

Sepsis is a global problem and laboratory methods must be optimized to effect rapid, positive patient outcomes. The proverbial reference standard method for diagnosis, the actual culture of blood, suffers from a variety of preanalytical issues such as sufficient blood volume, prior antimicrobial treatment, time from sampling to incubation, and analytical issues of prolonged turnaround time until final identification and availability of antimicrobial susceptibility testing results. Advancements have occurred in molecular methods to provide rapid microorganism identification and resistance gene detection (e.g., mecA in Staphylococcus spp.) from positive blood culture broths. In a meta-analysis of these new methods, a significant reduction in time to targeted treatment was observed ( 1 ). However, these methods are dependent on having a positive blood culture, which is subject to the preanalytical issues described above. The ability to directly identify pathogens from blood specimens would greatly reduce the time to identification and time to optimization of therapy, as well as potentially contributing to avoiding unnecessary antimicrobials and/or discontinuation of treatment in patients with negative testing results. Direct detection methods for viruses from patient specimens have benefited from advances in molecular methods, which is largely because of the high viral loads present in those specimens. In contrast, the majority of septic patients have a paucity of bacteria or fungi per milliliter of blood, necessitating the need for larger blood volumes to increase blood culture sensitivity ( 2 – 4 ).