Sampling Theory for Molecular- and Blood Culture-Based Techniques

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Background Septicemia is associated with high mortality. Inadequate antimicrobial therapy leads to poor outcomes. Pathogen detection aids in clinical decision making by directing appropriate treatment. Sampling volume and in vivo microbial blood concentration affect analytical sensitivity and represent a significant design challenge for future point-of-care pathogen detection technologies. Methods We applied the Poisson distribution to identify the probability of collecting no pathogens from a given sampling volume and bloodstream pathogen concentration for existing blood culture and whole-blood polymerase chain reaction (PCR) pathogen detection technologies. The concept of “effective target pathogen concentration” is introduced to account for the unique detection of both cell-free and intracellular DNA by PCR methods. Results The probability for a 10-mL blood culture or a 1.5-mL PCR specimen not collecting pathogens was 4.54 × 10−5 and 0.22, respectively, when pathogen bloodstream concentrations were at 1 colony-forming unit/mL. The probability of collecting no pathogens decreased as pathogen bloodstream concentrations increased. Accounting cell-free target pathogen DNA, the PCR sampling probability substantially improved and exceeded blood culture performance. Conclusions The probability of not isolating pathogens from a PCR or blood culture specimen was highly dependent on sampling volume and pathogen concentration. Cell-free pathogen DNA enhances PCR performance despite smaller sampling volumes. The impact of sampling volume and target pathogen concentrations should be considered when developing point-of-care pathogen detection solutions that may be constrained to smaller sampling volumes. Further in vitro studies are warranted to validate this theory during dynamic sepsis conditions.