Introduction
Sepsis is a leading cause of death worldwide; in the US it kills 250,000 people/yr and in Australia about 3000 people/yr . Sepsis can be difficult to distinguish from other non-infectious conditions that have similar clinical signs. Further, diagnosis of sepsis relies on blood cultures which are positive in only 10-30% of patients suspected of sepsis, and take at least 18 hours. The lack of sensitivity and timeliness of blood culture can lead to excessive use of broad-spectrum antibiotics in patients suspected of infection. It has been shown that most clinicians can make a correct narrow-spectrum antibiotic choice based on Gram-stain. We have developed a molecular Gram-stain assay that quickly and reliably detects and differentiates sepsis causing pathogens in whole blood and blood culture material based on Gram status.
Methods
SeptID® consists of a broad-range, 16S rDNA, multiplexed RT-qPCR capable of detecting and differentiating over 5400 bacteria based on Gram status, including most human pathogens. The assay relies on the use of two labelled probes, and one 16S rDNA Single Nucleotide Polymorphism, to differentiate Gram-positive from Gram-negative bacteria.We have previously demonstrated sensitivity of the assay using whole blood to be 1-10 cfu/mL. To determine SeptID® specificity we tested 225 positive and 100 negative blood culture samples (1mL). Gram status calls (Gram-positive, Gram-negative, mixed, negative) based on Ct values were made by two independent operators, and prior to any knowledge of clinical microbiology.
Results
Very high concordance was found for SeptID® results with clinical microbiology. Specifically, SeptID® and Gram-positive correlation was 97.99%; SeptID® and Gram-negative correlation was 98.61%. Further, SeptID® was able to determine whether a blood culture contained a mix of Gram-positive and Gram-negative organisms.
Conclusion
SeptID® can quickly, accurately and sensitively differentiate sepsis-causing pathogens based on Gram status. Nucleic acid based detection and differentiation of microbes in whole blood in patients suspected of sepsis holds promise for both decreasing time to diagnosis and aiding in antibiotic choice.