Discussion
The important findings of the RADICAL study are that PCR/ESI-MS detected BSI pathogens with high overall sensitivity and NPV; PCR/ESI-MS was three times more likely to identify an organism than standard culture; and, if available, PCR/ESI-MS results may have altered the treatment regimen in as many as 57% of patients.
Sepsis affects a large proportion of the critically ill population. Despite improvements in recent years, morbidity and mortality rates remain high. The importance of initiating treatment as soon as possible has been highlighted and shown to be associated with improved outcomes, yet this finding needs to be balanced against the direct risks and stewardship issues arising from overzealous or inappropriate antibiotic use.
Rapid diagnosis of severe infection or sepsis is thus crucial not only to optimize a patient's chances of survival but also to encourage responsible antibiotic use. However, diagnosing infection accurately in critically ill patients is challenging. Characteristic clinical and laboratory signs of severe infection, such as tachycardia, fever, and altered WBC count, are nonspecific and are often present in other acute conditions. Biomarkers, such as C-reactive protein and procalcitonin, are also nonspecific and are of more value in ruling out infection than in making a definite diagnosis. Microbiological culture results are negative in many patients with sepsis, largely because prior antimicrobial therapy affects ex vivo growth in culture medium. Certain microorganisms are also particularly difficult to culture, requiring specific growth media or a particular environment. As culture results often require several days to become available, patients with suspected severe infection are, therefore, often started on empiric broad-spectrum antibiotics to increase the likelihood that a pathogenic organism will be adequately covered. This approach, although valid in terms of preventing delays in starting treatment with currently available diagnostic techniques, has several negative aspects, including the potential for toxicity with multiple antibiotics, the high-associated costs, and the effects of antibiotic pressure on the development of antimicrobial resistance.
Availability of a technique that could provide more rapid pathogen identification directly from patient samples could, therefore, represent a marked improvement in terms of enabling more rapid diagnosis and earlier initiation of appropriate antimicrobial therapy, with associated beneficial effects on outcomes, antimicrobial resistance, costs, and toxicity. Various methods have been suggested for this purpose, including single pathogen assays, which are of limited use in patients with suspected sepsis in whom multiple organisms may be involved; selected-pathogen assays, which use specific molecular targets to identify some 20–35 species; and broad-range pathogen assays, which use universal or conserved targets to identify many hundreds of species, but for which earlier versions lacked sensitivity due to the small volumes of blood extracted for analysis.
The PCR/ESI-MS test used in RADICAL can detect more than 800 bacterial and candida species from a single 5-mL EDTA blood sample, with results available within 6 hours from when the sample enters the testing laboratory. This is much faster than standard culture techniques that frequently take 48–96 hours for full identification with susceptibility profile. As expected, the PCR/ESI-MS assay identified significantly more positive samples than standard culture, possibly because it does not rely on bacterial growth and thus can identify organisms even in the presence of ongoing antimicrobial therapy. In the present study, over 75% of the patients were exposed to antibiotic treatment prior to enrolment (Table 1). This may explain in part the significant lack of microbiological growth in a large number of the PCR/ESI-MS positive cases. The simple κ statistic was used to estimate the agreement between culture and PCR/ESI-MS, but it underrepresents this agreement because of the requirement that the PCR/ESI-MS assay and blood culture results matched in terms of organism identity in order to be considered a true positive in the contingency table. The data were simplified and samples categorized as either an organism match or no match; hence, all organism agreement can be considered as being unlikely to have happened by chance. Furthermore, the paucity (< 1–10) of microbial colony-forming units/mL of blood, as reflected by higher culture yields through taking large volumes of blood, could be expected to significantly increase the risk of a false-negative result with a 20-mL blood sample. Despite this, the PCR/ESI-MS and culture methods both showed similar agreement on replicate testing using the κ statistic.
Importantly, in 41% of cases, the panel of independent experts would have recommended a change in management, including initiation of therapy, altered antimicrobial spectrum, and/or change in duration of therapy, based on the PCR/ESI-MS results. This percentage increased to 57 when PCR/ESI-MS tests were positive.
This study has several limitations. First, PCR/ESI-MS cannot provide detailed antimicrobial susceptibility information, unlike culture techniques. Hence, although it can provide sensitive and rapid identification of causative microorganisms, PCR/ESI-MS cannot currently replace culture methods. The results reported here need to be confirmed in studies that can directly determine the impact of this approach on clinical and economic outcomes, including length of stay and survival, but also on resistance patterns.
Second, the greater detection rate of E. coli, S. aureus, E. faecium, C. albicans, and Klebsiella pneumoniae by PCR/ESI-MS compared with routine culture was unanticipated, and the explanation is unclear. Prior to study inclusion, most patients were exposed to combinations of two or more antibiotics active against Gram-positive and Gram-negative organisms and were often receiving one or more antifungals in addition. As stated above, the bacterium/fungus may have been largely cleared with preexisting antibiotics, hence the negative culture results, but remaining DNA remnants in the circulation may have been sufficient to give a positive PCR/ESI-MS. The sensitivity of the technique increases the risk of identifying contaminants and commensals; however, the pathogens most frequently detected in the study are those associated with infection. Accepting the validity of these data, the PCR/ESI-MS test could be of importance to help target antimicrobial therapy in patients who have already started antimicrobials and have negative cultures (salvage microbiology). Further, ideally interventional, studies are warranted to confirm and further explore these findings.