mNGS is broadly applied for detecting pathogens and especially for the timely and accurate diagnosis of critical illness due to suspected etiology microbes, such as sepsis, a severe condition that brings about poor outcomes for ICU patients. Herein, we conducted a comprehensive analysis on the diagnostic performance of mNGS for detecting pathogens among septic patients in comparison with routine culture-based diagnostics. We found that elderly patients were more commonly complicated by sepsis in the ICU, and the lungs were the major source of infection that caused sepsis, which might be partly attributed to anatomical features and age-associated comorbidities, including hypertension, diabetes, chronic cardiac dysfunction, and chronic respiratory disease. All septic patients received empirical antibiotic treatments at ICU admission mainly based on signs of infection in routine blood tests, including elevated counts in white blood cells and neutrophils, and increased levels of blood CRP and PCT. Indeed, these septic patients in our study presented with severe conditions, as evidenced by high SOFA and APACHE II scores, and needed further advanced treatments, including mechanical ventilation, infusion of red blood cells, and renal replacement therapy. Of note, septic patients had a high ICU mortality rate, sharing the same clinical characteristics with severe sepsis patients from a previous report by Xie et al. [4].
In the current study, mNGS showed obviously higher positive rates of pathogen detection than culture-based diagnostics in all samples as well as in various types of specimens, such as blood, BALF, and CSF. For example, the positive rate of blood culture was merely 14.7%, which was much lower than that of mNGS (78.3%) for detecting pathogens with systematic exposure. This was also applied to determine local infection, as noted, with higher positive rates in both BALF and CSF samples, indicating the general applicability of mNGS for pathogenic detection, even in samples with relatively low positive rates by culture-based diagnostic procedures. These benefits were noted in previously reported studies, which suggested that mNGS exerted a valuable diagnostic platform for determining relevant pathogens [11]. We further compared the diagnostic performance between mNGS and culture-based diagnostic procedures for isolating different kinds of microbes. mNGS was capable of identifying various pathogens with negative results by culture-based diagnostics and showed higher positive rates in common pathogens for the development of sepsis, including Acinetobacter baumannii, Pseudomonas aeruginosa, and Klebsiella pneumoniae. These commonly identified pathogens in this subset of patients conformed with previously published reports on etiological microorganisms for septic patients in ICUs, suggesting that Gram-negative organisms were the main cause for the development of in-hospital sepsis [4, 12]. In fact, the positive rates of culture-based diagnostics were less than 50% of that by mNGS, which was mainly due to the administration of empiric antibiotics. However, a study by Grumaz and colleagues [11] revealed that the divergent distribution of pathogen infection in postoperative septic patients by mNGS, such as Escherichia coli, Enterococcus faecium, and Bacteroides fragilis, was partly due to different sources of septic patients as well as sample types. For the results showed that culture positive but NGS negative in some patients, maybe due to the load of pathogens is under the detection threshold. Moreover, the noteworthy double-positive rates between mNGS and culture-based diagnostics shed light on the fairly good diagnostic performance of mNGS. In addition, for the mismatch results with double samples, we would introduce a third-party detection method for verification, and will be shown in the subsequent study. Remarkably, 19 of those 33 patients showed mNGS-guided clinical responses.
The early identification of fungal infection is of clinical significance with the use of mNGS and has been confirmed by many previous studies [9, 13]. In this study, Candida and Aspergillus were the major fungi isolated by both mNGS and culture-based diagnostics. However, the positive rates of fungus identification with mNGS were markedly lower than those with culture-based diagnostics, which showed similar results compared to previous studies that reported low-positive rates in fungi detection by mNGS in septic patients [9, 11]. In addition, a total of 36 samples were found to have viral infection by mNGS, indicating extensive pathogenic information for clinical practice by mNGS in septic patients. It has been demonstrated that reactivation of latent viruses is frequently complicated in prolonged sepsis and is critically involved in the progression and outcome of septic patients [14].
The application of mNGS is mainly restricted to identifying clinically relevant pathogens based on data from coverage, depth, and reads. Currently, the read values of mNGS are commonly used for the interpretation of distinct pathogenic infections after optimization [15, 16]. However, cut-off reads for diagnosing distinct microbes by mNGS and their clinical applications in septic patients remain unclarified. In this study, we applied ROC analysis to determine the optimal cut-off read values for the three most commonly detected pathogens based on the results of culture-based diagnostics. The optimal cut-off read values for these bacteria were relatively high, from 892.5 to 2893, and showed acceptable sensitivity and specificity. To our knowledge, this is the first report to identify the cut-off reads for diagnosing distinct pathogens, especially for patients with sepsis, which is indeed favorable for the clinical application of mNGS. In fact, mNGS has been used to isolate distinct microbes in various diseases, such as Streptococcus pneumoniae in pediatric bacterial meningitis, Ebola virus disease, and arthritis caused by Legionella micdadei as well as Staphylococcus aureus [15, 17, 18]. Although these reports show good performance in terms of pathogen detection, few provide optimal cut-off reads for each pathogen. The definite cut-off reads for pathogens indeed facilitate the extensive application and optimize the data interpretation of mNGS.
Nevertheless, some issues should be taken into consideration when interpreting our results. First, this study was conducted by means of retrospective analysis, which limited comprehensive data analysis and further information on the use of antibiotics. Recently, we registered and performed a prospective study for the evaluation of mNGS in pathogen detection and antibiotic administration in septic patients from the ICU. Second, a relationship between the read values and prognoses of septic patients was absent in this observation due to the relatively small sample size of patients with distinct pathogen infections, which requires further investigation. Third, the mNGS was capable of exporting data on mixed infections of multiple microbes, especially for the reactivation of fungi and viruses, which were pivotal factors for the prognostic assessment of septic patients and should be considered in further studies with large sample sizes.