Study Design and Ethics
This is a retrospective database-based cohort study conducted in Nanjing Jinling Hospital. The establishment of the database was approved by the institutional ethics committee of Nanjing Jinling Hospital (2019NZKY009-01). Broad informed consent was obtained from each participant on using the clinical and laboratory data for academic research. The clinical and laboratory data were collected prospectively and stored in a web-based electronic database (Acute Pancreatitis Database).
Patient Selection
Adult subjects diagnosed with AP admitted to the center of severe acute pancreatitis (CSAP), Jinling Hospital (Nanjing, China) from October 2020 to October 2021 were screened. The diagnosis and severity of AP were defined according to the revised Atlanta classification 2012(Banks et al., 2013).
The inclusion criteria were as follows: 1. Plasma mNGS was performed when IPN was suspected but not yet confirmed. Suspected IPN was based on clinical manifestations like fever with elevated inflammatory markers. The decision for an mNGS test is made by the treating physician. 2. Sampling from AP onset < 35days. 3. Survived more than 14 days after sampling. The exclusion criteria were pregnancy and confirmed extra-pancreatic infectious complications at screening.
Metagenomic Next-generation Sequencing and Analysis
Whole blood samples (3-5ml) were sent for PACEseq mNGS analysis (Hugobiotech, Beijing, China). Human cells of each sample were removed by centrifugation. The supernatant was collected for DNA extraction using TIANGEN DP316 kit (TIANGEN, Beijing, China) based on its manual. NEBNext Ultra II DNA library Prep Kit (NEB, Ipswich, UK) was then used to construct the DNA libraries according to the manufacturer's instructions. The quality of all libraries were measured by Qubit (Thermo Fisher Scientific, MA, USA) and Agilent 2100 Bioanalyzer (Agilent Technologies, Palo Alto, USA). The qualified libraries were finally sequenced on a Nextseq 550 platform (Illumina, San Diego, USA). Adapters, as well as low quality, low-complexity, and short reads (less than 35bp) were removed from the raw data. SNAP and Burrow-Wheeler alignment was then applied to exclude human sequences by mapping the reads to the human reference genome (hg38). The screened sequences were finally mapped to the microbial genome database (NCBI: ftp://ftp.ncbi.nlm.nih.gov/genome). All parameters of the detected pathogenic microorganisms were calculated, including the sequence reads, relative abundance, genome coverage, and depth. Positive and negative controls were set for each batch during the experiments.
The mNGS-detected pathogens (including bacteria and fungi) were judged as positive when meeting the following two thresholds [19]: 1. The relative abundance of the microorganism (bacteria and fungi) is above 30%; 2. The coverage of bacteria is 10 times higher than that of any other microorganism, and the coverage of fungi is 5 times higher than that of any other fungi. Since the virus sequence was not verified by polymerase chain reaction (PCR), the cases in which only the virus sequence was detected were defined as mNGS negative.
Microbial Culture
Blood samples of all patients were sent for microbial culture at the same time when the mNGS test was performed. The blood samples were analyzed using the blood culture (BC) instrument BD BACTECTMFX40 (Becton Dickinson) according to the manufacturer's manual. Positive blood culture samples were speciated using the Vitek MS system (BioMerieux, version 1.7, France).
Pathogens are classified according to Gram-negative bacteria, Gram-positive bacteria, and fungus. Polymicrobial infection was defined as more than one pathogen detected in a sample.
Diagnosis of confirmatory IPN
In this study, confirmatory diagnosis of IPN within two weeks after sampling was considered the reference standard. IPN was confirmed when a positive microbial culture was obtained from (peri)pancreatic drains through percutaneous fine-needle aspiration or during drainage procedures and/or operative necrosectomy. Otherwise, sterile pancreatic necrosis (SPN) would be defined. The decision of invasive intervention is decided by the treating physician.
Clinical outcome and definitions
Clinical outcome measures include in-hospital mortality, length of hospital stay (LOS), requirement of ICU admission, new-onset organ failure based on the modified Marshall's score[20], new-onset sepsis, and septic shock defined according to the SEPSIS 3.0 definitions[21], management-related measures, and gastrointestinal fistula or abdominal bleeding requiring invasive intervention[22]. 'New-onset' in this study was defined as events that occurred after sampling and were not present 24 hours before sampling.
Data Extraction
Data were extracted using a data extraction form developed in advance. Data concerning demographic and baseline clinical characteristics, including age, gender, etiologies, laboratory biochemistry, and clinical scores like Acute Physiology and Chronic Health Evaluation II (APACHE II) score, Computed Tomography (CT) severity index, and sequential organ failure assessment (SOFA) score at screening were extracted from the database. For identification of the pathogens, mNGS results were collected based on the standard reports. Before analysis, cross-checking was done on the data by the principal investigators to ensure the quality of the data.
Statistical analysis
The overall results of plasma mNGS tests will be adjudicated as true positive (TP), false positive (FP), true negative (TN), or false negative (FN), as reported in previous studies[23, 24]. The overall result was considered as true positive if the plasma mNGS detected at least one IPN-relevant organism, while it was considered as false positive if the plasma mNGS detected pathogens that were not in accordance with the IPN dianoses. We used the abovementioned reference standard to estimate positive percent agreement (PPA) and negative percent agreement (NPA). Results were reported as percentages with 95% CI[25].
Continuous variables were reported as the median with interquartile range (25%, 75%). Categorical variables were expressed in frequencies and percentages. Mann-Whitney U tests were used to compare the differences between the groups for continuous variables. Fisher's exact tests were used for comparing categorical variables. SPSS 26 and Graphed Prism7 software have been applied for data analysis. All tests were two-tailed, and P-values of less than 0.05 were considered statistically significant.