Study Design and Patient Selection
This retrospective, single-centre cohort study included all consecutive in- and outpatients in the general internal medicine department, excluding intensive care unit and emergency department, of Dokkyo Medical University Hospital, Mibu, Tochigi, Japan, who underwent blood culture testing from 1 January to 31 December, 2018. Dokkyo Medical University Hospital is a tertiary teaching hospital. This study was conducted in accordance with the current version of the Declaration of Helsinki. The study protocol was approved by the institutional ethics committee of Dokkyo Medical University (No. R-20-18J).
From a total of 399 adult patients (age >15 years) who underwent blood culture testing in the general internal medicine department during the study period, 205 were excluded because of antibiotic use within 2 weeks prior to the blood culture sampling (n=178), steroid use (n=25), or haematological cancer (n=2). Five other patients were excluded because of a lack of data. The remaining 189 patients were enrolled in the study. A flow diagram of patient selection is shown in Figure 1. All blood cultures were drawn at the discretion of the treating physician.
Patient and Public Involvement
No patient involved.
Outcome and definition
The primary study outcome was a positive blood culture indicative of bloodstream infection. We defined a bloodstream infection as the presence of a pathogenic microorganism in at least one blood culture bottle. Samples with bacterial contaminants were counted as negative cultures. The contamination criterion was the presence of multiplying coagulase-negative Staphylococcus species, Bacillus species, Propionibacterium acnes or Corynebacterium species in a single set of blood cultures. These bacteria were previously identified as frequent contaminants(14). All such samples were excluded prior to the review of medical notes.
Absolute eosinopenia was differently defined in each research and does not have a universal definition(13, 15). In this study, the optimal cut-off was defined as an eosinophil count of <24.3 cells/mm3 from univariate analysis. The qSOFA, a recently developed measure for the rapid identification of infected patients at risk of mortality, was also applied(10, 16, 17). This bedside clinical score identifies adult patients with suspected infection and a higher risk of poor outcomes typical of sepsis as those who meet with at least 2 of the following clinical criteria: respiratory rate of ≥22/min, altered mentation, or systolic blood pressure of ≤100 mm Hg (10).
From each patient, the clinicians drew 10 mL of blood aseptically from a superficial vein and inoculated the sample into both aerobic and anaerobic cultures. They repeated the procedure using a different superficial vein to yield 2 sets of blood cultures for each patient(18). The cultures were incubated in blood culture bottles containing BACTEC resin-beads (Bactec Plus Aerobic/23F and Anaerobic/22F bottles; Becton Dickinson Instrument Systems, Sparks, MD, USA). The bottles were incubated at 35℃, sub-cultured daily, and inspected for bacterial growth for 6 days.
A fully automated BACTEC-FX blood culture incubation system (Becton Dickinson) was used to isolate bacteria from the blood cultures. Significant isolates were identified and tested for antimicrobial susceptibility according to the National Committee for Clinical Laboratory Standards guidelines(19). All bacterial species isolated from blood culture bottles were confirmed using matrix-assisted laser desorption ionisation–time of flight mass spectrometry.
Patients’ medical records were reviewed to ensure that 2 attending clinicians considered the detected micro-organisms to be pathological, rather than contaminants. All data in this study were collected by the treating clinicians in the context of clinical management and included age, sex, presence of chills, and vital signs (mental status, respiratory rate, and systolic blood pressure) at the time of blood culture sampling. The potential markers of bloodstream infection assessed in this study included the serum CRP concentration and total white blood cell, neutrophil, and eosinophil count. All markers were measured within 1 day of blood culture sample collection. Eosinophil count was determined using an automated method.
Continuous variables are presented as medians and interquartile ranges [25th–75th percentiles] and were compared using the Mann–Whitney U test. Categorical or binary variables are presented as numbers (percentages) and were compared using the chi‐squared test or Fisher’s exact test. The diagnostic accuracies of each univariate variable and the multivariable logistic regression models were assessed by calculating the corresponding area under each receiver operating characteristic curve (AUROC). A P value of <0.05 was considered statistically significant. The 95% confidence intervals (CIs) were used to quantify uncertainty.
Previous studies identified the CRP level as a powerful predictive marker of bloodstream infection(11, 20). In this study, we calculated the integrated discrimination index (IDI) and net reclassification improvement (NRI)(21) to assess whether the inclusion of eosinopenia into the model involving the baseline variables (age + sex) and CRP level would improve the predictive value. All statistical tests were performed using the R 3.6.0 and pROC package(22) for MacOS X (The R foundation for Statistical Computing, Vienna, Austria). Internal validation of the prediction models was conducted using ordinary nonparametric bootstrapping with 1,000 bootstrap samples and bias-corrected, accelerated 95% CIs(23).