Background

Bacterial bloodstream infection is responsible for the majority cases of community-acquired and hospital-acquired sepsis and septic shock [1]. It is associated with poor outcomes, especially in cases where patients do not receive appropriate and timely antimicrobial therapy [2,3,4]. Early adequate empirical antibiotic therapy is pivotal for patients’ survival [5]. However, increasing antibiotic resistance to commonly used antimicrobials poses a challenge to treatment. Early identification of the causative pathogen is important because this can enable physicians to choose appropriate antibiotic agents for therapy. Blood culture still serves as the gold standard for identification of the causative microorganism, but it is time-consuming and shows a high false-negative rate. Other rapid diagnostic tools for the early optimization of antimicrobial therapy, such as PCR-based tests, are limited by the number of PCR probes required or are dependent on positive culture samples [1].

The causative pathogen can be identified based on the levels of specific infectious biomarkers or inflammation cytokines, such as procalcitonin, interleukin (IL)-2, IL-4, IL-6, tumor necrosis factor-α, and interferon-γ [6,7,8,9], but these parameters are not commonly measured at primary-level hospitals or hospitals in low-income countries [10]. Routine laboratory parameters, including complete blood cell (CBC) counts, acute-phase proteins, electrolytes, and blood gas indicators, are commonly measured, and the data for these variables can be easily obtained from hospitals at different levels across different countries. Using these parameters to predict the causative pathogen may present a more practical, feasible, and time-saving strategy, especially for patients who are severely ill or admitted to lower-level hospitals.

Machine learning (ML) techniques have shown great potential in aiding the diagnosis of disease [11, 12]. In recent years, ML technologies have seen remarkable advancements and are being rapidly implemented in various medical fields. A series of ML-based models have been developed successfully and have demonstrated the feasibility and interpretability of ML in bacteremia prediction [13,14,15,16]. Beeler et al. [13] developed a model using the random forest (RF) algorithm to predict the risk of central line-associated bloodstream infections (CLABSIs). Further, Mahmoud et al. [14] used six ML algorithms, including RF, logistic regression (LR), decision trees (DT), naive Bayes (NB), artificial neural networks (ANN), and support vector machine (SVM), to discriminate between positive and negative blood cultures with high specificity. Tsai et al. [15] established models based on LR and SVM to predict bacteremia in febrile children. In another such study, Bhavani et al. [16] demonstrated that ML models based on data from electronic health records exhibited better performance in predicting bacteremia than previously published risk scores.

As far as we know, routine laboratory parameters have not yet been successfully integrated in an ML model for predicting gram-positive and gram-negative infections in patients with bacteremia. Therefore, in this study, we aimed to establish an ML model for early identification of gram-positive and gram-negative bacteremia based on routine laboratory parameters. 1–3-β-d-glucan test is a good diagnosis tool of invasive fungal infection with great sensitivity and specificity, so the BSI patients caused by fungus were not included in this research. An ML-based model utilizing the LR, SVM, ANN, RF, and convolutional neural network (CNN) algorithms was constructed and evaluated in the Medical Information Mart for Intensive Care (MIMIC) cohort (from the USA) and a local cohort (from China) in order to provide robust evidence for the application of this model.

Methods

Study design

As shown in Fig. 1, the MIMIC cohort was divided into a training set and a test set. Routine laboratory blood test results were extracted from the MIMIC dataset. Parameters with large amounts of data missing were removed, and the Pearson correlation test was used to remove redundant features. After feature selection, five ML algorithms (LR, SVM, ANN, RF, and CNN) were used to build the model based on the selected features. The built model was then evaluated on a hold-out test set of the MIMIC cohort. In addition, data collected from an independent test cohort from Qilu Hospital of Shandong University were used to evaluate the model further.

Fig. 1
figure 1

Schematic depiction of the study design

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Source of data and study population

MIMIC is an open critical care database that contains comprehensive clinical data of patients admitted to Beth Israel Deaconess Medical Center in Boston, Massachusetts [17,18,19]. MIMIC III contains data collected between June 2001 and October 2012, while data collected between 2008 and 2018 are recorded in MIMIC IV. The datasets used in the present study were extracted by Zhang and Wang, who have completed the collaborative institution training initiative program course (Record ID: 36181465 and 46463103).

As shown in Fig. 2, data for patients with a positive blood culture for a bacterial pathogen were retrieved. The exclusion criteria were age less than 16 years and the detection of microorganisms that were potentially contaminants, as pre-defined according to previous reports [20, 21]. For patients with multiple episodes of bacteremia, only data for the first episode were retrieved. Moreover, cases of mixed infection with two or three microorganisms detected in the same blood sample were also excluded. We retrieved laboratory results for blood examinations that were conducted closest to the onset of the bacteremia (defined as the collection time of the positive blood sample). Parameters for which data were missing in more than 40% of the patients were excluded.

Fig. 2
figure 2

Flowchart depicting the cohort selection process

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Another cohort from outside of the USA was used to evaluate the model further. This cohort was recruited from two mixed ICUs in Qilu Hospital of Shandong University in China. Patients with positive blood bacterial culture between 2019 and 2020 were included according to a protocol approved by the Ethics Committee of Qilu Hospital, Shandong University (approval no. KYLL-2018153). Written informed consent was waived by the ethics committees because the study did not involve any interventions.

Variables

Initially, 374 variables were obtained from routine laboratory blood tests, including CBC count, liver function test, renal function test, serum cardiac markers, serum electrolytes, blood gas analysis, coagulation-associated markers, and immune cell-associated markers. However, 340 variables for which data were missing in > 40% of the patients were eliminated, and the remaining 34 variables were utilized for development of the model. All these 34 variables can be measured by routine laboratory blood tests for emergency or critical patients, and they include CBC count, liver function test, renal function test, serum electrolytes, blood gas analysis, and coagulation function test.

Statistical analysis

The Mann–Whitney U-test was utilized to analyze differences between gram-positive and gram-negative cases in terms of numerical features. Pearson correlation test was utilized to assess the linear dependence between features. Redundant features (Pearson correlation coefficient \(\left|\gamma \right|\ge 0.8\)) were removed [22] to ensure that there was no high degree of correlation in the final selected features for the model. The Delong test was performed to compare the area under the curve (AUC) of the classifiers used to construct the model. P < 0.05 was considered to indicate statistical significance for all the analyses. Statistical analysis was performed using Python in Pycharm-Professional-2019.1.3.

Model development

The model was built based on five classifiers: LR, SVM with the radial basis function kernel, ANN, RF, and CNN. As LR, SVM, and ANN are sensitive to the dimension of features, Z-score analysis was performed before model training. Receiver operating characteristic (ROC) curve analysis was used to evaluate the performance of the built model, and AUC was computed. Accuracy, sensitivity, specificity, positive predictive value (PPV), and negative predictive value (NPV) were also calculated to assess the performance of the model.

In order to determine the importance of each feature, SHapley Additive explanation (SHAP) values [23] were computed based on each model. With SHAP, an additive interpretation model can be constructed in which all the features are regarded as contributors, and then the marginal contribution of a feature can be calculated for each sample when it is added to the model. Since a feature has different marginal contributions for different feature sequences, the mean value is computed as the SHAP value. Eventually, the mean of all the samples’ SHAP values for a feature was considered to indicate the feature’s importance. Furthermore, considering that all 34 variables used in the model may not always be available, a predictive decision tree (DT) model requiring fewer parameters was also developed for the differentiation of gram-positive and gram-negative bacteria.

The MIMIC cohort was randomly partitioned into the training set and the test set at a ratio of 8:2, and the training and test sets had the same distribution of gram-positive and gram-negative bacteremia cases as the original dataset. This process was repeated five times to diminish the influence of data partitioning. Model tuning was performed with ten-fold cross validation, and the tuned model was tested on the test set. The median performance after five rounds of testing was finally reported in our study. The entire process was performed in Pycharm-Professional-2019.1.3. We use scikit-learn library of 1.2.2 version and pytorch library of 1.11.0 version.

Results

Patient characteristics and variables

In total, 2118 patients whose records were deposited in the MIMIC-III and MIMIC-IV database were enrolled; this included 1251 patients with gram-positive and 867 patients with gram-negative bacteremia. The three most common pathogens identified in the gram-positive group were Staphylococcus aureus (n = 599), Enterococcus faecium (n = 252), and Enterococcus faecalis (n = 106), and the three most common gram-negative bacteria were Escherichia coli (n = 347), Klebsiella pneumoniae (n = 163), and Pseudomonas aeruginosa (n = 70). The median age of the two groups was comparable, and 61.19% (n = 1296) of the patients were male and 38.81% (n = 822) were female. We excluded variables with missing values exceeding 40%, and then we selected patients who did not have missing variables.

Figure 3 shows a heatmap of the correlation coefficients for the correlations between the variables. Variables with a high degree of correlation were eliminated, and 32 variables were eventually selected for input into the model. All 32 variables can be measured with routine blood tests in the emergency department or ICU. Table 1 shows patients’ characteristics, including age, gender, and the 32 selected variables.

Fig. 3
figure 3

Heatmap of Pearson correlation coefficients for the correlations among variables. A high absolute value of the Pearson correlation coefficient corresponds to a high degree of correlation. The dark red and dark blue squares indicate a high degree of correlation, while the light red and light blue squares indicate a low degree of correlation

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Table 1 Distribution of patients’ demographics characteristics and routine laboratory parameters
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Construction of the predictive model for differentiating between gram-positive and gram-negative bacteremia

The predictive model was constructed based on four ML classifiers, i.e., LR, SVM, ANN, RF, and CNN. Figure 4 presents the ROC curves of the model based on the five classifiers for the training set and the test set. As demonstrated in Fig. 4, in both sets, RF performed better (AUC = 0.768, 95% CI = 0.715–0.798) than LR, SVM, and ANN (p = 0.0011, p = 0.0001, and p = 0.0039, respectively) with regard to discriminating between gram-positive and gram-negative bacteremia, while the other three classifiers had comparable performances (p > 0.05). We also compared the five-fold cross-validation results with ten-fold cross-validation based on RF according to your suggestions. It is found that the ten-fold cross-validation performed better (AUC = 0.768, 95% CI = 0.715–0.798) than five-fold cross-validation (AUC = 0.762, 95% CI = 0.708–0.796).The performance of CNN (AUC = 0.828, 95% CI = 0.817–0.840) was slightly better than RF (p = 0.0043). However, considering the CNN model lacking interpretability, which is very important in clinical application, the RF model was finally chosen for further research analysis.

Fig. 4
figure 4

ROC curves depicting the predictive ability of the model in the training set and test set

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Figure 5 shows the performance of the model in the training and test sets based on accuracy, sensitivity, specificity, PPV, and NPV. As shown in Fig. 5, RF achieved higher accuracy, specificity, PPV, and NPV than the other ML algorithms. Although both SVM and ANN had higher sensitivity than RF, the sensitivity of RF was still high at 75.20% in the test set and was effective for distinguishing between gram-positive and gram-negative bacteremia. Thus, the model was eventually built based on the RF classifier to predict gram-positive and gram-negative bacteremia.

Fig. 5
figure 5

Performance of the models based on their accuracy (A), sensitivity (B), specificity (C), PPV (D), and NPV (E)

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In order to identify the most important components in the predictive model, SHAP values were calculated for each variable. Figure 6 shows the variables’ importance in the model based on RF. The six most important variables were lactate (Lac), alkaline phosphatase (AKP), asparate aminotransferase (AST), total bilirubin (TBIL), white blood cell count (WBC), and base excess in extracellular fluid (BEecf): gram-positive bacteremia was associated with lower values of Lac, AKP, AST, and TBIL and higher values of WBC and BEecf. Among the six variables, Lac emerged as the most important one, as indicated in Fig. 6a. RF can also provide feature importance, as indicated in Fig. 6b. The feature importance obtained from RF is almost consistent with that computed using SHAP.

Fig. 6
figure 6

Importance of the variables in the model based on RF. a The left plot presents the SHAP value for each variable, with each point representing an individual sample. The vertical axis and horizontal axis represent the variables and the SHAP values, respectively. The color of each point represents the value of the variables, as shown in the color bar on the left. The SHAP values are directly associated with the model’s output. The right plot shows the mean of the absolute SHAP values for each variable as an indicator of the importance of the variable. Variables with higher values are thought to be more important. b The plot shows the mean of Gini index for each variable as an indicator of the importance of the variable. Variables with higher values are thought to be more important

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The ten most important variables selected from the RF model (shown in Fig. 6) were used as optional features to build a DT model. Cross-validation was used to select the optimal variable set for DT. Finally, only five variables were selected, as shown in Fig. 7: WBC count, percentage of basophils, alkaline phosphatase, lactate, and total bilirubin. The AUC of the built tree was 0.679 (95% CI = 0.632–0.723), and it had an accuracy of 66.75%, sensitivity of 66%, specificity of 67.82%, PPV of 74.66%, and NPV of 58.13%.

Fig. 7
figure 7

Decision tree for the prediction of gram-positive and gram-negative bacteremia. Blue squares: gram-negative samples, brown squares: gram-positive samples, and gray: leaf nodes. “Samples” refers to the number of samples in the current node. “Class” in the leaf nodes refers to the prediction of the built decision tree, which depends on the number of gram-positive and gram-negative samples. If the number of gram-positive samples is higher than the number of gram-negative samples in the leaf, then the prediction is gram-positive bacteremia

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Evaluation of the RF model in the Qilu Hospital cohort

An independent test cohort of 132 critical care patients was recruited from Qilu Hospital of Shandong University to evaluate the performance of the models. The patients’ characteristics and laboratory parameters are shown in Supplementary Table S1. Table 2 shows the performances of the proposed model based on RF and DT in the Qilu Hospital cohort, and Fig. 8 shows the ROC curves for the model. RF (accuracy = 64.39%, AUC = 0.666) outperformed DT (accuracy = 59.85%, AUC = 0.615). Although the performance of the models based on RF and DT decreased to a certain extent in the Qilu hospital cohort compared to the MIMIC dataset, both classifiers still showed acceptable performance in terms of predicting gram-positive and gram-negative bacteremia.

Table 2 The performances of the proposed model and DT in Qilu Hospital dataset
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Fig. 8
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ROC curves of the proposed predictive model based on RF (A) and DT (B) in the Qilu Hospital cohort

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A software was developed to make the RF- and DT-based prediction models easily accessible (we can provide the software if needed). Figure S1 shows the interface of the software: the input includes two parts that can function independently. Quick Prediction refers to the DT-based model, for which five input variables are required, while General Prediction refers to the RF-based model, which requires more input variables but provides a more precise prediction.

Discussion

The aim of the current study was to develop a predictive model based on ML algorithms to discriminate between gram-positive and gram-negative bacteremia in patients with severe bloodstream infection before the pathogenic test results are accessible. The model based on the RF algorithm showed satisfactory predictive performance in discriminating between gram-positive and gram-negative pathogens that cause bacteremia. In order to improve its applicability in real-life situations in which all routine blood test results might not be available on time, especially in emergency situations, a DT model was built using only five variables.

Empiric antimicrobial treatment of bacteremia is often problematic because of the increasing resistance of both gram-positive and gram-negative microbes against antimicrobial drugs. Gram-positive bacteria are a major concern, especially multidrug-resistant bacteria such as methicillin-resistant Staphylococcus Aureus, vancomycin-resistant Enterococcus faecium, and β-lactamase-resistant Streptococcus pneumonia [24, 25]. Further, multidrug-resistant gram-negative bacteria, such as Enterobacteriaceae, Pseudomonas aeruginosa, and Acinetobacter baumannii, also pose a serious and rapidly emerging threat, especially for patients in intensive care units [26]. The easy-to-use model proposed in the present study can be used to promptly predict gram-positive and gram-negative bacteremia and could contribute to the timely and adequate elimination of the implicated pathogen. Adequate empiric antimicrobial treatment for sepsis has been demonstrated to directly affect the mortality rate in the ICU [27]. With this model, the probability of gram-positive or gram-negative bacteremia can be calculated offline when the values of the 32 variables are input into the software provided. Further interventional studies based on this prediction model are necessary to verify its effectiveness based on patient outcomes.

Several laboratory blood test parameters have been proposed as potential predictive markers for the discrimination of gram-positive and gram-negative bacterial infections, and these are used to tailor empiric antimicrobial therapy before the results of the pathogen tests are obtained [6, 9, 28, 29]. However, there is no strong evidence for the ability of any of these parameters to predict the infection pathogens. The ML algorithm has been proved to be helpful in combining several variables to discriminate different subsets of patients. So far, there is no ideal ML model for predicting the pathogens that cause bacteremia. The ML model of Ratzinger et al. based on the K-star algorithm had a sensitivity of only 44.6% for detecting gram-negative bacteremia [30]. Although the AUC of their model (0.675) was comparable to that of the present study, it had poor sensitivity (44.6%) and specificity (79.8%). Ratzinger’s research also started with variables from routine laboratory tests, such as CBC count, liver function test, renal function test, serum electrolytes, and coagulation function test, but only seven variables (gender, count of lymphocytes, count of monocytes, percentage of monocytes, fibrinogen, creatinine, and C-reactive protein) were included in the final K-Star model. When building the current RF model, the results of blood gas analysis were also included. Moreover, 32 variables were entered into the RF model. The larger cohort of patients, the higher number of input variables, and the different algorithms used may explain why our model performed better.

Considering that measurements of the 32 variables input in the RF model may not be available in some areas, medical institutions, and units, a well-performing DT model was also constructed with only five routinely measured variables: WBC count, basophil percentage, alkaline phosphatase, and lactate. Gram-negative bacteremia is associated with a higher level of inflammatory response than gram-positive bacteremia [6]. Accordingly, the association of gram-negative bacteremia with increased levels of WBC has also been found in a previous report [31]. Additionally, as basophils are a type of WBC, the inclusion of basophil percentage as an indicator also makes sense. Gram-positive and gram-negative bacteria activate different receptor pathways [32] and cytokine production patterns in the host [33]. Certain cytokines (such as IL-3, IL-5, and GM-CSF) induced by gram-positive bacteria appear to be important developmental factors for basophils [34]. Further, lipopolysaccharide is found in abundance in the outer membrane of most gram-negative bacteria and plays a key role in host–pathogen interaction [35] by increasing lactatemia via enhanced glycolysis [36] and lactate production [35], as well as early and severe impairment of lactate clearance [37]. Furthermore, it causes hepatoxicity by induction of oxidative stress and consequent oxidative damage to biomolecules [38]. These functions of lipopolysaccharide may explain the significant increase in lactate levels and hepatic biomarkers (e.g., AKP and total bilirubin) in patients with gram-negative bacteremia.

Several limitations of this study must be considered. First, the laboratory blood test variables in the MIMIC database do not represent all commonly used infection-related parameters; for example, procalcitonin and C-reactive protein are not reported in the MIMIC database. Further, immune-related parameters, such as CD4, CD8, and HLA-DR, were rarely recorded in the MIMIC database and could not be included when developing the ML model. The exclusion of these parameters may limit the effectiveness of the ML algorithm. Second, blood cultures present varying degrees of false-negative rates, dependent on the specimen acquisition time, usage of antibiotics, and microbial culture techniques, etc. This ML prediction model was based on blood culture results, which may cause bias because the BSI patients with false-negative blood culture results were not included in the datasets. Third, there was limit data of antimicrobial resistance which could be used for machine learning models training or validation. Our model wasn’t helpful to predict the existence of antimicrobial resistance. Fourth, as the datasets were evaluated retrospectively, most of the laboratory blood test results were not obtained on the same day that bacteremia was suspected. As there is no standard turnaround time for laboratory test results, the applicability of the model may be limited in certain situations. Finally, the model needs to be evaluated using data from different regions and countries, as well as prospective cohorts.

Conclusion

The present ML-based models could effectively discriminate between gram-positive and gram-negative bacteremia based on routine laboratory blood test results. This simple model would be beneficial in terms of guiding timely antibiotic selection and administration in critically ill patients with bacteremia before their pathogen test results are available. This model would be especially useful for patients in developing countries or those admitted to lower-level healthcare centers.