Abstract
Background: Bronchiolitis is the most common cause of lower respiratory tract infection (LRTI) in the first year of life. We analyzed the association between complete blood count (CBC), c-reactive protein (CRP), and novel inflammatory indexes (NLR, PLR, MLR, ELR, LMR, NPR, LPR, LNR, PNR, SII, SIRI) in predicting bronchiolitis severity at hospital admission. Methods: We retrospectively collected data from 95 infants hospitalized for bronchiolitis in a third-level hospital during three epidemic seasons. Five outcomes of severity were analyzed: BRAS; pediatric intensive care unit (PICU) admission; ventilatory support; intravenous (IV) rehydration; and length of stay (LOS). Results: Lower age and weight at admission were statistically associated with four of the five severity outcomes. Prolonged LOS (≥6 days) was associated with high values of total white blood cells, lymphocytes, and eosinophils. Only three inflammatory indexes (PLR, MLR, and PNR) showed a significant association with one outcome (prolonged LOS). A new index (RBC/AiW/1000) was statistically associated with each severity outcome for a value > 350. Conclusions: We proposed a comprehensive analysis of the association between CBC, CRP, and novel inflammatory indexes and bronchiolitis severity. RBC/AiW/1000 could represent a future predictive marker of disease severity at hospital admission in infants with bronchiolitis.
1. Introduction
Viral bronchiolitis constitutes the most common cause of lower respiratory tract infection (LRTI) in infants in their first year of life, leading to a significant economic burden worldwide [1]. Among viral infections, Respiratory Syncytial Virus (RSV) is the most frequently detected, followed by Rhinovirus, Parainfluenza virus, and Metapneumovirus [1]. The diagnosis of bronchiolitis is based on clinical history and physical examination [2]; however, in children hospitalized for moderate to severe disease, laboratory tests are usually performed to identify the presence of dehydration or sepsis. Hospitalization is suggested mainly in infants younger than 3 months or with risk factors, such as prematurity, bronchopulmonary dysplasia (BPD), congenital heart diseases, and immunodeficiency [1,2].
Numerous clinical aspects have been studied to identify a possible correlation with bronchiolitis severity, resulting in the adoption of several clinical severity scores. However, most of these instruments are based on auscultation skills and clinical evaluations that, being subjective, can affect validity. In their systematic review, Rodriguez et al. [3] identified the most reported items in defining the severity of bronchiolitis and described as the best evidence-based available scoring system the bronchiolitis risk of admission score (BRAS) [4]. This score is based on anamnestic parameters (duration of symptoms and age at presentation) and vital signs (respiratory rate, heart rate, and oxygen saturation) [4], and it is frequently used in clinical practice and literature research.
The intersubject variability in determining some items reported in clinical scores has increased the interest in objective measurements, such as laboratory data. Viral respiratory infections significantly impact the immune system, perturbating the expression of blood cells and the host inflammatory status [5]. It is reasonable to hypothesize that some early laboratory markers can predict the disease severity in infants with bronchiolitis. Some studies reported a correlation between lymphopenia and bronchiolitis severity [6,7,8], while eosinophil and platelet count seem to be not associated with a negative disease outcome [9,10]. High c-reactive protein (CRP) levels seem to be associated with prolonged hospitalization, pediatric intensive care unit (PICU) admission, and the requirement for oxygen supplementation [11]. Recently, specific blood markers obtained from routine complete blood counts (CBCs), such as monocyte-to-lymphocyte ratio (MLR) and neutrophil-to-lymphocyte ratio (NLR), were suggested as potential biomarkers for the identification of infants with RSV infection at major risk for severe outcomes [12]. Moreover, novel inflammatory markers have been analyzed to predict morbidity and mortality in different conditions [13,14].
To date, a comprehensive analysis of early changes in CBC, CRP, and novel inflammatory markers in determining bronchiolitis severity in infants and children is not reported in the literature. The purpose of our study was to describe the laboratory picture of infants with bronchiolitis in relationship to clinical severity outcomes at admission in a pediatric ward (PW) or a PICU of a third-level hospital.
2. Materials and Methods
2.1. Study Design and Participants
We performed a monocentric, retrospective cohort study analyzing data from infants aged < 12 months admitted with a diagnosis of bronchiolitis to the Pediatric Unit or the Pediatric Intensive Care Unit (PICU) of the “G. Martino” Hospital, Messina (Italy). We collected data over three epidemic seasons, from October to May 2021 to 2024. Data were extracted using patients’ electronic medical records through the hospital platform. However, only patients who performed nasal swabs for microbiological analysis were initially selected for the study.
Inclusion criteria were children aged < 12 months with a clinical diagnosis of bronchiolitis. Bronchiolitis was defined as an LRTI characterized by respiratory distress, cough, wheezing, and/or crackles [15]. For the purpose of our study, we included both the first and subsequent episodes of bronchiolitis. Only patients with viral infections were included in our study.
Exclusion criteria were children with co-morbidities (chronic neurological or conditions, hemodynamically significant congenital heart diseases, chronic lung diseases, immunodeficiency, blood disorders, trisomy 21), and being extremely preterm (<28 weeks). Moreover, we excluded children receiving treatment with systemic corticosteroids, antibiotics, and antivirals before the hospital admission. Bacterial infection at nasal swabs was also considered an exclusion criterion.
2.2. Study Procedures
We retrospectively collected anamnestic (age, gender, weight, presence of co-morbidities, gestational age, duration of symptoms) and clinical data (respiratory rate, heart rate, oxygen saturation). We calculated BRAS [4] according to the patients’ clinical status at admission. At admission, all children performed laboratory tests for CBC, CRP, and nasal swabs for pathogen detection using the Filmarray system (multiplex polymerase chain reaction (PCR) system). The respiratory panel for viral and bacterial infections (Filmarray system) comprised: Adenovirus; Coronavirus 229E, HKU1, NL63, and OC43; Severe Acute Respiratory Syndrome Coronavirus 2 (SARS-CoV-2); Influenza A subtypes H1 and H3; Influenza B; Metapneumovirus; Parainfluenza virus type 1, type 2, type 3, and type 4; RSV; Rhinovirus; Enterovirus; Bordetella pertussis; Chlamydia pneumoniae; Mycoplasma pneumoniae; Haemophilus influenzae; Streptococcus pneumoniae. CBC was measured by flow cytometry, while CRP quantitative determination was detected by immunoturbidimetry.
We obtained the following inflammatory indexes from the absolute cell counts performed at admission: NLR (neutrophils/lymphocytes ratio), PLR (platelets/lymphocytes ratio), MLR (monocytes/lymphocytes ratio), ELR (eosinophils/lymphocytes ratio), LMR (lymphocytes/monocytes ratio), NPR (neutrophils/platelets ratio), LPR (lymphocytes/platelets ratio), LNR (lymphocytes/neutrophils ratio), PNR (platelets/neutrophils ratio), SII (systemic immune-inflammation index, NLR*platelets), and SIRI (systemic inflammation response index, MLR*neutrophils) [13,14]. We also calculated two new indexes: the ratio between age in weeks and hemoglobin levels (AiW/Hb); and the ratio between red blood cells and age in weeks divided by 1000 (RBC/AiW/1000).
To define the severity of bronchiolitis, we considered the following data: BRAS at admission; admission to PICU; need for supplementary oxygen (oxygen mask or nasal cannula), high flow nasal cannula (HFNC), continuous positive air pressure (CPAP), or non-invasive ventilation (NIV); intravenous (IV) fluids administration; and length of stay (LOS).
2.3. Statistical Analysis
For the analysis of the descriptive data, we used percentages (%), ranges, mean, and standard deviation (SD), or median and interquartile range (IQR). We analyzed the demographic and clinical characteristics of our sample, such as age, preterm birth, gender, weight at admission, viral detection at Filmarray, admission to PICU, IV rehydration, ventilatory support, LOS in the hospital, and BRAS at admission.
We derived the mean and SD of each blood parameter, represented by blood white and red cell count, platelet count, neutrophils, lymphocytes, eosinophils, monocytes, hemoglobin, and CRP. We also calculated mean and SD of novel inflammatory indexes (NLR, PLR, MLR, ELR, LMR, NPR, LPR, LNR, PNR, SII, and SIRI) as well as AiW/Hb and RBC/AiW/1000.
Student’s t-distribution was used to assess the association between blood parameters (including inflammatory indexes) and severity outcomes during hospitalization. When a statistical association between blood parameters and severity outcomes were found, we established a cut-off value and calculated sensitivity, specificity, positive and negative predictive values (PPV and NPV), Odds Ratio (OR) and Relative Risk (RR) associated with that, to evaluate its possible prognostic role.
The statistical analyses were performed using Microsoft Excel (2409 version) and MedCalc (2024 MedCalc Software Ltd., Ostend, Belgium). Statistical significance was considered p < 0.05.
2.4. Ethics
This study was conducted in accordance with the Helsinki declaration [16]. Informed consent was not required due to the retrospective nature of the study.
3. Results
3.1. Demographic and Clinical Characteristics
Among the 112 infants initially enrolled in the study, we included 95 patients with a median age of 12 weeks (IQR 8–20; F = 42%) (Figure 1). In total, 12% of our patients were born preterm, and 6% had co-morbidities. A viral respiratory infection was reported in all patients, and the pathogens detected by Filmarray system were RSV (82%), Rhinovirus (26%), Metapneumovirus (13%), Adenovirus (4%), and other pathogens (19%). Considering the severity outcomes during hospitalization, 39% of patients needed PICU admission, 65% required ventilatory support or oxygen supplementation, and 59% needed IV rehydration. The BRAS median value at admission was 2 (IQR 2–4), and the median LOS was 8 (IQR 6–10). The demographic and clinical characteristics are reported in Table 1.
Figure 1.
Flowchart of patients included in the study.
Table 1.
Demographic and clinical characteristics of our cohort (n = 95) *.
3.2. Laboratory Parameters and Inflammatory Indexes
We divided our patients into two groups based on five items (PICU admission, use of ventilatory support, IV rehydration, LOS ≥ 6 days or ≤5 days, BRAS ≥ 3 or ≤2) and evaluated if there were any significant statistical differences among the pairs of groups regarding demographic data, CBC values, CRP, or novel inflammatory markers.
Comparing the group of patients who required PICU admission (n = 37) with the group of patients who did not require it (n = 58), there was a statistically significative difference between the age in weeks and the weight at admission (p < 0.001), with inferior values for patients admitted to PICU; no statistical difference was found for gestational age at birth and gender. By analyzing CBC values, higher values of hemoglobin and lower values of red blood cells were significantly connected to PICU admission (p < 0.05). A value of RBCs ≤ 4,050,000/mmc was found to have NPV 71% and RR 1.5. AiW/Hb < 0.85 and RBC/AiW/1000 > 350 were associated with PICU admission, with a similar NPV (83 and 82, respectively) and an RR equal to 2.9 and 2.2, respectively. No difference was found for CRP and novel inflammatory markers. Data are synthesized in Table 2 and Table 3.
Table 2.
Analysis of patients admitted to PICU vs. not admitted *.
Table 3.
Analysis of significant parameters influencing PICU admission *.
Comparing the group of patients who required ventilatory support (n = 62) with the group of patients who did not require it (n = 33), there was a statistically significative difference between the age in weeks and the weight at admission (p < 0.001), with inferior values for patients who required ventilatory support; no statistical difference was found for gestational age at birth and gender. By considering CBC values, higher values of red blood cells were significantly related to no need for ventilatory support (p < 0.05). Considering the population requiring ventilatory support, a value of RBCs ≤ 4,300,000/mmc had a PPV of 72% and a RR of 1.4. Moreover, RBC/AiW/1000 > 350 and AiW/Hb < 1 had a PPV of 83% and 81%, respectively (with a RR of 2.7 and 2.3, respectively). No difference was found for CRP and novel inflammatory markers. Data are synthesized in Table 4 and Table 5.
Table 4.
Analysis of patients who required ventilatory support vs. who not required it *.
Table 5.
Analysis of significant parameters influencing ventilatory support *.
Comparing the group of patients who required IV fluid administration (n = 56) with the group of patients who did not require it (n = 39), there was a statistically significative difference between the age in weeks and the weight at admission (p < 0.001), with inferior values for patients that required intravenous fluids; moreover, higher values of red blood cells were significantly related to no need for IV fluids (p < 0.05). Considering the population requiring IV fluids, a value of RBCs ≤ 4,300,000/mmc had a PPV of 70% and an RR of 1.6. RBC/AiW/1000 > 350 and AiW/Hb < 1 had a PPV of 78% and 79%, respectively (with an RR of 2.5 and 2.6, respectively). No difference was found for CPR and novel inflammatory markers. Data are synthesized in Table 6 and Table 7.
Table 6.
Analysis of patients who required intravenous rehydration vs. who not required it *.
Table 7.
Analysis of significant parameters influencing intravenous rehydration *.
Comparing the group of patients with LOS ≥ 6 days (n = 76) with the group of patients with LOS ≤ 5 days (n = 19), there was any significant statistical difference between the age in weeks, weight at admission, gestational age, and gender between the two groups. By analyzing CBC values, higher values of total white blood cells, lymphocytes, and eosinophils were related to a prolonged LOS (p < 0.05). Specifically, WBC ≥ 9300/mmc had higher sensitivity (72%), a PPV of 90%, and an RR of 2.3; lymphocytes ≥ 4900/mmc had higher specificity (79%), PPV of 91%, and an RR of 2.7; eosinophils ≥ 110/mmc had a PPV of 89% and an RR of 2.0. We found an association between LOS ≥ 6 days and RBC/AiW/1000 > 350, with a PPV of 88% and an RR of 1.8. By considering the inflammatory indexes, lower values of PLR, MLR, and PNR were found in the group of patients with a prolonged LOS (p < 0.05), with an RR of 1.4, 1.4, and 1.2, respectively (considering as cut-off values ≤ 112, ≤0.15, and ≤99, respectively). Data are synthesized in Table 8 and Table 9.
Table 8.
Analysis of patients who required prolonged LOS vs. who not required it *.
Table 9.
Analysis of significant parameters influencing prolonged LOS *.
Comparing the group of patients with a BRAS ≥ 3 (n = 40) with the group of patients with a BRAS ≤ 2 (n = 55), there was a significant statistical difference between the age in weeks and the weight at admission (p < 0.001), with lower values in patients with higher a BRAS; no statistical difference was found for gestational age at birth and gender. By considering CBC values, higher values of hemoglobin were significantly related to a BRAS ≥ 3 (p < 0.05), with an NPV of 61% and an RR of 1.1 for a hemoglobin cut-off value > 11.2 g/dL. For BRAS ≥ 3, AiW/Hb < 0.85 had a PPV of 65%, an NPV of 77%, and an RR of 2.6, while RBC/AiW/1000 > 350 had a PPV of 62%, an NPV of 80%, and an RR of 2.2. No difference was found for CPR and novel inflammatory markers. Data are synthesized in Table 10 and Table 11.
Table 10.
Analysis of patients with BRAS ≥ 3 vs. BRAS ≤ 2 *.
Table 11.
Analysis of significant parameters influencing BRAS ≥ 3 *.
4. Discussion
Our study provides a comprehensive analysis of the early changes in CBC, CRP, and novel inflammatory markers in children with bronchiolitis. We collected data over three epidemic seasons of children hospitalized in the PW or PICU of a third-level hospital to assess the relationship between early laboratory findings and severity outcomes.
Lower age and weight at admission significantly impacted disease severity, especially for four of the five items (PICU admission, ventilatory support, IV fluids administration, and BRAS). In our cohort, these two demographic variables did not present association with LOS. Previous studies have found that younger infants with bronchiolitis were at higher risk of needing oxygen supplementation and PICU admission [17,18]. Moreover, the presence of different severity profiles of bronchiolitis was proposed in the analysis of affected patients, in which younger infants (especially those aged < 6 months) had the most severe clinical presentation and increased incidence of respiratory distress [19,20]. This age-related susceptibility could be associated with different immune responses observed in younger infants: in children < 6 months of age, decreased levels of interferon (IFN)-γ, IFN-γ–inducible protein (IP)-10, and eotaxin were linked to higher rates of hospitalization, while in children > 6 months of age higher levels of IFNs were found and were related to lower rates of hospitalization [21,22]. The other demographic variables analyzed in our study (i.e., gestational age at birth and gender) were not associated with disease severity. In fact, in the PICU admission group, the gestational age was higher than that of the other group, but it was not statistically relevant (p = 0.23). Among our 11 patients with a history of prematurity, 3 of them (27%) needed PICU admission, while 8 of them (73%) did not require PICU admission, without any relevant statistical difference between the two groups (p = 1).
The analysis of CBC revealed that several parameters may predict disease severity in infants hospitalized with bronchiolitis. High values of hemoglobin and/or low values of RBCs were linked to more severe disease, particularly considering PICU admission, ventilatory support, IV fluid administration, and a BRAS ≥ 3. Our result was in contrast with a previous study that described an association between anemia (defined by a hemoglobin level < 10 g/dL) and a higher use of CPAP as well as prolonged respiratory support in infants with viral bronchiolitis [23]. However, this study only analyzed infants younger than 16 weeks and with heterogeneous conditions (e.g., 15% were born premature, 5% had cardiological diseases, 5% had a diagnosis of anemia before the bronchiolitis onset) [23]. Conversely, in our analysis, we reported children aged < 12 months without important co-morbidities, which could affect the interpretation of the hematological results. Iron deficiency anemia was considered protective for viral infections in infants because high iron concentration facilitates viral replication. In fact, iron and hemoglobin reduction during viral infection is considered a physiological reaction of the immune system to contrast viral replication [24,25]. It could be supposed that a high hemoglobin concentration may predict the low ability of the immunological system in some infants to respond early to viral infections, increasing the disease severity. However, further studies are needed to clarify this issue. Viral infections may also compromise the morphology, deformability, and aggregability of RBCs, reducing their count in contrast to bacterial disease [26,27]. The presence of low values of RBCs could be interpreted as an early marker of viral infection.
Due to the statistically significant association that emerged during our research, we calculated two new parameters: AiW/Hb and RBC/AiW/1000. AiW/Hb < 0.85 was significantly associated with PICU admission and BRAS ≥ 3, AiW/Hb < 1 was significantly associated with ventilatory support and IV rehydration, and RBC/AiW/1000 > 350 was significantly associated with all the five severity outcomes (PICU admission, ventilatory support, IV fluids administration, LOS ≥ 6 days, and BRAS ≥ 3). These two new indexes may identify an increased risk of severe disease in relationship to younger age and higher values of hemoglobin and/or RBC count. Further studies are required to assess the validity of these indexes.
In our cohort, we showed that high values of total white blood cells, lymphocytes, and eosinophils are associated with a prolonged LOS (≥6 days). These parameters were not significantly related to the other severity outcomes. The presence of high values of leucocytes and lymphocytes is in line with the diagnosis of viral infection in our patients. Viruses induce the production of IFN-γ, which stimulates the immune response by activating B and T lymphocytes and inhibiting viral replication [28]. However, other studies in infants with severe bronchiolitis have described the association with a specific viral agent and lymphopenia. Matera et al. described a correlation between low lymphocyte count and bronchiolitis severity, which was statistically significant related to a worse clinical severity score, a more frequent need for oxygen supplementation, prolonged hospitalization, and more frequent ICU admission. However, these authors described how the low lymphocyte count was mostly associated with RSV infection, which was prevalent in their sample [8]. RSV seems to induce lymphopenia by increasing lymphocyte apoptosis in infants with bronchiolitis [29]. Moreover, in infants with severe RSV infection, neutrophils create lung inflammatory infiltrates, but whether these cells contribute or not to determine the disease severity is still debated [30,31]. For our study, we did not analyze the impact of each virus on the blood count, also considering the wide viral heterogeneity of our cohort. Eosinophilia was previously described in association with prolonged hospitalization and with increased prevalence of mechanical ventilation [32]. Eosinophils may induce bronchoconstriction due to mucus secretion, cellular desquamation, and airway hyperresponsiveness [33,34]. These phenomena may enhance disease severity and predispose to developing wheezing and asthma in children [35].
In recent years, the literature research has analyzed the role of novel inflammatory indexes derived from blood cell counts as early predictors of severity outcomes in several inflammatory diseases, such as lung infections, cardiovascular diseases, and sepsis [13,14]. To the best of our knowledge, only two studies have already investigated their role in infants with bronchiolitis, and limited markers were considered in these analyses with contrasting results. A single-centre retrospective study on children aged < 2 years hospitalized with moderate-to-severe bronchiolitis (n = 155) did not find any statistically significant association between bronchiolitis severity and NLR or SII values. However, in this study, the disease severity was evaluated by considering only LOS and oxygen supplementation, and patients who required mechanical ventilation were not included in the final analysis [36]. The second one was a single-centre prospective study on children aged < 2 years hospitalized with bronchiolitis (n = 74). This original research showed that neutrophil count, NLR, and SII values increased significantly with bronchiolitis severity, with SII reaching the highest area under the curve (AUC) in predicting patients with high disease severity [37]. Our study did not find any significant association between disease severity and NLR or SII. Each severity outcome did not correlate with the analyzed novel inflammatory indexes, except for prolonged LOS and lower values of PLR, MLR, and PNR. These data must require future assessments to determine their validation and their possible use in the early prediction of bronchiolitis severity in infants.
Our study has some limitations. First, this was a single-centre study, affecting the generalizability of the results. Secondly, this was a retrospective study and potentially confounded by selection bias. However, we carefully checked the inclusion/exclusion criteria in our sample, and our selection method was rigorous. Thirdly, we did not analyze the influence of each pathogen on the blood parameters, due to the excessively different prevalence of each virus in our sample and the high prevalence of co-infection (Table 1). Fourthly, we did not include a control group. Lastly, we did not include the evaluation of the treatment provided during the hospitalization, which may have a positive or negative impact on the severity of the outcomes. Further studies, even multi-centre and prospective, may be helpful to increase the scientific value of our results.
5. Conclusions
Our study aimed to evaluate the association between CBC and novel inflammatory indexes with the severity of bronchiolitis at hospital admission. We did not find any association between novel inflammatory indexes and bronchiolitis severity, except for LOS and lower PLR, MLR, and PNR values. CBC may be used as a prognostic tool for assessing the severity of the disease in infants hospitalized with bronchiolitis. The most important CBC parameters reported associated with disease severity were RBCs, hemoglobin, white blood cells, lymphocytes, and eosinophils. Two new indexes, AiW/Hb and RBC/AiW/1000, were proposed in our study. Particularly, RBC/AiW/1000 > 350 showed a statistically significant association with each severity outcome and could represent a future blood biomarker for predicting the severity of bronchiolitis in hospitalized infants. It would be helpful to conduct further studies to explore and validate the potential of the identified biomarkers for application in clinical practice.
Author Contributions
Conceptualization, S.M. (Sara Manti), E.G. and M.W.; methodology, S.M. (Salvatore Mollica), A.G. and S.M. (Sara Manti); data curation, E.R., F.X. and A.L.P.; writing—original draft preparation, A.G. and S.M. (Salvatore Mollica); writing—review and editing, A.G., S.M. (Salvatore Mollica) and S.M. (Sara Manti); supervision, S.M. (Sara Manti), M.V., M.F.M., A.V. and G.Z. All authors have read and agreed to the published version of the manuscript.
Funding
This research received no external funding.
Institutional Review Board Statement
The study was conducted in accordance with the Declaration of Helsinki and was registered with the following EUDRACT code: 2018-002980-25.
Informed Consent Statement
Not applicable.
Data Availability Statement
Data are available on request from the corresponding author.
Conflicts of Interest
The authors declare no conflicts of interest.
List of Abbreviations
| Abbreviations | Definition |
| AiW/Hb | Age in Weeks-to-Hemoglobin ratio |
| AUC | Area Under the Curve |
| BPD | Bronchopulmonary Dysplasia |
| BRAS | Bronchiolitis Risk of Admission Score |
| CBC | Complete Blood Count |
| CPAP | Continuous Positive Air Pressure |
| CRP | C-Reactive Protein |
| ELR | Eosinophils-to-Lymphocytes ratio |
| HFNC | High Flow Nasal Cannula |
| IQR | Interquartile Range |
| IV | Intravenous |
| LMR | Lymphocytes-to-Monocytes ratio |
| LNR | Lymphocytes-to-Neutrophils ratio |
| LOS | Length of Stay |
| LPR | Lymphocytes-to-Platelets ratio |
| LRTI | Lower Respiratory Tract Infection |
| MLR | Monocyte-to-Lymphocyte ratio |
| NIV | Non-Invasive Ventilation |
| NLR | Neutrophil-to-Lymphocyte ratio |
| NPR | Neutrophils-to-Platelets ratio |
| NPV | Negative Predictive Value |
| OR | Odds Ratio |
| PCR | Polymerase Chain Reaction |
| PICU | Pediatric Intensive Care Unit |
| PLR | Platelets-to-Lymphocytes ratio |
| PNR | Platelets-to-Neutrophils ratio |
| PPV | Positive Predictive Value |
| PW | Pediatric Ward |
| RBC/AiW/1000 | Red blood cell-to-Age in Weeks-to-1000 ratio |
| RR | Relative Risk |
| RSV | Respiratory Syncytial Virus |
| SARS-CoV-2 | Severe Acute Respiratory Syndrome Coronavirus 2 |
| SD | Standard Deviation |
| SII | Systemic Immune-Inflammation Index |
| SIRI | Systemic Inflammation Response Index |
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