Importance of respiratory syncytial virus as a predictor of hospital length of stay in bronchiolitis

Introduction : Bronchiolitis is the leading cause of hospitalization in children. Estimate potentially preventable variables that impact the length of hospital stay are a priority to reduce the costs associated with this disease. This study aims to identify clinical variables associated with length of hospital stay of bronchiolitis in children in a tropical middle-income country Methods: We conducted a retrospective cohort study in 417 infants with bronchiolitis in tertiary centers in Colombia. All medical records of all patients admitted through the emergency department were reviewed. To identify factors independently associated we use negative binomial regression model, to estimate incidence rate ratios (IRR) and adjust for potential confounding variables Results : The median of the length of hospital stay was 3.68 days, with a range of 0.74 days to 29 days, 138 (33.17%) of patients have a hospital stay of 5 or more days. After modeling and controlling for potential confounders age <6 months, comorbidities (CHD or neurological), BPD, chest indrawing, detection of RSV, and C-reactive protein were independent predictors of LOS Conclusions : Our results show that in infants with bronchiolitis, detection of RSV, age <6 months, comorbidities (CHD or neurological), BPD, chest indrawing, and C-reactive protein were independent predictors of LOS. As a potentially modifiable risk factor, efforts to reduce the probability of RSV infection can reduce the high medical cost associates with prolonged LOS in bronchiolitis.


Introduction
Bronchiolitis is the most frequent lower respiratory tract infection in infants 1,2 . One of the variables with more incidence in the financial burden of this disease is the hospital length of stay (LOS) 3 . The hight medical cost associates with prolonged LOS in bronchiolitis imposes an economic burden, especially in tropical middle-income countries 4 . LOS is a direct measure of the quality of health service 5 .
Some models have identified predictors of LOS such as age, underlying conditions (congenital heart disease, chronic lung conditions, immunocompromised states), low birthweight, male gender, clinical characteristics at admission, prematurity, detection of RSV 6 . However, many of these models lack accuracy 7 or were made in patients without significant comorbidities 8,9 . In this context, there is a critical need to explore predictors of LOS, improving their accuracy of current models. This information will allow risk management for healthcare and prioritize care strategies in groups with a high probability of prolonged hospital stay to reduce their impact on hospital costs and morbidity. This study aims to identify clinical variables associated with LOS of bronchiolitis in children in Colombia.

Methods
We conducted a retrospective cohort study that included all infants with bronchiolitis younger than two years of age admitted to tertiary centers in Rionegro, Colombia, from January 2019 to December 2019. The municipality of Rionegro had a total population of 101,046 inhabitants, with two tertiary referral hospitals 10 . Inclusion criteria were defined as children younger than two years of age admitted to the pediatric ward diagnosed with bronchiolitis, according to the national clinical guideline of bronchiolitis (first wheezing episode younger than 24 months of age) 11 . Patients without lower respiratory compromise, with positive bacterial cultures on admission, confirmed whooping cough (culture or PCR) were excluded. The study protocol was reviewed and approved by the Institutional Review Board of the University of Antioquia (No 18/2015). Informed consent was obtained from all parents or caregivers of the patients included in the study, following the clinical research standards in Colombia, and prior approval by the ethics committee.

Procedures
We collected the following variables: age, sex, weight, height, signs, and symptoms on admission (including fever, chest indrawing, chest auscultation, %SpO 2 ), vaccination scheduled chart for age, exposure to cigarette smoking, history of prematurity and bronchopulmonary dysplasia confirmed by a neonatologist at the time of discharge from the NICU, comorbidities (congenital heart disease, neurological disease), diagnostic tools as chest X rays, hemograms, etc. Additionally, we collected variables related to outcomes of care or disease-severity parameters such as length of hospital stay. In our hospitals, bronchodilators and systemic steroids are discourage according to national clinical guidelines of bronchiolitis 11 . Nasopharyngeal aspirate (NPA) was taken immediately upon admission to the emergency department within 48 hrs of admission using standardize technique. RSV was confirmed using direct immunofluorescence (Light Diagnostics TM Respiratory Panel 1 DFA, Merck-Millipore Laboratory). NPA data for other viruses were not available in our institution consistently.

Statistical analysis
Continuous variables were presented as mean ± standard deviation (SD) or median (interquartile range [IQR]), whichever appropriate. Categorical variables are shown as numbers (percentage). Differences between continuous variables were analyzed using the unpaired t-test or Wilcoxon's signed-rank test, whichever was appropriate. Associations between categorical variables and the outcome variable were analyzed using the chi-square test or Fisher's exact test, as needed. To identify factors independently associated with length of hospital stay, we used a Poisson regression model, or negative binomial regression model in case of the presence of overdispersed count data, to estimate incidence rate ratios (IRR) and adjust for potential confounding variables. We only include initially variables associated with LOS with values of p <0.2 or that change the effect estimate by more than 10% after their inclusion. The variable selection and modeling processes were made following the recommendations of Greenland 12 . The goodness of fit of the model was evaluated using Hosmer-Lemeshow test and area under curve in Poisson regression or Akaike information criterion (AIC), Bayesian information criterion (BIC) in negative binomial regression. All statistical tests were two-tailed, and the significance level used was p < 0.05. The data were analyzed with Stata v15.0 (Stata Corporation, College Station, TX).

Study population
During the study period, 417 cases of bronchiolitis were included. A total of 66% of the patient was less than 6 month, most of them males (60%), with supportive O 2 (83%). RSV was detected in 200 patients (48%). Of these, 81 patients had a history of premature birth and 17 of them with BPD. A total of 20 patients had some cardiac or neurological disease and 10 of them with a history of use of palivizumab. The term 'CRP level of more than 40 mg/lit' was clarified in the document and in the tables. There are a typo error, CRP> 40 mg/lit was considered increased. All the corrections suggested by the reviewer were modified.
Any further responses from the reviewers can be found at the end of the article

Discussion
The main purpose of this study was to determine the independent clinical variables associated with LOS of bronchiolitis in children in tropical middle-income countries. Our study shows that RSV, age <6 months, comorbidities (CHD or neurological), BPD, chest indrawing, and C-reactive protein were independent predictors of LOS Our results emphasize the importance of knowing the presence of RSV. While some predictors of LOS, such as age, comorbidities, and potentially initial signs of respiratory distress, can not be modified, others as detection of RSV are potentially modifiable by interventions such as futures vaccines or palivizumab in a high-risk population 14,15 . Previous studies in populations with seasonality had revealed the importance of RSV as a predictor of hospital stay. DeVicenzo et al., in a sample of 141 infants <24 months old without previous chronic cardiac or lung disease or prematurity, in Tennessee described a longer LOS with a higher amount of RSV in secretions. A 1-log higher RSV load was associated with a 0.8-day longer hospitalization, reflects the higher RSV load that occur earlier  Preventive strategies such as the use of palivizumab in a high-risk population or the use of future vaccines that confer immunity in children under 6 months against RSV; will constitute possibly effective interventions in reducing the economic burden of this disease.
Several predictive models had reports consistently the chest indrawing as predictive of prolonged LOS that is which is biologically plausible and expected due that this sign also is a universal marker of severity of the disease, as well as the presence of underlying conditions (congenital heart disease, chronic lung conditions, immunocompromised states) 3,6-8,21-23 or C-reactive protein (CRP) as a biomarker of severity and bacterial co-infection in patients hospitalized for bronchiolitis [24][25][26] Our study has limitations. First, since this study was based on medical records review, we cannot include other variables such as environmental pollution and genetic factors, and residual confounding cannot be excluded. Second, respiratory syncytial virus was confirmed using direct immunofluorescence, which may underestimate the real burden of viral infection. However, despite this possible underestimation, RSV infection was positively associated, and it is possible that the magnitude of the IRR is even greater. The detection of other respiratory viruses was not homogeneous in all patients, so to avoid information bias we decided not to include them in the analysis. We cannot rule out that other respiratory viruses have equal or greater association with our dependent variable in the study. Third, the study was conducted in a tertiary referral hospital, and therefore the patients included represent the high spectrum of severity, limiting the generalization of results to other contexts. However, the similarity of our population in terms of clinical characteristics, risk factors, and seasonality of bronchiolitis in our country with previous reports suggest strength and consistency in our results 3,4

Conclusion
Our results show that in infants with bronchiolitis, RSV, age <6 months, comorbidities (CHD or neurological), BPD, chest indrawing, and C-reactive protein were independent predictors of LOS in a tropical middle-income country. As a potentially modifiable risk factor, efforts to reduce the probability of RSV infection can reduce the hight medical cost associates with prolonged LOS in bronchiolitis.

Ethics approval
The study protocol was reviewed and approved by the Institutional Review Board of Clinica Somer (No 281015) and the University of Antioquia (No 18/2015).

Consent for publication
All authors consent this paper for publication Chronic heart disease (CHD)

Authors' contributions
All the authors contributed in the same way from conception of the work to the publication of results. All Authors read and approved the manuscript.
A CRP of 4 mg/L is completely normal. The authors can still claim that a higher CRP predicted a longer LOS but they need to note that this was not necessarily an elevated CRP.

Competing Interests:
No competing interests were disclosed.

Reviewer Expertise: Pediatric infectious diseases
I confirm that I have read this submission and believe that I have an appropriate level of expertise to confirm that it is of an acceptable scientific standard, however I have significant reservations, as outlined above.
You can see that all grammar suggestions were corrected according to your advice. When we place in our corrections the word "corrected" it means that the text was changed as required by your advise. We do not consider the need to place the multiple grammar corrections in other text , when these can be see in the text with the track changes in word.
The cut-off of the CPR estimated in our mathematical model associated with our outcome was 4 mg/L, and this just is the value of our paper. This variable was managed continuously and no preset cut-off point ( eg dichotomic , etc). As was stated in the discussion these values should be alert to the doctor in conjunction with other variables that may be is a patient with a longer hospital stay or seen another way patient with a lower value together with other variables can help to the pediatrician to predict the lack for examen of intensive use of resources by the patient not having a high risk of prolonged hospital staly The authors analyzed data from all children less than 2 years of age admitted in 2019 to two hospitals in Columbia to establish factors identifiable at admission that predict a longer length of stay (LOS). The factors that they identified include RSV, age <6 months, comorbidities (CHD or neurological), BPD, chest indrawing, and a higher C-reactive protein.
The main question is whether their findings have practical implications. Some of the risks that they identified are not well established (in particular the higher CRP). They make the point that identification of these risk factors for prolonged LOS may inform their palivizumab program and eventually an RSV immunization program. The risk factors that would be helpful for such programs are age < 6 months and comorbidities, both of which are well recognized as risk factors for RSV hospitalization but less so for LOS. Nonetheless, both are currently widely used in palivizumab programs worldwide.
Minor points: Abstract -What does "admitted to the emergency department" mean. Do the authors perhaps mean "admitted through the emergency department"? 1.
"Among inpatients with bronchiolitis, approximately a quarter undergo a prolonged length of stay". There is no standard definition of prolonged LOS, so it is difficult to know what the authors mean by this.

2.
The high medical cost associates with prolonged LOS in bronchiolitis imposes an economic burden, especially in tropical middle-income countries." Why? Surely it is even a greater burden in low income countries. I do not understand why it is a special problem in "tropical countries". 3.
"Some models have identified predictors of LOS such as age, underlying conditions (congenital heart disease, chronic lung conditions, immunocompromised states), low weight, male gender, clinical characteristics at admission, prematurity, RSV isolation" -I think that the authors mean "low birthweight". Throughout the paper, I do not know what they mean by "RSV isolation" as almost no centers do viral culture anymore. They need to change it to "detection of RSV".

4.
"Otherwise, in tropical areas in addition to genetic differences, the respiratory syncytial virus (RSV), generates differences in the burden of morbidity and mortality given the nonseasonality of these areas" -I do not know what this sentence means.

5.
By "in tertiary centers", do the authors mean "admitted to tertiary centers"? Patients can also be in the emergency department of a hospital.

6.
Do the national guidelines encourage or discourage use of bronchodilators and systemic steroids? I think that most discourage both these days.

7.
Please reword "NPA data for other viruses were no available in our institution consistently." I think that the authors mean "not" rather than "no". 8.
In Table 1, are these parameters at admission? For example, "%SpO2, median(ds)", is this the value on admission without oxygen? What does "ds" mean? 9.
Throughout the paper, I would round all percentages to the nearest whole number or provide only one decimal point. It makes it easier for the reader and the sample size does not justify two decimal points.

10.
Table 3 and the abstract need to specify which CRP was considered to be significant. 11.
The DeVicenzo study described a longer LOS with a higher amount of RSV in secretions. This is quite different from showing that RSV itself increases the LOS.

12.
"Mansbasch et al., in a prospective cohort of 2207 infants of 16 US hospital without excluding patients previous chronic cardiac or lung disease or prematurity, also found that patients with RSV have a higher proportion of patient with prolonged LOS (>3 days) than patients with only HSV infection, but less than RSV+HRV co-infection (48% vs 28% vs 54%, p<0.001" -I do not understand why HSV would be compared to RSV.

Are sufficient details of methods and analysis provided to allow replication by others? Yes
If applicable, is the statistical analysis and its interpretation appropriate? I cannot comment. A qualified statistician is required.
The authors analyzed data from all children less than 2 years of age admitted in 2019 to two hospitals in Columbia to establish factors identifiable at admission that predict a longer length of stay (LOS). The factors that they identified include RSV, age <6 months, comorbidities (CHD or neurological), BPD, chest indrawing, and a higher C-reactive protein. The main question is whether their findings have practical implications. Some of the risks that they identified are not well established (in particular the higher CRP). They make the point that identification of these risk factors for prolonged LOS may inform their palivizumab program and eventually an RSV immunization program. The risk factors that would be helpful for such programs are age < 6 months and comorbidities, both of which are well recognized as risk factors for RSV hospitalization but less so for LOS. Nonetheless, both are currently widely used in palivizumab programs worldwide.  ○ "Some models have identified predictors of LOS such as age, underlying conditions (congenital heart disease, chronic lung conditions, immunocompromised states), low weight, male gender, clinical characteristics at admission, prematurity, RSV isolation" -I think that the authors mean "low birthweight". Throughout the paper, I do not know what they mean by "RSV isolation" as almost no centers do viral culture anymore. They need to change it to "detection of RSV".