SOFIA^®RSV: prospective laboratory evaluation and implementation of a rapid diagnostic test in a pediatric emergency ward

Respiratory syncytial virus (RSV) is a common ubiquitous pathogen responsible for mild upper respiratory tract infection in most children and healthy adults. In infants, RSV is mainly associated with acute lower respiratory infections (ALRI, which includes bronchiolitis and pneumonia) during the winter months in temperate regions of the northern hemisphere. In elderly and immunocompromised patients, RSV has been known to cause severe respiratory failure, extended hospitalizations, higher mortality, with symptoms similar to those associated with seasonal influenza [1, 2]. If 98% of all infants have been infected by RSV at least once by the age of two [3], primary infection does not protect against reinfection but likely lessens the severity of later infections. In infants and young children, RSV is more likely to move into the lower respiratory tract, and infect ciliated followed by non-ciliated cells [4].

RSV is a major public health problem worldwide, since its circulation generates a significant excess activity in pediatric departments, including pediatric emergency departments (ED). In 2005, 3.4 million young children were hospitalized worldwide for severe ALRI associated with RSV [5]. In a 5-year prospective data analysis, the RSV hospitalization rate was 5.2 per 1000 infants, among whom infants under 1 month of age were at highest risk of hospitalization [6]. A previous epidemiologic study conducted in 2009 at Caen University Hospital in infants with respiratory symptoms during winter season had already highlighted significant associations between RSV infection, higher risk of hospitalization and higher clinical severity scores [7]. In a meta-analysis including 82,000 patients who had been infected by RSV before 3 years old (y/o), Regnier et al. [8] found that 21.9% of children hospitalized for an RSV infection during their first year of life develop asthma before 5 y/o. However, diagnosing RSV infection based only on respiratory symptoms is not possible since the symptoms most often found in RSV infections are nonspecific, requiring virological confirmation [3].

Rapid diagnostic tests (RDT) for RSV are frequently used in emergency departments to guide and establish the most accurate and quickest RSV diagnosis possible, despite their insufficient sensitivity [9, 10]. A second generation, automated RDT, SOFIA Fluorescent Immunoassay for RSV (SOFIA^®RSV, QUIDEL, San Diego, CA), was introduced in France in 2013. This assay allowed for quick RSV detection and enabled better decision-making in bed-management, thus enhancing infection control. These newer standardized tests are an improvement over the performances of classical manual RDT [11]. Indeed, they include a cell lysis phase, a time-controlled incubation phase inside the machine as well as an objective automated lecture. Their use at point-of-care testing (POCT) should enhance patient care, which begins in the pediatric ED.

This report combines two studies, study 1 and study 2, each having its own objective. The objective of study 1 was to assess the analytical performances of SOFIA^®RSV tests under “real-life laboratory” conditions, including samples from patients of all ages. This prospective study was conducted in 2013. The objective of study 2 was to assess the performance of the SOFIA^®RSV test implemented at POCT in a pediatric ED during four winter months (from November 2013 to March 2014).

In study 1, which was conducted in the laboratory, we showed that SOFIA^®RSV is a sensitive and specific method for infants with high negative predictive value (96%), positive and negative likelihood ratios of 95 and 0.05, respectively. Its performance proves equivalent to PCR tests in infants. In the other age groups, the negative predictive values were similar to those calculated with the other direct detection techniques (DFA RSV and MRC5 RSV). This can be explained by the fact that among children younger than 2 y/o, viral replication during first infection is higher since they have no prior immunity [3]. Finally, study 1 is a prospective evaluation in a “real-life laboratory”, which provided the advantage of comparing the performance of several direct detection tests to PCR and allowed the calculation of positive, and especially negative predictive values. These data are often missing in recently published studies, where only values of sensitivities and specificities are shown [13‐16].

Study 2 is an observation of the implementation, for the first time in France, of SOFIA^®RSV at POCT in a pediatric ED. This RSV test is easy-to-use, including a printed report that describes the quality requirements for biological diagnosis. It is also an efficient method for making a quick and accurate diagnosis, which can save significant amount of time during a RSV epidemic period. Compared to PCR, this SOFIA^®RSV test is faster, feasible at POCT, especially in smaller hospitals where no laboratory would be available and close to the pediatric ED. Diagnosing RSV at POCT is possible and highly informative. In fact, we showed significant differences in bed-management in infants: compared to SOFIA^®RSV negative children, SOFIA^®RSV positive children were more likely to be admitted to the PICU (P < 0.001) or to the general pediatrics ward (P < 0.001) (Table 3). There were also significant differences between infants with delayed RSV diagnosis (group 2) and with RSV diagnosis at POCT (group 3), concerning bronchiolitis diagnosis (P < 0.001) as well as aerosol therapy (P < 0.05) (Table 4). However, a treatment protocol has been implemented in the ED during the study 2 period, which recommended doctors to prescribe epinephrine aerosols to infants above 6 weeks of age who were diagnosed with bronchiolitis. Consequently, epinephrine aerosols cannot be taken into account in study 2, since the final diagnosis of bronchiolitis was particularly significant in group 3 (RSV infection diagnosed at POCT). In study 2, clinicians properly targeted the population at greatest risk of developing RSV disease (under 6 months of age, particularly under 3 months of age), in accordance with the national 2012 French guidelines which identify the target population during epidemics as infants under 4 months old [17]. There was no significant reduction in prescriptions for antibiotics or corticosteroids, whether the emergency physicians were aware of the presence of RSV or not, except for aerosols. These results may provide a rationale for clinical reflection, especially since the national guidelines [17] have highlighted the overuse of inappropriate (bronchodilators, corticosteroids) or deleterious (antitussive) medications from results of national surveys. These recommendations are also in the NICE 2015 guidelines [18]. That being said, this non-compliance with the guidelines, especially for antibiotic prescriptions, has been previously reported [19]. In Table 4, among the significant differences between groups 2 and 3, there were age under 6 months and the presence of signs of respiratory distress. These results suggest a viral load differential associated with whether the RSV diagnosis is delayed or made at POCT. Infants from group 3 had probably presented a higher viral load in the upper respiratory tract, consequently making the SOFIA^®RSV more likely to be positive. Moreover, these patients presented initially with more severe symptoms at first examination. Previous studies showed that high viral load is a prognostic factor of severity [20, 21], since most children have apnea and lower weight at presentation. A young age has already proven to be a severity factor in RSV infections [22, 23]. We assume that RNA RSV load may have a positive correlation with longer hospitalizations and a greater use of intensive care.

The sensitivity of the SOFIA^®RSV test in infants performed in study 1 conditions (collected in 1.5 mL of UTM) in the laboratory is 95% compared to 74.8% in study 2, in which tests were performed at POCT (collected in 3 mL of UTM). This higher volume of collection in study 2 could explain the decrease of sensitivity from the laboratory to the pediatric ED. Furthermore, we assume that what is not done in the laboratory can be associated with lower values of sensitivity. At POCT, the conditions are not those recommended by the manufacturer and could affect sensitivity. However, we chose to do differently for research purposes. Five studies from the United States [14, 15], Europe [16, 24] and Asia [25] have evaluated the SOFIA^®RSV test performance between 2014 and 2015, in hospitals and clinics. Among them, 3 studies focused only on a pediatric population (under 18 y/o); the 2 other studies examined an adult population. No study has been conducted solely on infants, yet they form the most critical target population for RSV infection. Finally, only one study was conducted at POCT on a pediatric population, described by the mean and the median ages [24]. In that study, the SOFIA^®RSV tests were performed on nasopharyngeal swabs placed in 1 mL of UTM, and showed a sensitivity of 81.8% compared to PCR in infants, whose proportion in the study population was not defined. Two other studies from Kanwar et al. [15] and Bruning et al. [16] showed sensitivity values equivalent to our results, for respiratory samples placed in 1 mL in the Kanwar study but not specified in the Bruning study. However, our results cannot be compared with these studies, since one was conducted in a laboratory and the other did not show the age distribution among the pediatric patients. The study of 348 respiratory samples reported by Jang et al. [25] was conducted on patients aged from birth to 98 y/o, with a mean age of 28 y/o, in South Korea. These data did not separate patients into age groups and therefore cannot be compared to our study, which is focused on infants. The study reported by Leonardi et al. [14] compared the performances of 4 rapid diagnostic tests with 230 samples placed in 3 mL of UTM, but the sampled population is also insufficiently described for comparison with our study. In 3 of the 5 studies, the samples were placed in 3 mL of UTM, similar to the modified protocol we used in study 2. Finally, our virological results are not outliers in terms of sensitivity: they are robust for infants, when nasopharyngeal swabs are placed in 1.5 mL of UTM. Despite the difficulties in comparing above mentioned studies, our results of sensitivity of 74.8% at POCT are similar to Bruning study (sensitivity of 75%, age non-specified). The robustness of the test guarantied accurate results even under poor sampling conditions.

The main limitation of study 2 was the retrospective clinical data collection, performed by different students, even though this collection was standardized with clear instructions. In contrast, study 1 was a prospective evaluation of SOFIA^®RSV performance.

Our work emphasizes the satisfactory performance level of SOFIA^®RSV tests targeted in infants at POCT. Further reflection on the use of SOFIA^®RSV tests in the pediatric ED would optimize patient management, when clinicians are globally satisfied. SOFIA^®RSV tests will be very useful when new specific treatments become available, for example antiviral or immunomodulators, to reduce viral load and clinical severity scores. Further studies are needed to assess the economic impact of SOFIA^®RSV within the national health system since RSV infections are associated with considerable costs [26].