key: cord-0773925-du8wmh6n authors: Clifford, Vanessa; Curtis, Nigel title: Saliva testing for SARS-CoV-2 in children date: 2021-06-06 journal: Clin Microbiol Infect DOI: 10.1016/j.cmi.2021.05.046 sha: 72731a4b453d68e1a6ba0a4d9fca95c33c5e6474 doc_id: 773925 cord_uid: du8wmh6n If found to have adequate sensitivity and specificity, saliva would be an ideal sample type for identifying SARS-CoV-2 and other respiratory viruses in children, as its collection is less invasive than other standard upper respiratory tract sample types such as nasopharyngeal swab (NPS), deep nasal/oropharyngeal swab or nasopharyngeal aspirate (NPA). At present, the majority of commercial SARS-CoV-2 assays do not list saliva as an acceptable sample type for nucleic acid testing (NAT) and therefore validation studies assessing the use of this sample type are welcome. Children are less likely than adults to suffer symptomatic or severe disease from SARS-CoV-2 but they have nevertheless been significantly impacted by the COVID-19 pandemic (1). In particular, they have suffered from the physical, emotional and mental health consequences of the various public health restrictions needed to manage the pandemic, including lengthy lockdowns and school closures. In high transmission settings, it has been suggested that frequent surveillance testing within school settings may allow children and their teachers to safely return to their classrooms. If found to have adequate sensitivity and specificity, saliva would be an ideal sample type for identifying SARS-CoV-2 and other respiratory viruses in children, as its collection is less invasive than other standard upper respiratory tract sample types such as nasopharyngeal swab, deep nasal/oropharyngeal swab or nasopharyngeal aspirate. Collection of nasopharyngeal swabs or aspirates involves both discomfort for the child and potential risk to the health care collector, who may be exposed to SARS-CoV-2 through aerosol generating procedures and behaviours. The use of saliva could also reduce time and cost for sample collection (2) . Prior to the COVID-19 pandemic, studies in adults reported that saliva might be an adequately sensitive sample type for detection of respiratory viruses (e.g. influenza and RSV), either on its own (2, 3) or as an adjunctive sample type to complement nasopharyngeal swab testing (4). However there have been surprisingly few studies in children. At present, the majority of commercial SARS-CoV-2 assays do not list saliva as an acceptable sample type for nucleic acid testing (NAT) and therefore validation studies assessing the use of this sample type are welcome. In this edition of Clinical Microbiology and Infection, Al Suwaidi et al report on a prospective evaluation of saliva as an alternative upper respiratory tract sample type for the diagnosis of SARS-CoV-2 infection in children (5) . In a study of 476 children attending a screening clinic, 485 paired samples were collected, comprising saliva collected in a dribble pot and a nasopharyngeal swab. These were tested for SARS-CoV-2 by RT-PCR using a commercially available extraction and diagnostic testing platform. The median age of children in the study was 10 years; the youngest was three years old. A total of 87 samples were positive from either saliva or NPS, with an overall sensitivity of saliva compared to nasopharyngeal swab of 88%. Paired samples were concordantly positive in 71 cases (82%) and discordant in 16 samples: in six only saliva was positive and in 10 only nasopharyngeal swab was positive. Although there have been several recent large studies in adults comparing saliva with other upper respiratory tract sample types for the diagnosis of SARS-CoV-2 (6), this is the largest published study evaluating this in children. Studies in adults have found variable concordance between saliva and other upper respiratory tract sample types, with saliva usually reported as slightly less sensitive than other samples types (7) . The relatively few studies done specifically in children (8) (9) (10) , or that included children within a larger cohort (10, 11) are summarised in table 1. Although small, these studies suggest that the sensitivity of saliva as a paediatric diagnostic sample type is acceptable compared to nasopharyngeal swabs, but not perfect. Interestingly, whilst missing a few cases detected in nasopharyngeal swab samples, SARS-CoV-2 may be additionally detected in some saliva samples where nasopharyngeal swab is negative (10) (11) (12) . Interpreting these positive NAT results from saliva samples remains difficult in the absence of a defined gold standard comparator test. Concordance between the two samples types is likely impacted by a variety of pre-analytic factors such as patient age, timing of onset of symptoms in relation to sample collection, and severity of illness. In a small study in children hospitalised with COVID-19 in Singapore, the majority of saliva and nasopharyngeal samples were concordant in the first few days after onset of symptoms, but the sensitivity of saliva testing declined rapidly after that (13) . Test sensitivity is also impacted by the method used to collect saliva: published studies have used a variety of methods including dribble pots, suck swabs and specialised saliva collection kits (11) . Location of sampling (buccal swabs (14) versus posterior oropharynx saliva (15) ) may also affect test sensitivity. Other differences that may impact test sensitivity include the volume of sample collected and use of transport medium, pre-testing storage conditions and dilution of the sample for testing (which may be required due to viscosity of some saliva samples). In the study by Al Suwaidi et al, the cycle threshold (Ct) values were higher for saliva samples (2-3 cycles) than for paired nasopharyngeal swabs, consistent with other studies (9), suggesting a reduced analytic sensitivity of the test, possibly due to the presence of inhibitory factors in saliva. Choice of extraction and testing platforms (and variations in test interpretation based on one or more gene targets positive) may also explain heterogeneity between study findings. Overall, the findings of the study by Al Suwaidi et al and other smaller paediatric studies suggest that neither nasopharyngeal swab nor saliva are perfect sample types for the diagnosis of SARS-CoV-2. In symptomatic children in the clinical setting, both sample types would ideally be collected to maximise virus detection, in addition to lower respiratory tract samples if possible. Saliva offers a stand-alone option for testing in situations where a child is significantly distressed by the collection of a standard nasopharyngeal swab, particularly early in the course of infection (16) . These include children obliged to undergo frequent testing due to exacerbations of a chronic respiratory condition, or those requiring frequent testing prior to repeat surgical procedures (as occurs commonly for oncology patients). It should be noted that saliva has already been adopted as an acceptable sample type in several countries (16) . There is insufficient evidence to determine the role of saliva testing in infants and pre-school age children, as very few children of this age have been included in published studies to date, although the expected high SARS-CoV-2 viral load in very young children suggests that saliva may prove to be an adequate sample type (5) . The primary use of saliva as a diagnostic sample type may be as a tool for frequent surveillance testing in high-risk asymptomatic populations. Saliva, even if a slightly less sensitive sample type than nasopharyngeal swabs, may still be an ideal option for settings where children require frequent testing, as collection of multiple sequential test samples will compensate for minor reductions in test sensitivity (17) . Modelling suggests that for successful surveillance programs testing frequency is more important than the sensitivity of the test (18) . Future studies should evaluate saliva for the detection of other common respiratory viruses (such as influenza and parainfluenza viruses and respiratory syncytial virus) in children. This might allow inclusion of saliva as an acceptable sample type in commercial multiplex respiratory virus NAT assays that include SARS-CoV-2, and has the potential to both simplify and increase public health surveillance capacity in the future. VC drafted the commentary. NC provided oversight and revised the manuscript for critically important intellectual content. Why is COVID-19 less severe in children? 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No external funding was received for this commentary.