key: cord-0922019-2i8mbsxp
authors: Bouzid, Donia; Vila, Jordi; Hansen, Glen; Manissero, Davide; Pareja, Josep; Rao, Sonia N; Visseaux, Benoit
title: Systematic review on the association between respiratory virus real-time PCR cycle threshold values and clinical presentation or outcomes
date: 2021-09-23
journal: J Antimicrob Chemother
DOI: 10.1093/jac/dkab246
sha: 0987ca49c30d9892075ed7ba5f9c41ebcc57f26c
doc_id: 922019
cord_uid: 2i8mbsxp

OBJECTIVES: It is unclear whether real-time (rt)-PCR cycle threshold (Ct) values can be utilized to guide clinical and infection-control decisions. This systematic review assesses the association between respiratory pathogen rt-PCR Ct values and clinical presentation or outcomes. METHODS: We searched MEDLINE, EMBASE and Cochrane library databases on 14–17 January 2020 for studies reporting the presence or absence of an association between Ct values and clinical presentation or outcomes, excluding animal studies, reviews, meta-analyses, and non-English language studies. RESULTS: Among 33 studies identified (reporting on between 9 and 4918 participants by pathogen), influenza (n = 11 studies; 4918 participants), human rhinovirus (HRV, n = 11; 2012) and respiratory syncytial virus (RSV, n = 8; 3290) were the most-studied pathogens. Low influenza Ct values were associated with mortality in 1/3 studies, with increased disease severity/duration or ICU admission in 3/9, and with increased hospitalization or length of hospital stay (LOS) in 1/6. Low HRV Ct values were associated with increased disease severity/duration or ICU admission in 3/10 studies, and with increased hospitalization or LOS in 1/3. Low RSV Ct values were associated with increased disease severity/duration or ICU admission in 3/6 studies, and with increased hospitalization or LOS in 4/4. Contradictory associations were also identified for other respiratory pathogens. CONCLUSIONS: Respiratory infection Ct values may inform clinical and infection-control decisions. However, the study heterogeneity observed in this review highlights the need for standardized workflows to utilize Ct values as a proxy of genomic load and confirm their value for respiratory infection management.

Common viral causes of respiratory tract infections in humans are influenza virus, respiratory syncytial virus (RSV), parainfluenza virus (PiV), metapneumovirus (HMPV), adenovirus (HAdV), rhinovirus (HRV), bocavirus (HBoV), and coronavirus (HCoV). [1] [2] [3] Most of these viruses demonstrate seasonal patterns of circulation, particularly in temperate regions. 1, 4 During the development of this systematic review, severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) became a substantial pandemic; however, the long-term impact and whether this virus becomes a seasonal infectious disease is yet to be understood. 5 Real-time PCR is increasingly used to detect and identify respiratory pathogens. The PCR cycle threshold (Ct) value represents the number of amplification cycles required for amplification of the amplicon to exceed a basal fluorescence threshold level. It is inversely related to the amount of copies of the target region in the sample, meaning that a low Ct value corresponds with a high pathogen load. However, the use of Ct values as a proxy of genomic load is influenced by the assay (e.g. sample volume, chemical conditions and single copy or multi-copy region design) and factors within the sample (e.g. quality of sample, and inhibitors) that impact amplification efficiency. The absence of a standard curve in most assays means correction of most of these limitations is not possible. However, Ct values still provide semi-quantitative information. Results of diagnostic tests are usually reported as a positive or negative result using a specified cut-off, based on Ct value or integrated by an automatic algorithm interpreting different parameters of the potential amplification. Caution should be noted when interpreting Ct values, as they are an integer scale that represents a log result measurement of a log biological process.

Ct values are not normally reported, and it is unclear whether they can be utilized to guide clinical and infection-control decisions. Several studies have attempted to elucidate the clinical utility of Ct values, including contradictory reports that Ct values are associated with disease severity, ICU admission and length of hospital stay (LOS). [6] [7] [8] Ct values may also have utility in clarifying diagnostic uncertainty when several respiratory pathogens are identified, particularly when using multiplex PCR respiratory panels. 9, 10 Notably, the unprecedented challenges from the COVID-19 pandemic have raised the level of interest in the clinical and diagnostic utility of Ct values, particularly whether rapid understanding of Ct values could be leveraged as an early measure of disease severity or utilized to guide patient management or public health decisions. [11] [12] [13] [14] A previous systematic review on the utility of Ct values in the management of patients with SARS-CoV-2 infection showed that lower Ct values are potentially associated with worse outcomes in patients with COVID-19, and correlate with increased probability of progression to severe disease, increased disease severity, increased mortality and presence of biochemical and haematological markers. 14 Furthermore, observed changes in population distributions of SARS-CoV-2 Ct values can potentially improve estimates of an epidemic's trajectory, enabling real-time outbreak management and response. 15 This systematic review assesses the global medical literature for associations between Ct values of respiratory pathogens and clinical presentation or outcomes and therefore, whether they could provide valuable information to clinicians for more tailored decisionmaking. This systematic review does not include SARS-CoV-2 because the searches were carried out before COVID-19 and the literature on the relationship between SARS-CoV-2 Ct values and clinically relevant outcomes is still maturing. 14 

This review was undertaken according to the principles outlined in the Cochrane handbook and guidance published by the Centre for Reviews and Dissemination. The original protocol (PROSPERO CRD42020167239) included broad search terms, unrestricted by pathogen or disease type. For pragmatic purposes, this report focuses on respiratory pathogens.

Between 14-17 January 2020, literature searches of MEDLINE, EMBASE and the Cochrane Library using search tools at ncbi.nlm.nih.gov/pubmed, embase.com and cochranelibrary.com were undertaken to identify studies reporting on the presence or absence of an association between real-time PCR Ct values and clinical presentation or outcomes (see Table S1 , available as Supplementary data at JAC Online, for the PubMed search strategy). The search terms were specific for articles referring to quantitative real-time PCR Ct values. All randomized and non-randomized studies were eligible for inclusion; animal studies, systematic reviews, non-systematic reviews, and meta-analyses were excluded. When assessing the full text of articles, those reporting genomic load by measures other than Ct values were excluded unless a clear association between genomic load and Ct value was described. For reasons of feasibility, searches were limited to English language studies. We also carried out manual citation searches for additional articles.

Two reviewers independently screened titles and abstracts for inclusion, based on the eligibility criteria, and then independently assessed full texts of potentially relevant publications; a third reviewer resolved conflicts. Key data from all included studies were captured using a data extraction form. All extracted data were verified by an independent reviewer. Outcomes were reported in a narrative format and meta-analyses were deemed inappropriate due to a wide variation in reported outcomes. Outcomes were broadly divided into the following categories: mortality, severity of symptom, duration of symptoms, ICU admission, hospitalization, and LOS. We also captured information related to single versus co-infections and transmissibility.

The quality and risk of bias of each study was assessed using a tool relevant for each study design (Newcastle Ottawa Scale for cross-sectional, cohort and case-control studies and the Cochrane Risk of Bias tool for randomized controlled trials).

Literature searches identified 991 unique records. After screening and restricting to respiratory pathogens, 33 eligible studies were included ( Figure 1 ). Detailed characteristics, methodology and variables are provided in Table S2 . Influenza, HRV and RSV were the most studied pathogens ( Figure 1 ). The total number of participants included by pathogen ranged from 9 to 4918; and 59 to 4918 when restricted to viral pathogens. The majority (18/33, 55%) of studies were clearly identifiable as being in a hospital setting, and a large percentage of studies (15/33, 45%) were solely in children. There was wide variation across the studies with regards to sampling, sample handling and analysis. Gaps in reported methodology were identified in 12/31 (39%) studies; excluding two studies that were congress abstracts only (Table S2) . We inferred this to mean 19/31 (61%) studies provided clearly defined methodology.

Using Newcastle-Ottowa scales, all cross-sectional studies and nearly all cohort studies were classed as being of low quality (Tables S3 and S4 , respectively). This was generally due to a lack of comparability between groups, insufficient or unjustified sample sizes, the use of non-representative samples (often hospitalized patients or age-specific populations) and a lack of detail regarding patient follow-up or non-response; ascertainment of exposure and outcome was usually appropriate, although blinding was not often utilized. One cohort study (Clark et al. 16 ) was considered to be of 'fair' quality and one (Kim et al. 17 ) was Bouzid et al.

iii34 considered to be of 'good' quality. Both case-control studies were considered of 'good' quality (Table S5) , 6, 18 as were the two randomized, controlled trials (Table S6) . 19, 20 Influenza Eleven studies investigated associations between Ct value of influenza infection and clinical presentation or outcomes, Table 1 . The total number of participants across studies was 4918.

Three studies investigated the association of Ct value with mortality, of which two reported no significant associations, 21, 22 and one reported significantly higher viral load in patients with radiographic evidence of pneumonia who died versus those who survived; however, this analysis included a small number of patients. 6 Seven studies investigated association of Ct value with symptom severity or duration. In a study by Spencer et al. 23 in patients 3 years old (n = 1660 influenza A; n = 806 influenza B), low Ct values (25) compared with high Ct values (30) were associated with moderate to high self-rated illness severity for influenza A and to a lesser extent with influenza B. Furthermore, patients with influenza A and low Ct values were significantly more likely to report fever/feverishness. Two smaller studies reported significant associations between low Ct values and CURB65 score or duration of symptoms. 16, 24 In contrast, 4/7 studies reported no significant associations of Ct value with symptom severity, 6, 8, 21, 25 including two studies with >200 patients and clearly defined methodology (see Table S2 ). 21, 25 In Lalueza et al., 21 viral load was not significantly associated with multiple measures of disease severity, including pneumonia, respiratory failure, acute respiratory distress syndrome (ARDS), invasive ventilation, SOFA score and duration of invasive ventilation. However, patients with low Ct values (20) were more likely to have baseline abnormal chest X-rays compared with Ct values of >20-30 or >30 (P = 0.004), and more likely to record low lymphocyte counts (<1000 lymphocytes/lL; P = 0.035).

Three studies investigated the association of Ct value with ICU admission, 21, 26, 27 all of which reported no significant associations. In Reina et al., 26 Ct values were lower for patients in the ICU, but this did not reach statistical significance.

Four studies investigated the association of Ct value with hospital admission, all of which showed no significant association between low Ct values and hospitalization. 8, 18, 26, 27 The study by Reina et al. 26 reported no significant variation in Ct values between ICU, emergency department (ED) or hospital admission, and Zou et al. 27 reported no difference in Ct values between inpatients and outpatients. Conversely, the study by Fuller et al., 18 which provided clearly defined methodology (Table S2) , reported higher median Ct values for inpatients (Ct 35.0) than outpatients (Ct 29.8), with increasing Ct values increasing the odds of being an inpatient versus an outpatient.

Three studies investigated the association of Ct value and LOS. 8, 16, 21 Lalueza et al. 21 and Wishaupt et al. 8 reported no significant associations. In contrast, Clark et al. 16 reported LOS was longer in patients with high viral loads (Ct 20 versus other groups combined; P = 0.005). Notably, in comparison with Lalueza et al. 21 and Wishaupt et al., 8 the study by Clark et al. 16 reported a more clearly defined standardized protocol and potentially lower risk of bias (Tables S2 and S4) .

Eleven studies investigated associations between Ct value of HRV infection and clinical presentation or outcomes, summarized in Table 2 . The total number of participants across studies was 2012.

Only one study investigated viral load and mortality, and found no association in paediatric patients with radiographic evidence of pneumonia. 6 Ten studies investigated association of Ct value and symptom severity or duration. In a cohort study of 1421 paediatric patients by Feikin et al., 6 with clearly defined and standardized methodology, lower Ct values were reported in patients with radiographic evidence of pneumonia compared with control participants with detectable HRV Furthermore, in patients with radiographic evidence of pneumonia, viral load was significantly higher for very severe versus severe pneumonia. 6 Two other studies also reported association between Ct value and symptom severity; 28,29 however, the remaining seven studies (all with <100 patients each) reported no association between Ct values and symptom severity, duration of symptoms and ICU admission. 8, 16, 19, 24, [30] [31] [32] Hospitalization and length of stay Three studies investigated association of the Ct value with hospital admission and/or LOS. Similar to the findings with influenza patients, Clark et al. 16 reported significant association between LOS and low Ct value. The other two studies 8, 33 (n 32 patients) reported no associations between Ct value and hospitalization or LOS; however, methodology for sampling and sample handling was less clear than for Clark et al. 16 

Eight studies investigated associations between Ct value of RSV infection and clinical presentation or outcomes, summarized in Table 3 . Notably, all except one study (Fuller et al., 18 which split cohorts into <5 years and 5 years) were in paediatric populations. The total number of participants across studies was 3290.

Only one study investigated viral load and mortality, and found no association with radiographic evidence of pneumonia in paediatric patients. 6 Six studies investigated the association of Ct value and symptom severity or duration. In a study of 1764 paediatric patients hospitalized with bronchiolitis by Hasegawa et al., 7 which used standardized protocols for sampling and sample handling, there was no significant association between Ct values and vital signs, including oxygen saturation and severity of retractions. The same study reported that patients with low Ct values (<20.8) were at higher risk of needing ICU care compared with high Ct values (24.3); OR 1.43 (95% CI, 1.03-1.99), P = 0.03; however, significance was lost in a sensitivity analysis using a restrictive definition of bronchiolitis. Three other studies also investigated Ct values and disease severity and found no significant associations. 8 

Continued Bouzid et al.

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In line with the studies of influenza, HRV and RSV, evidence was mixed as to the association of Ct values and clinical presentation or outcomes for other respiratory pathogens (Tables S7-S12). In the paediatric study by Feikin et al., 6 lower Ct values were associated with radiographic evidence of pneumonia compared with control participants for HBoV, HAdV, HMPV, PiV1 and PiV3. However, associations were not observed for HCoVs (229E, OC43, NL63 and HKU1), PiV2, PiV4 and parechovirus/enterovirus. None of these viruses had higher viral load with very severe versus severe pneumonia. 6 Across other HCoV studies (excluding Middle East respiratory syndrome coronavirus; MERS-CoV), three studies reported significant associations between low Ct values and symptom severity or duration, 8, 24, 34 but no associations between Ct value and hospitalization or LOS. 8 Two studies of MERS-CoV reported significant associations between low Ct values and mortality, symptom severity, ICU admission and transmissibility. 17, 35 Among three HBoV studies, notably smaller in size than Feikin et al., 6 none reported significant associations between Ct values and disease severity. 8, 19, 36 Across HAdV studies other than Feikin et al., 6 Schjelderup Nilsen et al. 37 2019 reported that Ct values <30 were associated with respiratory tract infection (RTI), adjusted for age, gender and presence of other viruses; however, three other studies reported no associations between Ct values and symptom severity, hospitalization or LOS. 8, 18, 28 Aside from Feikin et al., 6 no further associations were reported across studies with HMPV and PiV, with exception of Fuller et al., 18 in which hospitalized paediatric patients (<5 years old) with PiV reported lower Ct values than controls (P = 0.047), and conversely, HMPV infections had lower Ct values in outpatients versus inpatients (P = 0.030), similar to the outcome for influenza. 8, 28, 38 In one study of herpes simplex virus (HSV), higher viral load was associated with increased mortality, disease severity and ICU admission; however, there were no other HSV respiratory studies identified in this review. 39 Among the limited number of bacterial studies identified, three studies investigated Ct values in patients with Bordetella pertussis, of which two reported significant associations between low Ct values and hospitalization or LOS. [40] [41] [42] There were too few studies in other bacterial pathogens to draw any conclusions.

Among the eligible studies in this review, nine investigated differences between Ct values between mono-and co-infections (Table  S13) . Associations were identified between lower Ct values in single infections versus co-infections; however no consistent trend Clearly defined methodology (see Table S2 

To the best of our knowledge, this is the first review to systematically assess and consolidate available evidence on associations between respiratory virus Ct values and clinical presentation or outcomes. Respiratory viruses were more commonly studied than bacterial infections, and among respiratory viruses, influenza, HRV and RSV were the most studied. Across all studies included, there was contradictory evidence regarding the association between Ct value and clinical presentation or outcomes. However, for some pathogens, there are trends that warrant further investigation. In studies of RSV, several studies reported associations between low Ct values and clinically relevant outcomes, including hospitalization, 8, 18 ICU requirement and LOS, 7 and radiographic evidence of pneumonia. 6 However, other studies of RSV viral load (Ct values not reported, hence not eligible for inclusion in this review) have failed to identify associations with disease severity or ICU admission. 46, 47 Among HRV studies, despite numerous studies showing no associations, the three studies with cohorts >100 patients reported significant associations between low Ct values and symptom severity and hospital LOS. 6, 16, 29 A similar observation can be made with HBoV studies, particularly as other studies of HBoV viral load (Ct values not reported, hence not eligible for inclusion in this review) have shown significant associations with disease severity. 48, 49 Although few HCoV studies showed significant associations, the two studies identified for MERS-CoV suggest low Ct values are associated with disease severity and transmissibility, 17, 35 in line with reports of viral load studies for SARS-CoV-1 and SARS-CoV-2. 14, 50 Interestingly, influenza studies gave conflicting reports for associations between Ct values and hospitalization or ICU admission. However, the largest influenza study identified (Spencer et al. 23 ) reported that patients with low Ct values were significantly more likely to self-report moderate to high disease severity and fever/feverishness. A similar observation was made in a study of influenza A viral load (Ct values not reported, hence not eligible for inclusion in this review) where patients with pneumonia had higher copies/mL than those with upper RTIs. 51 When interpreting the studies presented in this literature review, consideration must be given to various factors. 8, 52, 53 Many of the studies included patients in hospital settings, which potentially corresponds to greater disease severity and a less-striking difference in viral load. Indeed, studies of RSV and PiV in this review reported significantly lower Ct values for paediatric inpatients versus outpatients or controls. 8, 18 Considering the fact that many studies identified significantly lower Ct values in single versus co-infections, the lack of clarity in studies as to whether reports are sole infections has potential implications. Future studies investigating Ct values should consider the quantification of all pathogens present to obtain more detailed and consistent information on the impact of co-infections. The use Clearly defined methodology (see Table S2 ) (1) (2) (3) (4) There is evidence that genomic load in respiratory infections is affected by age and comorbidities. 23, 41, 46, 54 It is therefore important to consider the age and comorbidity profile of each of the studies when making comparisons, and future studies should consider adjusting comparisons to account for the impact of age and comorbidities.

Differences in study procedures and PCR workflow are likely to impact viral load measurement, including: specimen source, collection method, transport media type and volume; stability, quality of the sample and time of sampling versus onset of infection; inter-and intra-variability in assay platforms, and whether they were single or multiplex systems. Study procedures varied widely between studies. Many studies had gaps in reported methods relating to sampling, sample handling and analysis, and most studies did not mention the use of standardized protocols and procedures; therefore within-study variability may have limited the ability to detect associations. Consistent approaches to standardize a given workflow (e.g. sample collection, pre-treatment, realtime PCR methodology) should be implemented to utilize Ct values as a proxy of genomic load linked to a given methodology. Alternatively, normalized genomic load/copies in the test media, using reference material or international standards when available, should be attempted, 55 and cross study comparisons where this is not clear should be avoided without normalization. Importantly, raw Ct values across studies should not be interpreted as a unit of genomic load as standard curves are needed to calculate the link between Ct values and genomic load in any given study. 52 Furthermore, trends in Ct values reported in many studies, including those within this review, are population data and caution should be taken if applying this to individual patients. 

Overall (n = 120 infants with LRTI) statistically significant, but very weak correlation between Ct value and length of hospital stay (mean Ct value 28.5 ± 4.9, median hospital stay 4 days, rho = 0.19, P = 0.04). In the RSV-LRTI group, this correlation was significant but weak (rho = 0.28, P = 0.05)

Ct, cycle threshold; IQR, interquartile range; LRTI, lower respiratory tract infection; NPA, nasopharyngeal aspirate; NPS, nasopharyngeal swab; OPA, oropharyngeal swab; RSV, respiratory syncytial virus; RTI, respiratory tract infection.

iii47

This systematic review has several limitations. The protocol was restricted to consideration only of articles referring to Ct values as a measure of genomic load, meaning numerous studies that define genomic load other than using Ct values were excluded. Furthermore, it is inevitable that some studies investigating Ct values as secondary or post-hoc analyses were missed in the screening process. However, considering no aggregated outcomes were generated in this review, and no definitive conclusions made, the absence of these studies is unlikely to be a major consideration. Additionally, the high heterogeneity of reported outcomes meant it was not possible to conduct aggregated/meta-analyses, reducing the clinical utility of this review. A single reviewer conducted the data extraction and a second reviewer checked all the data points. Whilst this approach is generally accepted as adequate, it is acknowledged the optimal approach is double independent reviewer data extraction with a third reviewer resolving any discrepancies. However, given the large number of studies and outcomes identified, this was a pragmatic approach to ensure feasibility. Despite these limitations, the studies included provide insights into the potential clinical utility of respiratory pathogen Ct values, and future areas of research.

In summary, Ct values for some respiratory tract infections could be clinically useful in guiding treatment, hospital, or wider healthcare decisions. However, large-scale clinical trials, designed to minimize the limitations and disparities highlighted, are required to draw definitive conclusions.

Global patterns in monthly activity of influenza virus, respiratory syncytial virus, parainfluenza virus, and metapneumovirus: a systematic analysis

The etiological role of common respiratory viruses in acute respiratory infections in older adults: a systematic review and meta-analysis

The burden of respiratory syncytial virus in adults: a systematic review and meta-analysis

Seasonality of respiratory viral infections

Projecting the transmission dynamics of SARS-CoV-2 through the postpandemic period

Is higher viral load in the upper respiratory tract associated with severe pneumonia? Findings from the PERCH study

Respiratory syncytial virus genomic load and disease severity among children hospitalized with bronchiolitis: multicenter cohort studies in the United States and Finland

Pitfalls in interpretation of CT-values of RT-PCR in children with acute respiratory tract infections

Multicenter evaluation of QIAstat-Dx respiratory panel V2 for detection of viral and bacterial respiratory pathogens

Multicenter evaluation of the QIAstat respiratory panel-a new rapid highly multiplexed PCR based assay for diagnosis of acute respiratory tract infections

Viral kinetics of SARS-CoV-2 in asymptomatic carriers and presymptomatic patients

Implementing rapid diagnostics for COVID-19

SARS-CoV-2 viral load predicts COVID-19 mortality

A systematic review of the clinical utility of cycle threshold values in the context of COVID-19

Estimating epidemiologic dynamics from cross-sectional viral load distributions

Viral load is strongly associated with length of stay in adults hospitalised with viral acute respiratory illness

Risk factors for transmission of Middle East respiratory syndrome coronavirus infection during the 2015 outbreak in South Korea

Association of the CT values of realtime PCR of viral upper respiratory tract infection with clinical severity, Kenya

Quantitation of respiratory viruses in relation to clinical course in children with acute respiratory tract infections

Xpert MTB/RIF assay shows faster clearance of Mycobacterium tuberculosis DNA with higher levels of rifapentine exposure

Influence of viral load in the outcome of hospitalized patients with influenza virus infection

Clinical, virological, and histopathological manifestations of fatal human infections by avian influenza A(H7N9) virus

Factors associated with real-time RT-PCR cycle threshold values among medically attended influenza episodes

Prospective evaluation of a novel multiplex real-time PCR assay for detection of fifteen respiratory pathogens-duration of symptoms significantly affects detection rate

Pandemic A(H1N1)2009 influenza virus detection by real time RT-PCR: is viral quantification useful?

Usefulness of Ct value in acute respiratory infections caused by respiratory syncytial virus A and B and influenza virus A (H1N1)pdm09, A (H3N2) and B

Comparison of the Cepheid Xpert Xpress Flu/RSV assay and commercial real-time PCR for the detection of influenza A and influenza B in a prospective cohort from China

Association between semi-quantitative microbial load and respiratory symptoms among Thai military recruits: a prospective cohort study

Viral load in hematopoietic cell transplant recipients infected with human rhinovirus correlates with burden of symptoms

Disease severity and viral load are correlated in infants with primary respiratory syncytial virus infection in the community

Viruses as sole causative agents of severe acute respiratory tract infections in children

Human rhinovirus infections in hematopoietic cell transplant recipients: factors determining progression to lower tract disease

Equal virulence of rhinovirus and respiratory syncytial virus in infants hospitalized for lower respiratory tract infection

Human coronavirus in hospitalized children with respiratory tract infections: a 9-year population-based study from Norway

Association of higher MERS-CoV virus load with severe disease and death, Saudi Arabia

Human bocavirus in hospitalized children under 5 years with acute respiratory infection

Human adenovirus in nasopharyngeal and blood samples from children with and without respiratory tract infections

Nasal immune responses to human metapneumovirus

The clinical interest of HSV1 semiquantification in bronchoalveolar lavage

Correlation of real time PCR cycle threshold cut-off with Bordetella pertussis clinical severity

Molecular detection and quantification of pertussis and correlation with clinical outcomes in children

Marked difference between adults and children in Bordetella pertussis DNA load in nasopharyngeal swabs

High detection rates of human bocavirus in infants and small children with lower respiratory tract infection from Croatia

Human bocavirus infection as a cause of severe acute respiratory tract infection in children

Diagnosis of pediatric acute adenovirus infections: is a positive PCR sufficient?

Clinical disease and viral load in children infected with respiratory syncytial virus or human metapneumovirus

Clinical characteristics and viral load of respiratory syncytial virus and human metapneumovirus in children hospitaled for acute lower respiratory tract infection

Human bocavirus in children: mono-detection, high viral load and viraemia are associated with respiratory tract infection

Clinical significance of different virus load of human bocavirus in patients with lower respiratory tract infection

Viral loads in clinical specimens and SARS manifestations

Correlation of pandemic (H1N1) 2009 viral load with disease severity and prolonged viral shedding in children

The raw Ct values from RT-PCR detection are not viral load quantitation units

Pitfalls of quantitative real-time reverse-transcription polymerase chain reaction

Viral loads and duration of viral shedding in adult patients hospitalized with influenza

Cellular DNA quantification in respiratory samples for the normalization of viral load: a real need

The authors would like to acknowledge Sarah Rossall, PhD and Stephanie Cumberworth, PhD, of Ashfield MedComms, an Ashfield Health company, for assistance with literature screening that was funded by Qiagen Manchester Ltd.

This study was funded by Qiagen Manchester Ltd. Medical writing support for the development of this manuscript, under the direction of the authors and in accordance with Good Publications Practice (GPP3) guidelines (http://www.ismpp.org/gpp3), was provided by Tom Hudson, PhD, of Ashfield MedComms, an Ashfield Health company, and funded by Qiagen Manchester Ltd. Tom Hudson also provided assistance with protocol development and literature screening.

D.B. reports personal fees from Qiagen, outside the submitted work. J.V. and G.H. declare no conflicts of interest. D.M. is an employee of Qiagen and owns shares in Qiagen. J.P. is an employee of STAT-Dx Life, a QIAGEN company, and owns shares in Qiagen. S.N.R. is an employee of Qiagen. B.V. reports grants, personal fees, and non-financial support from Qiagen, personal fees and non-financial support from BioMérieux, personal fees from Hologic, personal fees from Gilead, outside the submitted work. This article forms part of a Supplement sponsored by QIAGEN.

All authors contributed to protocol development, data interpretation and writing of this report.

Tables S1 to S13 are available as Supplementary data at JAC Online.