Background
Acute respiratory infection (ARI) is the most common reason patients
seek healthcare worldwide. Uncomplicated ARIs in the outpatient setting
are often of viral origin [acute bronchitis (90%), pharyngitis
(85%), and sinusitis (98%)] or are self-limiting and tend to resolve
without antibiotics.1 Reliable differentiation between
uncomplicated and self-limiting viral from bacterial ARIs remains
challenging, primarily due to the non-specific overlapping clinical
manifestations which can be present in both clinical scenarios, and
secondly because many patients are carriers of or are colonized with
bacterial or viral pathogens.
The COVID-19 pandemic is putting an extraordinary strain on healthcare
resources. To date, molecular reverse transcriptase polymerase chain
reaction (rRT-PCR) has been used for screening and initial diagnosis
despite long turnaround times that can take upwards of 48 hours from
sample collection to result. Some patients, including those with high
likelihood COVID-19 infection (e.g. symptomatic with recent exposure and
chest imaging consistent with COVID-19 infection), test negative on the
initial RT-PCR test which requires multiple subsequent tests to return
an eventual positive result.2-4 Variations of sampling
techniques, viral load and analytical sensitivity of the manufacturer
test kit may impact sensitivity.5 Molecular tests are
also impacted by efficiency of viral sample transfer to the test and can
differ depending on sampling technique (oropharyngeal vs.
nasopharyngeal).6,7 This may offer an explanation as
to why early data from China reported test sensitivities ranging from
66-70%.5,8 Recently, studies comparing rRT-PCR to a
composite of radiological plus clinical findings (signs/symptoms,
epidemiological evidence of exposure) have reported that that chest
imaging improves initial diagnosis of COVID-19 and is associated with
fewer false negatives.2,9 Therefore, a comprehensive
clinical diagnosis inclusive of clinical exam (symptoms/signs),
laboratory findings, confirmatory testing (i.e. RT-PCR), and chest
imaging may also be considered as indictors of COVID-19 to reduce false
negative rate of molecular testing.9-12
Rapid host response assays have been proposed for initial triage as
components of a comprehensive COVID-19 diagnostic strategy that also
includes molecular and antibody testing. This is in an effort to
streamline patients for confirmatory testing, quarantine and facilitate
hospitalisation or discharge.13 FebriDx, (Lumos
Diagnostics, Sarasota, FL, USA) is a point-of-care (POC) immunoassay
that rapidly (10 minutes) assesses a host immune response to an ARI and
differentiates viral from bacterial infection through detection of both
Myxovirus resistance protein A (MxA) and C-reactive protein (CRP) from a
fingerstick blood sample.
CRP is a nonspecific, acute-phase protein that is upregulated due to
acute inflammation, including response to infection and is predominately
produced by the liver in response to inflammatory cytokines such as
IL-6. 14-22 MxA is an intracellular protein that is
exclusively induced by type I IFN and not by other cytokines expressed
during bacterial infection (e.g. IFN-gamma, IL-1,
TNF-alpha.23 Type I IFNs are produced by many
different cell types, specifically monocytes and macrophages, in
response to a wide range of viral infections and are found to be
elevated in the presence of most acute viral
infections.24-26 Therefore, MxA is elevated in
response to an acute viral infection and remains low in bacterial
infections.24,25 It is hypothesised that SARS-CoV-2
may initially suppress type I IFN production causing loss of viral
containment early in of infection followed by an influx of neutrophils,
macrophages and excessive production of type I IFN.27However, considering that MxA is exclusively expressed by type 1 IFNs
and similar viruses such as MERS-CoV and SARS-CoV have been found to
elevate MxA, it is likely that MxA would also increase in response to
SARS-CoV-2 infection.28
FebriDx, utilizes monoclonal anti-MxA and anti-CRP antibodies to detect
elevated levels of MxA and CRP respectively. MxA elevation with or
without an elevation in CRP is consistent with a viral infection. An
elevation in CRP without MxA is consistent with a bacterial infection
that may require antibiotic therapy. Two multicentre trials demonstrated
that FebriDx was sensitive and specific to differentiate viral from
bacterial and non-infectious conditions in patients presenting to
General Practice offices as well as Hospital or Emergency Department
(ED) settings, with non-specific ARI symptoms.24,25
Therefore, we hypothesised that FebriDx would provide an early
indication of a host immune response in suspected COVID-19 cases
presenting to Hospital / ED. Identifying patients as having a bacterial
or viral infection or non-infectious condition could significantly
decrease time to presumed diagnosis and allow for appropriate isolation
from the outset. The primary objective was to assess the FebriDx assay
ability to identify COVID-19 patients as viral infections in order to
inform clinical management strategies and initial isolation procedures
until confirmatory testing results are available. As such, diagnostic
performance of FebriDx was measured against the Case Definitions for
Bacterial Infection, COVID-19 Viral Infection29,
Non-COVID-19 Viral Infection and Non-Infectious conditions and were
confirmed by an independent physician (Figure 1). In addition, the
diagnostic performance of SARS-CoV-2 rRT-PCR was also compared to the
COVID-19 Case Definition.30,31