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