Validation of a MALDI-TOF MS method for SARS-CoV-2 detection on the Bruker Biotyper and nasopharyngeal swabs. A Brazil - UK collaborative study

We had developed a MALDI-TOF mass spectrometry method for detection of SARS-CoV-2 virus in saliva-gargle samples using Shimadzu MALDI-TOF mass spectrometers in the UK. This was validated in the USA to CLIA-LDT standards for asymptomatic infection detection remotely via sharing protocols, shipping key reagents, video conference and data exchange. In Brazil, more so than in the UK and USA, there is a need to develop non-PCR dependent rapid affordable SARS-CoV-2 infection screening tests, which also identify variant SARS-CoV-2 and other virus infections. Travel restrictions necessitated remote collaboration with validation on the available Clinical MALDI-TOF – the Bruker Biotyper (microflex® LT/SH) – and on nasopharyngeal swab samples, as salivary gargle samples were not available. The Bruker Biotyper was shown to be almost log10^3 more sensitive at detection of high molecular weight spike proteins. A protocol for saline swab soaks out was developed and duplicate swab samples collected in Brazil were analysed by MALDI-TOF MS. The swab collected sample spectra varied from that of gargle-saliva in three additional mass peaks in the mass region expected for IgG heavy chains and human serum albumin. A subset of clinical samples with additional high mass, probably Spike-related proteins, were also found. Spectral data comparisons and analysis, subjected to machine learning algorithms in order to resolve RT-qPCR positive from RT-qPCR negative swab samples, showed a 78% agreement with RT-qPCR scoring for SARS-CoV-2 infection.