Raman spectroscopic analysis of human serum samples of convalescing COVID-19 positive patients

Rapid screening, detection and monitoring of viral infection is of critical importance, as exemplified by the rapid
spread of SARS-CoV-2, leading to the worldwide pandemic of COVID-19. This is equally the case for the stages of
patient convalescence as for the initial stages of infection, to understand the medium and long terms effects, as
well as the efficacy of therapeutic interventions. Optical spectroscopic techniques potentially offer an alternative
to currently employed techniques of screening for the presence, or the response to infection. In this study, the
ability of Raman spectroscopy to distinguish between samples of the serum of convalescent COVID-19 positive
patients and COVID-19 negative serum samples, and to further analyse and quantify systemic responses, was
explored. The study included serum samples of patients who had been tested for SARS-CoV-2 specific IgG and
IgM responses between 25 and 134 days after the infection was identified. Both COVID-19 positive and negative
groups included males and females who ranged in age from 21 to 81 years old. No correlation was apparent
between the specified SARS-CoV-2 specific IgG and IgM immunoglobulin levels of the positive group, their sex, or
age. Raman spectroscopic measurements were performed at 785 nm, in liquid serum, thawed from frozen, and
spectra were pre-processed to remove the contribution of water, normalising to the water content. Principal
components analysis of the spectral dataset over the range 400–1800 cm-1 provided no clear indication of a
difference between normal serum and SARS-CoV-2 positive serum. A selection of 5 of the samples, which were
available in sufficient volume, were fractionated by centrifugal filtration, and the 100 kDa, 50 kDa, 30 kDa, and
10 kDa concentrates similarly analysed by Raman spectroscopy. Partial least squares regression analysis revealed
a negative correlation between the spectral profile of the 30 kDa fractions and SARS-CoV-2 specific IgG antibody
levels, potentially indicating an association with depleted glutathione levels. The study supports a potential role
of Raman screening of blood serum for monitoring of SARS-CoV-2 infection, but also in longitudinal studies of
disease progression, long term effects, and therapeutic interventions.