Serum [beta].sub.2-microglobulin levels in Coronavirus disease 2019

[beta].sub.2 -microglobulin ([beta].sub.2 -m), a 11.8 kDa protein, pairs non-covalently with the [alpha]3 domain of the major histocompatibility class (MHC) I [alpha]-chain and is essential for the conformation of the MHC class I protein complex. Shed [beta].sub.2 -m is measurable in circulation, and various disorders are accompanied by increases in [beta].sub.2 -m levels, including several viral infections. Therefore, we explored whether [beta].sub.2 -m levels could also be elevated in Coronavirus disease 2019 (Covid-19) and whether they predict disease severity. Serum [beta].sub.2 -m levels were measured in a cohort of 34 patients infected with SARS-CoV-2 on admission to a tertiary care hospital in Riyadh, Saudi Arabia, as well as in an approximately age-sex matched group of 34 uninfected controls. Mean [beta].sub.2 -m level was 3.25±1.68 mg/l (reference range 0.8-2.2 mg/l) in patients (mean age 48.2±21.6) and 1.98±0.61 mg/l in controls (mean age 48.2±21.6). 17 patients (mean age 36.9± 18.0) with mean [beta].sub.2 -m levels of 2.27±0.64 mg/l had mild disease by WHO severity categorization, 12 patients (mean age 53.3±18.1) with mean [beta].sub.2 -m levels of 3.57±1.39 mg/l had moderate disease, and five patients (of whom 2 died; mean age 74.4±13.8) with mean [beta].sub.2 -m levels of 5.85±1.85 mg/l had severe disease (P < = 0.001, by ANOVA test for linear trend). In multivariate ordinal regression [beta].sub.2 -m levels were the only significant predictor of disease severity. Our findings suggest that higher [beta].sub.2 -m levels could be an early indicator of severity of disease and predict outcome of Covid-19. As the main limitations of the study are a single-center study, sample size and ethnicity, these results need confirmation in larger cohorts outside the Arabian Peninsula in order to delineate the value of [beta].sub.2 -m measurements. The role of [beta].sub.2 -m in the etiology and pathogenesis of severe Covid-19 remains to be elucidated.