SPP1 and COVID-19: On the other hand, Varim et al. [161], in a prospective cohort study with hospitalized patients, found OPN values of 13.75 ng/mL (11.30–17.07 ng/mL) in hospitalized COVID-19 adult patients in critical condition and 9.85 ng/mL (8.32–19.23 ng/mL) in non-critical patients.