GDF15 and COVID-19: Multivariable analysis showed that age, but not sex nor presence of comorbidities, was a confounding factor of the association between GDF-15 and disease severity (Table 2), because GDF-15 was associated with age (Manova p < 0.0001) but not Severity (Manova p = 0.89) (Table 2) Age-adjusted regression analysis increased association between plasma GDF-15 and COVID-19 severity (unadjusted β: 6.106, adjusted β: 0.222).