Useful information about a subject's level of systemic inflammation is obtained by quantitatively measuring the amount of fibrinogen and the hematocrit and or hemoglobin in the subject's whole blood. The fibrinogen measurement, when combined with an hematocrit or hemoglobin measurement, provides a systemic Inflammation Index value for the donor. The method is not affected by blood variables which are not related to the presence of inflammation, which blood variables are known to invalidate an erythrocyte sedimentation rate, which is the most frequently used blood test for detecting systemic inflammation in humans.