Disclosed is an in vitro method for assessing (analyzing) the risk of potential adverse effects for a human patient mediated by the administration of a CD19xCD3 bispecific antibody to said patient comprising determining the ratio of B cells to T cells determining in a sample from said patient, wherein a ratio of about 1:5 or lower is indicative for a risk of potential adverse effects for said patient. Further disclosed is the use of a CD19xCD3 bispecific antibody for the preparation of a pharmaceutical composition for treating malignant CD19 positive lymphocytes and ameliorating and/or preventing an adverse effect mediated by the administration of a CD19xCD3 bispecific antibody in a human patient identified as having a B : T cell ratio of about 1:5 or lower, wherein said B : T cell ratio can be determined by the following steps: a) determining the total B cells in a peripheral blood sample from said patient； b) determining the total T cells in a peripheral blood sample from said patient； c) calculating the ratio of the total B cells of step a) and the total T cells of step b) in order to obtain a B : T cell ratio； d) wherein said composition is formulated for administration in a first dose of a CD19xCD3 bispecific antibody for a first period of time to said human patient； and e) wherein said composition is formulated for consecutive administration in a second dose of said antibody for a second period of time to said human patient； wherein said first period of time is between 3 days and 10 days, and said second period of time is between 3 days and 10 days.
