A method for selecting a variant antibody having amino acid alterations in the antibody-Fab region (Fab-variant antibody) with increased or decreased in vivo half-life relative to its parent antibody comprising the following steps: a) determining the retention time of the Fab-variant antibody and its parent antibody on an FcRn affinity chromatography column with a positive linear pH gradient elution in the presence of a first salt concentration, b) determining the retention time of the Fab-variant antibody and its parent antibody on an FcRn affinity chromatography column with a positive linear pH gradient elution in the presence of a second salt concentration, whereby in case of selecting a Fab-variant antibody with increased in vivo half-life relative to its parent antibody a Fab-variant antibody is selected that has i) a retention time determined in step a) that is by at least 5 % longer than the retention time of its parent antibody determined in step a), and ii) a retention time determined in step a) that is within 5 % of the retention time determined in step b), whereby in case of selecting a Fab-variant antibody with decreased in vivo half-life relative to its parent antibody a Fab-variant antibody is selected that has i) a retention time determined in step a) that is by at least 5 % shorter than the retention time of its parent antibody determined in step a), and ii) a retention time determined in step a) that is within 5 % of the retention time determined in step b), wherein the positive linear pH gradient is from pH 5.5 to pH 8.8, wherein the salt is sodium chloride, wherein the first salt concentration is between 50 mM and 200 mM, and wherein the second salt concentration is between 300 mM and 600 mM, wherein the antibody is a full length antibody. a) determining the retention time of the Fab-variant antibody and its parent antibody on an FcRn affinity chromatography column with a positive linear pH gradient elution in the presence of a first salt concentration, b) determining the retention time of the Fab-variant antibody and its parent antibody on an FcRn affinity chromatography column with a positive linear pH gradient elution in the presence of a second salt concentration, whereby in case of selecting a Fab-variant antibody with increased in vivo half-life relative to its parent antibody a Fab-variant antibody is selected that has i) a retention time determined in step a) that is by at least 5 % longer than the retention time of its parent antibody determined in step a), and ii) a retention time determined in step a) that is within 5 % of the retention time determined in step b), whereby in case of selecting a Fab-variant antibody with decreased in vivo half-life relative to its parent antibody a Fab-variant antibody is selected that has i) a retention time determined in step a) that is by at least 5 % shorter than the retention time of its parent antibody determined in step a), and ii) a retention time determined in step a) that is within 5 % of the retention time determined in step b), wherein the positive linear pH gradient is from pH 5.5 to pH 8.8, wherein the salt is sodium chloride, wherein the first salt concentration is between 50 mM and 200 mM, and wherein the second salt concentration is between 300 mM and 600 mM, wherein the antibody is a full length antibody.