Methods of predicting an in vivo serum concentration of an antibody with a post-translational modification of interest after administration of the antibody are provided, as are methods for predicting a subject's exposure to post-translational variants of the antibody. The methods include predicting a percentage of the antibody with the post-translational modification of interest using an in vivo rate constant determined for the post-translational modification, and multiplying the predicted percentage of the antibody with the post-translational modification of interest by the in vivo concentration of the antibody to determine the concentration of the antibody with the post-translational modification of interest.