Abstract: Provided herein are in vitro methods of assaying an in vivo level of high molecular weight (HMW) species of a therapeutic protein. In exemplary embodiments, the method comprises (a) incubating a mixture comprising (i) a sample comprising the therapeutic protein and (ii) serum, or a depleted fraction thereof; and (b) assaying the level of HMW species of the therapeutic protein present in the mixture at one or more time points after step (a). Also methods of determining the in vivo reversibility of HMW species of a therapeutic protein are provided herein. In exemplary instances, the method comprises (A) assaying the in vivo level of high molecular weight (HMW) species of a therapeutic protein according to a presently disclosed in vitro method, and (B) comparing the level(s) of HMW species present in the mixture to the level of HMW species present in the sample prior to the incubating step.