The invention is based in part on the observation that a CHO cell oxidizing enzyme, particularly QSOX1, can survive a seemingly rigorous antibody purification process to reduce subsequent conjugation efficiency of the antibody to a drug. Whether the oxidizing enzyme survives the purification procedure depends on which purification techniques are employed which can vary from one antibody to another. With knowledge that contamination with a CHO cell oxidizing enzyme is a potential problem for subsequent conjugation, a suitable purification scheme can be devised for any antibody that eliminates or at least reduces CHO oxidizing enzyme(s) to an acceptable level.