Purification fold $= \dfrac{\text{Specific activity after step (units/mg protein)}}{\text{Specific activity at start (units/mg protein)}}$. Specific activity = total enzyme units / total protein. As purification proceeds, total protein decreases while enzyme units are (ideally) maintained, so specific activity increases. A high purification fold indicates effective removal of contaminating proteins. It is tracked alongside yield (% recovery of enzyme units) in a purification table.