Indeterminate for acute rejection is defined by variable presence of the three main features of acute rejection (infiltration by a mixed but primarily mononuclear inflammatory population including blastic or activated lymphocytes; crypt injury and inflammation; and an increase in crypt apoptotic bodies) which are usually focal and do not meet the criteria for mild acute rejection. The inflammatory infiltrate is usually minimal and localized. Although the mucosa is intact, crypt epithelial injury is often present. There is a variable increase in crypt epithelial apoptosis, but usually with less than 6 apoptotic bodies/10 crypts.

Indeterminate for acute rejection should only be used when the biopsy shows features of acute rejection with degrees of inflammation, epithelial injury and apoptosis less prominent than mild acute rejection. It should not be applied to other non-rejection processes when one is not sure about the diagnosis.