Minimal residual disease (MRD) is the presence of residual CLL cells in a patient who has otherwise achieved a complete remission (CR) in accordance with National Cancer Institute (NCI) criteria.

When complete remissions are achieved, physicians can order a variety of tests for MRD using polymerase chain reaction (PCR) or flow cytometry. When minimal residual disease is present, it is predictive of a shorter event-free period of remission. For this reason, some physicians will attempt to intensify complete remissions by treating residual disease with drugs like rituximab (Rituxan) and alemtuzumab (Campath-1H).

Complete remissions with no evidence of minimal residual disease--sometimes called PCR negative--are thought to be the most durable CR's.