Polymerase Chain Reaction, (PCR) is a laboratory process that was developed in 1985. In PCR, a particular DNA segment from a mixture of DNA chains is rapidly replicated, (between 10,000 and 1,000,000 copies can be made, depending on the procedure) producing a large, readily analyzed sample of a piece of DNA. The process is sometimes called DNA amplification. PCR has had an immense impact on biology and medicine, especially genetic research.

PCR is commonly used in CLL to test for minimal residual disease (MRD) in patients who have achieved complete remissions. This technique is very sensitive and is capable of detecting a single CLL cell in 100,000 cells.

PCR can be performed on any number of specimens: tissue, bone marrow, peripheral blood, and fluids. It is usually performed on the specimen that is thought to harbor the cell line of interest. In CLL, PCR is typically performed using the bone marrow or the peripheral blood (a normal blood draw). Because the leukemias arise in the marrow, some physicians prefer to perform PCR using a marrow specimen.

The presence of minimal residual disease in patients who have achieved complete remissions is predictive of a shorter event-free remission. For this reason, when minimal residual disease is detected, further treatment aimed at eradicating residual CLL cells may be recommended.