Definition of remission, minimal residual disease, and relapse in hairy cell leukemia bone marrow biopsy histology and immunohistology specimens.

No consensus has been reached in the definitions of complete remission (CR) and minimal residual disease (MRD) on bone marrow biopsy specimens of patients with hairy cell leukemia (HCL). Large comparative studies correlating clinical outcome with detection of MRD by immunohistochemistry (IHC), flow cytometry, and molecular diagnostics are lacking. The presence of normal bone marrow B-lymphocytes and the lack of immunohistochemical stains that are both sensitive and specific can make it challenging to differentiate normal lymphocytes from hairy cells. The use of CD20 and/or DBA.44 immunohistochemical stains, in combination with either tartrate resistant acid phosphatase (TRAP), CD11c-5D11, or annexin A1, improve specificity and facilitate evaluation. The interobserver concordance in quantifying the percentage of residual hairy cells is poor. Performing cell counts in multiple fields improves reproducibility. The prolonged hematologic remissions of subsets of patients with MRD emphasize the need to define clinically relevant thresholds that correlate with long-term responses and clinical endpoints. The development of internationally accepted, reproducible, and clinically relevant CR, MRD, and relapse criteria are of paramount importance in evaluating and comparing clinical trials and in providing optimal patient care.