Hairy Cell Leukemia Workup

• The typical hairy cells of hairy cell leukemia are so named because of their characteristic cytoplasmic projections, which appear as fine (hairlike) microvilli when seen by light microscopy, phase-contrast microscopy, and electron microscopy. These are mononuclear cells with eccentric or centrally placed nuclei.
• Hairy cells have a mature B-cell phenotype and typically express single or multiple immunoglobulin light chains and pan–B-cell antigens, such as CD19, CD20, CD22, and CD79b, but not CD21 (late B-cell marker). The cells also typically express CD103, CD11c, and CD25 but usually not CD5, CD10, or CD23. Hairy cells strongly express CD45, seen as a bright signal, with increased forward and side scatter resembling large lymphocytes and monocytes. Immunophenotypic analysis helps distinguish hairy cell leukemia from other low-grade B-cell malignancies. A study by Venkataraman et al confirmed that CD123 and CD103 are useful in the differential diagnosis of B-cell lymphoproliferative disorders.^[6]
• Monoclonal BLy-7 has high sensitivity and specificity for HCL. CD22 stains at higher intensity in hairy cells than in normal B cells.^[7] Hairy cells can be identified immunophenotypically in 92% of cases, even when the cells represent less than 1% of the circulating lymphocytes.
• Cytochemical evaluation is important for diagnostic confirmation of the morphologic findings. Hairy cells demonstrate strong positivity for tartrate-resistant acid phosphatase (TRAP) staining (see image below). A positive TRAP stain in conjunction with a characteristic bone marrow biopsy is essentially diagnostic of hairy cell leukemia. Blood film at × 1000 magnification. This image demonstrates tartrate-resistant acid phosphatase (TRAP) activity of lymphocytes. Photographed by U. Woermann, MD, Division of Instructional Media, Institute for Medical Education, University of Bern, Switzerland.
• The peripheral blood cell counts show pancytopenia with decreased cell counts in all 3 cell lines.

  • Anemia is usually severe and normochromic-normocytic in character.
  • Neutropenia and monocytopenia are usually present in hairy cell leukemia, but an elevated white blood cell count (hairy cells) is found in 20% of cases.
  • Thrombocytopenia is found in more than 80% of patients.
• The bone marrow aspirate is usually unsuccessful due to a "dry tap." Infiltration of the bone marrow by hairy cell leukemia makes aspirating cells through a needle difficult.

  • Core biopsy of the bone marrow shows a pattern of hairy cell infiltration with a single round or oval nucleus separated by abundant cytoplasm in a fine fibrillar network.^[8]
  • The cell appears separated, resulting in the characteristic fried-egg appearance.^[8]
• Clonal cytogenetic abnormalities are present in two thirds of patients, and the involvement of chromosomes 1, 2, 5, 6, 11, 14, 19, and 20 have been described. Chromosome 5 abnormality is most frequent (in 40% of patients) with trisomy 5 and pericentric inversions and interstitial deletions of band 5q13.

• Most patients with hairy cell leukemia have massive splenomegaly such that imaging studies are unnecessary to appreciate its presence.
• In milder forms, a liver and spleen scan or ultrasound measurement may detect some mild forms of organomegaly that may be missed by abdominal palpation.

• Difficult cases can be confirmed by using immunophenotypic analysis of the buffy coat cells or by performing electron microscopy on suspected cells.
• Soluble interleukin-2 receptor levels are elevated in patients with hairy cell leukemia and may provide additional supportive data for the diagnosis.
• The somatically acquired V600E mutation of the BRAF gene is present in all patients with hairy cell leukemia and represents a reliable marker.^[9] A study by Tiacci et al examined the use of a test for genetics-based diagnosis of hairy cell leukemia. The molecular assay determines the presence of the BRAF-V600E mutation in order to differentiate between hairy cell leukemia and other disorders (eg, splenic marginal zone lymphoma, hairy cell leukemia variant). The study found that the molecular assay was a powerful tool for enhancing the diagnostic accuracy.^[10]

The findings of pancytopenia and splenomegaly in the presence of circulating cells that are TRAP positive and a dry bone marrow aspirate with biopsy material showing infiltration with a mononuclear cells that have a fried-egg appearance are diagnostic of hairy cell leukemia.