Approach Considerations
The diagnosis of hairy cell leukemia (HCL) is based on the following [13] :
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Morphological evidence of hairy cells
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An HCL immunologic score of 3 or 4 based on CD11C, CD103, CD123, and CD25 expression
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Trephine bone marrow biopsy and/or aspiration, which makes it possible to specify the degree of tumoral medullary infiltration and the presence of BRAF V600E somatic mutation
International consensus guidelines recommend performing immunohistochemistry (IHC) or flow cytometry to establish a diagnosis of HCL and distinguish classic HCL from other histologically similar peripheral small B-cell lymphoid neoplasms. In addition to the BRAF V600E mutation, testing should include assays for the following [15, 16, 17] :
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CD19
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CD20
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CD5
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CD10
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CD11c
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CD22
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CD25
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CD103
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CD123
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CD200
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Cyclin D1
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Annexin A1
On flow cytometry, classic HCL is negative for CD5 and CD10, and positive for CD20 (bright), CD11c (bright), CD22 (bright), CD25, CD103, CD123, CD200, cyclin D1, and annexin A1. In contrast, HCL variant is typically negative for CD25, CD123, and annexin A1, as well as negative for BRAF V600E. [15, 18]
Laboratory Studies
Complete blood count and peripheral blood smear
A complete blood cell count (CBC) and careful review of a peripheral blood smear are the first steps in the identification of hairy cells. The typical hairy cells of hairy cell leukemia (HCL) 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.
The CBC in patients with HCL typically shows pancytopenia, with decreased cell counts in all three cell lines, as follows:
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Anemia is usually severe and normochromic-normocytic in character
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Neutropenia and monocytopenia are usually present, but an elevated white blood cell count (hairy cells) is found in 20% of cases
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Thrombocytopenia is found in more than 80% of patients
Trephine bone marrow biopsy and/or aspiration
The immunophenotypic profile of HCL is characterized by the clonal expansion of B‐cells with bright CD19, CD20, CD22, and CD200 expression. Hairy cells are usually negative or dim for CD5, CD23, CD10, CD79b, and CD27 but positive for CD11c, CD103, CD123, and CD25. A proposed immunological score gives one point to each of the last four markers when they are expressed. A score of 3 or 4 is observed in 98% of HCL cases, whereas in other HCL‐like disorders, the score is usually 0 or 1. [13]
The somatically acquired V600E mutation of the BRAF gene is present in nearly all patients with HCL and represents a reliable marker. [19] BRAF V600E can be reliably detected at the protein level by immunohistochemical stain using a mutant protein–specific antibody. [20]
A study by Tiacci et al examined the use of a test for genetically-based diagnosis of HCL. The molecular assay determines the presence of the BRAF V600E mutation in order to differentiate between HCL and other disorders (eg, splenic marginal zone lymphoma, HCL variant). The study found that the molecular assay was a powerful tool for enhancing diagnostic accuracy. [21]
Genomic analysis of de novo vemurafenib-resistant classic HCL identified a novel gain-of-function mutation in IRS1 and losses of NF1 and NF2, each of which contributed to treatment resistance. [22]
Imaging Studies
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 mild organomegaly that could be missed by abdominal palpation.
Histologic Findings
In a patient with pancytopenia and splenomegaly, the presence of circulating cells that are tartrate-resistant acid phosphatase (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.
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Blood film at × 400 magnification. This image demonstrates a lymphocytosis and an absence of any other type of blood cell (pancytopenia). The characteristic cytoplasmic projections are already visible. Photographed by U. Woermann, MD, Division of Instructional Media, Institute for Medical Education, University of Bern, Switzerland.
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Blood film at × 1000 magnification. This image demonstrates lymphocytes with characteristic cytoplasmic projections. Photographed by U. Woermann, MD, Division of Instructional Media, Institute for Medical Education, University of Bern, Switzerland.
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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.
