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  How is hairy-cell leukemia diagnosed by flow cytometry?

Hairy cell leukemia (HCL) is a relatively rare low-grade B-cell neoplasm, predominantly occurring in middle-aged to elderly patients, especially men. Despite its rarity, it is important to recognize it by flow cytometry because it shares immunophenotypic features with other B-cell neoplasms and it is important to take the necessary steps to distinguish them. This is particularly true because HCL is a relatively indolent disease with a very effective low-intensity treatment. Thus, distinguishing it from other B-cell neoplasms can both reassure the patient and physician and prevent excessive therapy.

The basic phenotype of HCL is relatively non-specific. It is positive for CD19 and C20, monotypic for kappa or lambda light chain, and negative for CD5 and CD10 (see Figure 1A-C). In this regard, it is similar to that of marginal zone lymphoma, lymphoplasmacytic lymphoma, and the less-common splenic B-cell lymphoma/leukemia, unclassifiable entities (splenic diffuse red pulp small B-cell lymphoma and hairy cell leukemia-variant). The distinguishing hallmark of HCL is co-expression of CD25 and CD103, which is seen in nearly all cases (see Figure 1D). Bright CD123 also has been reported as a specific marker of HCL. However, since these markers are often not present in screening panels for B-cell lymphoma, other clinical and phenotypic clues are needed to signal the laboratory that HCL is likely.

These additional clues fall into three categories. The first is forward and side scatter (see Figure 1E. The neoplastic cells of HCL are slightly larger, and thus exhibit higher forward scatter compared to normal small lymphocytes. The increased cytoplasm and membrane irregularities of these cells also increase the side scatter properties. Often, these cells occupy the same position as monocytes on a forward/side scatter plot. The second clue is markedly increased (bright) expression of CD11c, CD22, and CD200 compared with normal B cells (see Figure 1F-G). Thus, at least one of these markers should be in routine screening panels to alert to the possibility of HCL in the right immunophenotypic context. The third clue is a relative paucity of monocytes. HCL is one of the few conditions that consistently causes monocytopenia. The presence of one or more of these phenotypic clues, especially in the right clinical and morphologic context, should trigger additional evaluation with a panel containing CD25 and CD103.

A number of these markers are important for distinguishing HCL from other entities in the differential diagnosis. Other lymphomas may express CD25 or CD103, but not both. For example, splenic marginal zone and lymphoplasmacytic lymphomas are usually negative for CD103 and hairy cell leukemia-variant is typically negative for CD25. Amongst CD5(-)/CD10(-) B-cell neoplasms, uniformly bright CD200 expression is also fairly unique to HCL. Thus, taken together, this combination of immunophenotypic markers is strong evidence of hairy cell leukemia.

  • Foucar K, Falini B, Catovsky D, Stein H. Hairy cell leukemia. In: WHO Classification of Tumours of Haematopoietic and Lymphoid Tissues, 4th ed. SH Swerdlow, et al., eds. Lyon, France:IARC. 2008. pp. 188-190.
  • Stetler-Stevenson M, Tembhare PR. Diagnosis of hairy cell leukemia by flow cytometry. Leuk Lymphoma. 2011;52(suppl 2):11-13.

Author: Adam Seegmiller