Flow cytometric evaluation of peripheral blood for suspected Sézary syndrome or mycosis fungoides: International guidelines for assay characteristics

A peripheral blood flow cytometric assay for Sézary syndrome (SS) or circulating mycosis fungoides (MF) cells must be able to reliably identify, characterize, and enumerate T‐cells with an immunophenotype that differs from non‐neoplastic T‐cells. Although it is also important to distinguish SS and MF from other subtypes of T‐cell neoplasm, this usually requires information in addition to the immunophenotype, such as clinical and morphologic features. This article outlines the approach recommended by an international group with experience and expertise in this area. The following key points are discussed: (a) At a minimum, a flow cytometric assay for SS and MF should include the following six antibodies: CD3, CD4, CD7, CD8, CD26, and CD45. (b) An analysis template must reliably detect abnormal T‐cells, even when they lack staining for CD3 or CD45, or demonstrate a phenotype that is not characteristic of normal T‐cells. (c) Gating strategies to identify abnormal T‐cells should be based on the identification of subsets with distinctly homogenous immunophenotypic properties that are different from those expected for normal T‐cells. (d) The blood concentration of abnormal cells, based on any immunophenotypic abnormalities indicative of MF or SS, should be calculated by either direct enumeration or a dual‐platform method, and reported.