In nonneoplastic conditions, the specimen comprises a mixture of T and B cells. In addition to CD19, CD20, and CD22, mature nonneoplastic B cells typically express polytypic surface immunoglobulin. B-cell lymphomas, however, express a single clonal light chain (also called light chain restriction ), so the kappa-to-lambda ratio is either substantially increased or decreased due to a significant lambda excess. Ki67 immunostaining on cytospin preparation (Fig. 4.​13) is often included in lymphoma workup to help tumor grading, to aid in the diagnosis of Burkitt lymphoma , or to confirm the blastoid nature of mantle cell lymphoma. The aberrant expression of some antigens in B-cell lymphomas may be of prognostic value. For example, CD38 expression in B-cell SLL/CLL is often associated with a more aggressive clinical course. Table 5.1 lists the key immunophenotypic and molecular features of mature B-cell neoplasms commonly encountered in cytology practice.

Although in most cases, the interpretation of flow cytometric data is quite straightforward, traps may be encountered. Correlation of flow cytometric data with clinical and cytologic findings and Ki67 staining is important. In rare cases, cytogenetic or molecular studies may also be needed. Interpreting a case solely on the basis of flow cytometric results may lead to an erroneous diagnosis. Negative findings do not necessarily indicate a benign process.