Flow cytometric immunotyping (FCI) characterizes cells based on size, internal complexity, and immunophenotype, enabling identification of cell types and some cellular abnormalities. Because it uses cells in suspension, it is particularly useful in examination of peripheral blood, allowing for a relatively non-invasive method of diagnosing some hematologic abnormalities. FCI requires manual labor, sophisticated analyzers, and expensive reagents, and thus is resource intensive.
Routine peripheral blood flow cytometry immunotyping (PB FCI) is best used to characterize morphologically abnormal cells identified by review of a peripheral blood smear. Examples include blasts and lymphoma cells, or in clinical settings with a high pre-test probability for a hematologic disorder, such as in a patient with a history of hematologic malignancy.
One of the best uses for FCI is to determine whether a lymphocytosis is reactive or represents a lymphoproliferative disorder (LPD). B-cell LPD can be detected by monotypic kappa/lambda expression, and/or by expression of aberrant antigens (eg, CD5, CD123). T-cell LPD can be detected by skewed CD4/CD8 expression, monotypic TRBC1 expression, loss of pan-T cell antigens (eg, CD3, CD7), and aberrant antigen expression (eg, CD10, CD25).
Another indication for PB FCI is the presence of increased peripheral blood blasts, FCI can confirm the presence of lymphoblasts or immunophenotypically aberrant myeloblasts consistent with acute leukemia.
PB FCI may also be helpful in some cases of neutropenia or monocytopenia. FCI can detect T-large granular lymphocytic leukemia or hairy cell leukemia, respectively.
Lymphocyte-variant hypereosinophilic syndrome is a rare cause of hypereosinophilia, but warrants use of PB FCI in the setting of persistently elevated eosinophils.
FCI can be used to monitor the effectiveness of monoclonal antibody therapy for leukemia and lymphoma. For example Rituxmab targets CD20, daclizumab CD 25, and Campath CD 52.
There are, however, other PB abnormalities in which PB FCI may have limited utility. PB FCI cannot distinguish normal mature neutrophils, basophils, red blood cells, or platelets from those arising from a neoplastic clone (with the exception of specialized testing for paroxysmal nocturnal hemoglobinuria). FCI is not indicated for cases of isolated neutrophilia, basophilia, polyclonal hypergammamglobulinemia, thrombocytosis, or erythrocytosis.
Left-shifted neutrophilia with basophilia is highly suggestive of chronic myeloid leukemia. This entity is better diagnosed by detection of BCR::ABL1 gene fusion than FCI.
References
Craig FE, Foon KA, Flow cytometric immunophenotyping for hematologic neoplasms, Blood, 2008;111(8):3941-3967.
Nakashima MO, et al. Peripheral Blood Flow Cytometry Testing, August 27, 2025, https://documents.cap.org/documents/PeripheralBloodFlowCytometry.FullModule.pdf
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