Peripheral blood and bone marrow immunophenotyping (know indications for flow cytometry, and the basic immunophenotypes of haematological cancers)

Role

Flow cytometry immunophenotyping identifies and characterises cell populations in peripheral blood or bone marrow by their surface/intracellular marker expression - essential for diagnosing and subtyping leukaemias and lymphomas, and for detecting minimal/measurable residual disease
A routine, high-yield component of haematological malignancy workup, performed alongside morphology and cytogenetics for full modern classification

Cells are labelled with fluorescently-tagged antibodies against specific surface/intracellular antigens (CD markers) and passed through a laser-based detection system, generating a multi-parameter profile of the cell population - the specific combination of markers expressed (or absent) defines the cell lineage and, often, the specific malignancy subtype (e.g. CD5+CD23+ supporting CLL, CD5+CD23-negative supporting mantle cell lymphoma)

Distinguishes lineage (myeloid vs lymphoid, B cell vs T cell) and identifies aberrant/clonal marker expression patterns characteristic of malignancy, distinguishing a malignant clonal population from a normal polyclonal immune response
Minimal/measurable residual disease (MRD) testing uses highly sensitive flow cytometry (or molecular methods) to detect very low levels of residual malignant cells after treatment, below the detection threshold of standard morphology - increasingly used to refine prognosis and guide treatment intensity/duration decisions in several leukaemias

Immunophenotype directly informs subtype-specific classification and, increasingly, targeted therapy selection (e.g. CD20 expression determining rituximab eligibility, CD30 expression determining brentuximab vedotin eligibility)
MRD status after treatment is an increasingly important prognostic and treatment-guiding marker in several leukaemias (e.g. ALL, CLL), with MRD-negative status generally associated with better long-term outcomes

Acute leukaemia lineage determination (myeloid vs lymphoid), CLL/mantle cell lymphoma distinction, minimal residual disease monitoring, targeted therapy marker identification (CD20, CD30, CD33)

Flow cytometry's ability to detect residual disease far below the sensitivity of standard morphology has meaningfully changed treatment paradigms in several leukaemias, where MRD status now directly influences treatment duration/intensity decisions rather than morphological remission alone being the endpoint

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