What does the presence of markers such as CD138, along with an abnormal Kappa (kappa light chain) to Lambda (lambda light chain) light chain ratio, indicate in the context of multiple myeloma?

Flow Cytometry Markers in Multiple Myeloma: Diagnostic Interpretation

The presence of CD138-positive plasma cells with an abnormal kappa to lambda light chain ratio (typically <0.26 or >1.65) confirms the diagnosis of a clonal plasma cell disorder, most commonly multiple myeloma when accompanied by ≥10% bone marrow plasma cells or other diagnostic criteria. 1, 2

Essential Diagnostic Framework

Primary Plasma Cell Identification

The comprehensive marker panel you've listed allows for systematic evaluation through a hierarchical approach:

Initial gating strategy:

• CD38 and CD138 serve as the primary markers for plasma cell identification, as these enable unequivocal identification of plasma cells among other hematopoietic cells 1, 2, 3
• CD45 should be included in the initial gating to distinguish plasma cells from other leukocytes 2, 3
• CD38 expression should be bright/intense in normal plasma cells, while decreased CD38 expression suggests aberrancy 1

Clonality Assessment: The Critical Determinant

Kappa and Lambda light chain ratio interpretation:

• A normal kappa:lambda ratio ranges from 0.26 to 1.65 in serum free light chains 1
• For bone marrow immunohistochemistry, ratios ≤1/7 or ≥9 indicate light chain restriction with the highest diagnostic accuracy 4
• More stringent historical cutoffs of ≤1/16 or ≥16 have slightly lower accuracy but remain clinically valid 4
• An abnormal ratio definitively establishes monoclonality and excludes reactive plasmacytosis 1, 2

Critical caveat: Extremely rare cases of dual kappa/lambda expression exist (<10 reported cases), which may show seemingly polytypic results but are actually monoclonal 5, 6. These cases typically present as light chain-only myeloma with high-risk cytogenetics and require additional investigation including cyclin D1 expression and FISH studies 6.

Phenotypic Aberrancy Markers

The extensive panel you've listed allows identification of aberrant plasma cells, which strengthens the diagnosis beyond clonality alone:

Core aberrancy markers (minimal panel):

• CD19 negativity: Present in 95% of myeloma cases (normal plasma cells are CD19-positive) 1, 3
• CD56 positivity: Found in 75% of myeloma cases (normal plasma cells are CD56-negative) 1, 3

Extended aberrancy markers (preferred panel):

• CD117 positivity: Seen in 30% of cases, absent in normal plasma cells 1, 3
• CD20 positivity: Present in 30% of cases, typically negative in normal plasma cells 1, 3
• CD28 strong positivity: Found in 15-45% of cases, normally negative or weak 1
• CD45 negativity or decreased expression: Aberrant pattern compared to normal plasma cells 1

Prognostic and Risk Stratification Markers

The ratio of aberrant to normal plasma cells predicts progression risk:

• ≥95% phenotypically aberrant plasma cells of total bone marrow plasma cells indicates significantly higher risk of progression from MGUS or smoldering myeloma to active myeloma 1, 2
• This ratio is an independent predictor of progression on multivariate analysis 1

Individual marker prognostic significance:

• CD56 expression correlates with specific genetic abnormalities 1
• CD45 expression pattern serves as a highly significant prognostic factor 1
• Low CD138 expression indicates immature phenotype, worse overall survival, and reduced sensitivity to lenalidomide 7

Additional Markers in Your Panel

Myeloid markers (CD11b, CD11c, CD13, CD14, CD16, CD33, CD64):

• These are typically negative in plasma cell myeloma and help exclude acute myeloid leukemia or other myeloid malignancies 1

CD34:

• Negative in mature plasma cells; positivity would suggest immature precursor involvement or alternative diagnosis 1

CD10:

• Not typically expressed in plasma cell disorders; helps exclude lymphoblastic processes 1

HLA-DR:

• Variable expression; not a primary diagnostic marker for myeloma 1

7-AAD:

• Viability marker to exclude dead cells from analysis 1

Diagnostic Algorithm

Step 1: Confirm plasma cell population using CD38+/CD138+/CD45-dim pattern 2, 3

Step 2: Assess clonality via kappa:lambda ratio - abnormal ratio (outside 0.26-1.65 or using stricter cutoffs) establishes monoclonality 1, 4

Step 3: Evaluate aberrant phenotype using CD19 (should be negative) and CD56 (should be positive) as minimal markers 1, 3

Step 4: Calculate percentage of aberrant plasma cells - ≥95% aberrant indicates high-risk disease 1

Step 5: Assess extended markers (CD117, CD20, CD28) for additional prognostic information and therapeutic target identification 1, 3

Critical Pitfalls to Avoid

Sample quality issues:

• Discrepancies between flow cytometry and morphology primarily reflect sample quality rather than technique limitations 1, 8
• First-pull bone marrow aspirate provides optimal results 8
• Samples should be washed twice in buffered saline before cytoplasmic immunoglobulin assessment to remove cytophilic immunoglobulin 3

Interpretation errors:

• Presence of CD19+ polyclonal plasma cells alongside CD19- monoclonal cells indicates residual disease, not remission 1
• CD19-negative plasma cells exist in normal individuals; both CD19+ and CD19- fractions must be assessed for clonality 1
• At least 100 neoplastic plasma cell events must be acquired for accurate enumeration 3

Rare dual expression cases:

• If seemingly polytypic results occur with clinical suspicion of myeloma, perform double immunohistochemistry to assess co-expression in individual cells 6
• Check for cyclin D1 overexpression and t(11;14) translocation, which are common in dual-expressing cases 6