Plasma cell disorders (dyscrasias) encompass a spectrum of clonal plasma cell and B-cell proliferations characterized by the production of a monoclonal immunoglobulin (M-protein, paraprotein). They range from the benign monoclonal gammopathy of undetermined significance (MGUS) to the malignant multiple myeloma. The clinical laboratory is central to screening, diagnosis, and monitoring through serum and urine protein electrophoresis, immunofixation, and serum free light chain assays.

Structure of Immunoglobulins

Immunoglobulins (antibodies) are Y-shaped glycoproteins composed of two heavy chains (γ, α, μ, δ, ε) and two light chains (κ or λ). The five immunoglobulin classes are IgG (70–75%), IgA (15–20%), IgM (5–10%), IgD (< 1%), and IgE (< 0.01%). A monoclonal gammopathy produces a single heavy chain class and a single light chain type (either κ or λ), resulting in a homogeneous (monoclonal) protein band on electrophoresis.

Serum Protein Electrophoresis (SPEP)

SPEP separates serum proteins into five fractions by charge in an alkaline buffer (pH 8.6): albumin (fastest), α₁-globulin, α₂-globulin, β-globulin, and γ-globulin (slowest). Capillary zone electrophoresis (CZE) is an increasingly used alternative to gel-based SPEP, offering automated, high-resolution separation of serum proteins with direct quantification. Normal gamma globulin produces a broad, polyclonal pattern. A monoclonal gammopathy appears as a tall, narrow, discrete peak (M-spike) in the gamma, beta, or occasionally alpha-2 region. The M-spike is quantified by densitometry. The size of the M-spike correlates with tumor burden but does not always reflect disease activity (in IgM monoclonal gammopathies, the M-spike overestimates concentration due to pentamer size).

Immunofixation Electrophoresis (IFE)

IFE is the gold standard for confirming and typing monoclonal proteins. Following SPEP, the serum proteins are separated by agarose gel electrophoresis, then overlaid with monospecific antibodies to IgG, IgA, IgM, κ, and λ light chains. A monoclonal protein appears as a sharp, localized band in one or more heavy chain lanes and one light chain lane (κ or λ, not both). IFE is more sensitive than SPEP (detection limit ~100 mg/L vs ~500 mg/L) and is essential for typing biclonal gammopathies and confirming the nature of small or obscured bands. Urine immunofixation (UPEP with IFE) detects Bence Jones protein (free monoclonal light chains) in concentrated urine.

Serum Free Light Chain (FLC) Assay

Serum free light chain (FLC) measurement by nephelometry or turbidimetry quantifies free κ and λ light chains not bound to heavy chains. The κ/λ FLC ratio is a sensitive marker for clonal light chain production. A normal ratio (0.26–1.65) indicates polyclonal production. An abnormal ratio indicates monoclonality: elevated κ/λ > 1.65 suggests monoclonal κ production, and decreased κ/λ < 0.26 suggests monoclonal λ production. The involved/uninvolved FLC difference (dFLC) correlates with disease activity and is used for response assessment. FLC is particularly valuable for diagnosing light chain myeloma (no heavy chain M-protein), AL amyloidosis, and achieving CR criteria in multiple myeloma.

Monoclonal Gammopathy of Undetermined Significance (MGUS)

MGUS is defined by serum M-protein < 3 g/dL, bone marrow plasma cells < 10%, and absence of end-organ damage (CRAB: hypercalcemia, renal dysfunction, anemia, bone lesions). MGUS is present in 3–4% of the population over age 50 and 6–7% over age 80. It is a premalignant condition with a 1% per year risk of progression to multiple myeloma or other lymphoproliferative disorder. Risk stratification for progression uses the Mayo Clinic model: M-protein ≥ 1.5 g/dL, non-IgG isotype (IgA or IgM), and abnormal FLC ratio. Monitoring with serum protein studies every 6–12 months is recommended.

Multiple Myeloma

Multiple myeloma (MM) is a malignant plasma cell neoplasm accounting for 1–2% of all cancers and 10–15% of hematologic malignancies (median age 69 years). Diagnostic criteria require ≥ 10% clonal bone marrow plasma cells (or biopsy-proven plasmacytoma) plus one or more myeloma-defining events: CRAB features (hypercalcemia, renal insufficiency, anemia, bone lesions), or biomarkers of malignancy (≥ 60% bone marrow plasma cells, involved/uninvolved FLC ratio ≥ 100, or > 1 focal bone lesion on MRI). The CBC often shows normocytic anemia, with rouleaux on blood smear (RBC stacking). The WBC differential may show mild neutropenia or lymphopenia. The platelet count may be decreased. Bone marrow aspirate shows increased plasma cells (≥ 10%), some with abnormal morphology (immature, binucleate, flame cells, Mott cells). Flow cytometry: CD38+, CD138+, CD56+, CD19−, CD45 (dim). Cytogenetics stratifies risk: hyperdiploidy and t(11;14) are standard risk; del(17p), t(4;14), t(14;16), and t(14;20) are high risk.

Laboratory Monitoring in Multiple Myeloma

Response assessment uses the International Myeloma Working Group (IMWG) criteria based on serum and urine M-protein, FLC ratio, and bone marrow plasma cell percentage. Complete response (CR) requires negative IFE of serum and urine, FLC ratio normal, and < 5% bone marrow plasma cells. Stringent complete response (sCR) additionally requires normal FLC ratio and absence of clonal plasma cells by flow cytometry or immunohistochemistry. Serum β₂-microglobulin (β₂M) and albumin are used for ISS/R-ISS staging.

Waldenström Macroglobulinemia

Waldenström macroglobulinemia (WM) is a lymphoplasmacytic lymphoma with IgM monoclonal gammopathy, characterized by bone marrow infiltration and IgM-associated symptoms. Unlike multiple myeloma, it is a B-cell disorder with MYD88 L265P mutation in over 90% of cases. The IgM M-protein can cause hyperviscosity syndrome (symptomatic at > 40–60 g/L), presenting with headache, blurred vision, mucosal bleeding, and retinopathy. Peripheral neuropathy (anti-MAG antibodies) and cold agglutinin disease may occur. Treatment includes rituximab, BTK inhibitors (ibrutinib), and BCL2 inhibitors (venetoclax).

AL Amyloidosis

Immunoglobulin light chain (AL) amyloidosis is caused by deposition of misfolded monoclonal light chains as amyloid fibrils in tissues, leading to progressive organ dysfunction. The most commonly involved organs are the heart (restrictive cardiomyopathy, heart failure, arrhythmias), kidneys (nephrotic syndrome, renal failure), liver, peripheral nerves, and soft tissues. Diagnosis requires tissue biopsy (abdominal fat pad, bone marrow, or affected organ) with Congo red staining showing apple-green birefringence under polarized light, and demonstration of clonal light chain deposition by mass spectrometry. Serum FLC assay is the most sensitive screening test; abnormal κ/λ ratio is present in over 90% of cases. Chemotherapy with bortezomib-based regimens aims to suppress the underlying clonal plasma cell disorder.