Serum Biomarkers in Plasma Cell Proliferative Disorders

#hemato #nonrad

Overview

Plasma cell proliferative disorders produce monoclonal immunoglobulins or fragments, which can be detected in serum and/or urine as abnormal proteins. These serve as essential diagnostic, monitoring, and prognostic biomarkers.

Key Serum & Urine Markers

Marker	Description	Where to Test	Clinical Use
M protein	Monoclonal immunoglobulin or fragment	SPEP / UPEP	Screening, diagnosis, monitoring
Free Light Chains (FLC)	Unbound κ or λ light chains	Serum FLC assay	Detects light chain myeloma, MRD tracking
Bence-Jones Protein (BJP)	Free κ or λ light chains filtered into urine	UPEP + IFE	Urine correlate of FLC, confirms excretion
κ/λ ratio (FLC ratio)	Involved/uninvolved light chain ratio	Serum FLC assay	Myeloma-defining biomarker

Reference Ranges & Diagnostic Thresholds

Normal κ/λ ratio: 0.26 – 1.65
(slight lab variation tolerated)

Myeloma-defining abnormalities:

FLC ratio ≥100 or ≤0.01 + clonal plasma cells ≥10% → diagnostic of active myeloma
M protein ≥3 g/dL + plasma cells 10–60% → suggests Smoldering myeloma

Clinical Scenarios

Disease	Common Pattern
Multiple myeloma	High M protein ± abnormal FLC ratio ± BJP
Light Chain Myeloma	No M spike, but abnormal FLC ratio and BJP positive
Nonsecretory Myeloma	No M protein or BJP; abnormal marrow biopsy
MGUS	Low M protein (<3 g/dL), no CRAB, normal FLC or mild skew
AL Amyloidosis	Often subtle M protein, markedly abnormal FLC ratio, low burden on SPEP

Linked Notes: