A 65-year-old man with multiple myeloma (IgA-λ type) presents with acute kidney injury (creatinine 3.2 mg/dL). To determine whether light chain cast nephropathy (myeloma kidney) is the cause, which investigation is most specific?
A. Serum free light chain assay with involved/uninvolved ratio
B. Renal biopsy with light microscopy and immunofluorescence
C. Urine dipstick and microscopy
D. 24-hour urine light chain quantification
Explanation
Diagnostic Investigation for Myeloma Kidney (Light Chain Cast Nephropathy)
Key Point
Renal biopsy with light microscopy and immunofluorescence is the gold standard for confirming light chain cast nephropathy (myeloma kidney) and distinguishing it from other causes of acute kidney injury in myeloma.
Pathology of Myeloma Kidney
Light chain cast nephropathy is characterized by:
Intratubular casts composed of monoclonal light chains (λ > κ)
Tubular epithelial cell injury and apoptosis
Interstitial inflammation and fibrosis (chronic cases)
Immunofluorescence shows monotypic light chain (κ or λ only, not both)
Diagnostic Approach to AKI in Myeloma
Table
Investigation
Findings in Myeloma Kidney
Diagnostic Value
Serum free light chain ratio
Elevated involved/uninvolved ratio
Supports diagnosis; not specific
24-hour urine light chains
Elevated (often >1 g/day)
Supportive; not diagnostic
Urine dipstick/microscopy
May show proteinuria, granular casts
Non-specific; casts may not be visible
Renal biopsy (LM + IF)
Intratubular casts + monotypic IF
Gold standard; definitive diagnosis
High-YieldNEET PG
Light chain cast nephropathy accounts for ~50% of AKI in myeloma. Other causes include:
Acute tubular necrosis (ATN) from hypercalcemia, dehydration
Immunofluorescence is crucial because it demonstrates monotypic light chain deposition (only κ or only λ), confirming clonal origin and excluding polyclonal proteinuria.
Warning
Serum and urine light chain assays are elevated in myeloma kidney but are NOT specific—they can be elevated in other kidney diseases and do not prove cast nephropathy. Biopsy is needed to visualize intratubular casts.
