Serum Free Light Chain Assay and Serum Immunofixation in AL Amyloidosis
Both serum free light chain (FLC) assay and serum immunofixation electrophoresis (IFE) are essential and complementary tests that must be performed together—along with urine IFE—to reliably detect the plasma cell dyscrasia in AL amyloidosis, achieving near-perfect sensitivity when combined.
Diagnostic Role: Initial Evaluation
The diagnosis of AL amyloidosis requires two components: (1) demonstration of tissue amyloid deposits and (2) evidence of a plasma cell dyscrasia 1. The combination of serum FLC assay, serum IFE, and urine IFE serves to identify the monoclonal protein that confirms the plasma cell dyscrasia.
Performance of Individual Tests
When used alone, these tests have important limitations:
- Serum IFE alone: 68% sensitivity 2
- Urine IFE alone: 77% sensitivity 2
- Serum FLC ratio alone: 82% sensitivity 2
- Serum + Urine IFE combined: 89% sensitivity 2
However, when all three tests are combined (serum IFE + urine IFE + serum FLC ratio), sensitivity reaches 99-100% 2, 3. This near-perfect detection rate makes the combination mandatory for excluding AL amyloidosis.
Critical Clinical Algorithm
When evaluating suspected cardiac amyloidosis:
Always order all three tests simultaneously: serum IFE, urine IFE, and serum FLC assay before proceeding to Tc-PYP/DPD/HMDP nuclear imaging 4
If any test is abnormal or equivocal, immediately involve a hematologist to distinguish between:
- Spurious findings from kidney dysfunction
- True MGUS (monoclonal gammopathy of undetermined significance)
- AL amyloidosis
- Multiple myeloma 1
Do not rely on a single test—the identification of amyloidogenic light chains cannot be made with one assay alone 3
Important Caveats and Pitfalls
Kidney Dysfunction Effects
Renal impairment can cause disproportionate urinary loss of lambda light chains, potentially creating abnormal FLC ratios that are spurious rather than indicative of a plasma cell dyscrasia 1. Critically, in these cases, IFE will not demonstrate a monoclonal protein, which helps distinguish true disease from artifact.
FLC Assay Non-Interchangeability
A major pitfall: Different commercial FLC assays (Freelite, N-Latex, Sebia) are not interchangeable and require method-specific reference intervals for the κ/λ ratio 5, 6. The Sebia assay, for example, reports lower involved FLC concentrations and has reduced detection rates compared to Freelite and N-Latex, particularly in λ-type disease 6.
Practical guidance: Select one FLC method and use it consistently for the same patient, always applying that assay's validated reference intervals, preferably performed in the same laboratory 5.
Monitoring Role
Once AL amyloidosis is diagnosed, serum FLC assay plays a central role in monitoring treatment response and disease progression 7. The difference between involved and uninvolved FLC (dFLC) and the FLC ratio track the burden of the plasma cell clone producing the amyloidogenic light chains.
Integration with Tissue Diagnosis
Even when monoclonal protein testing is positive, tissue confirmation remains mandatory. Over 10% of patients with a monoclonal gammopathy can have ATTR (transthyretin) deposits rather than AL deposits 1. Mass spectrometry (LC-MS/MS) of tissue is the gold standard for definitively identifying the amyloid precursor protein, with 88% sensitivity and 96% specificity 1.
Prognostic Value
Serum FLC measurements provide prognostic information in AL amyloidosis, with higher involved FLC concentrations and greater dFLC associated with worse outcomes 7. This makes the FLC assay valuable not only for diagnosis but also for risk stratification.