What blood markers are used to diagnose amyloidosis, specifically light-chain (SPEP) (Serum Protein Electrophoresis) and free light chain assays?

Blood Markers for Amyloidosis Diagnosis

The essential blood markers for diagnosing amyloidosis are serum free light chains (sFLC) with kappa/lambda ratio, serum immunofixation electrophoresis (SIFE), and urine immunofixation electrophoresis (UIFE)—this triad is mandatory and achieves 100% sensitivity for detecting the amyloidogenic light chain. 1, 2

Primary Screening Panel (Required for All Suspected Cases)

The American College of Cardiology and American Heart Association mandate that every patient with suspected cardiac amyloidosis must undergo a complete monoclonal protein screen before any other diagnostic testing, including nuclear scintigraphy. 1

The complete screening panel includes:

• Serum free light chain (sFLC) assay measuring kappa and lambda independently with kappa/lambda ratio 1
• Serum immunofixation electrophoresis (SIFE) to identify the specific immunoglobulin type (IgG, IgA, IgM) and light chain (kappa or lambda) 1, 2
• Urine immunofixation electrophoresis (UIFE) from a 24-hour urine collection 1, 2

Why This Combination Is Critical

Serum protein electrophoresis (SPEP) alone is inadequate and should NOT be used to exclude AL amyloidosis, as it fails to show a monoclonal spike in nearly 50% of AL amyloidosis cases due to the low levels of monoclonal protein produced. 1 This is a critical diagnostic pitfall—SPEP has insufficient sensitivity compared to immunofixation. 1

The combination of SIFE, UIFE, and sFLC ratio achieves 100% sensitivity for identifying the amyloidogenic light chain, whereas using only SIFE and UIFE detects 96% of cases, and the sFLC ratio alone detects only 76-79.5% of cases. 1, 3, 2

Interpreting Serum Free Light Chain Results

An abnormal kappa/lambda ratio indicates clonality:

• Elevated ratio (>1.65) suggests kappa light chain clonality 1
• Decreased ratio (<0.26) suggests lambda light chain clonality 1

Important caveats about sFLC interpretation:

• Renal impairment significantly affects sFLC levels and the normal reference range shifts to 0.34-3.10 in severe renal impairment (CKD stage 5 or greater), though even mild renal dysfunction impairs clearance 1
• Two major commercial assays exist (FreeLite and N Latex) with mathematically inconvertible results and different responses to renal impairment—the same assay must be used consistently for monitoring 1, 4
• If one assay is negative, checking with the alternative assay may be necessary given different performance characteristics 1
• An abnormal sFLC is found in 97% of AL amyloidosis patients at diagnosis 3, 5

Additional Blood Markers for Monitoring and Prognosis

Once AL amyloidosis is diagnosed, additional blood markers guide treatment monitoring:

• NT-proBNP for cardiac involvement assessment and monitoring cardiac response (cardiac response defined as >30% decrease and <300 ng/L if baseline >650 ng/L) 6, 7
• Difference in involved and uninvolved free light chains (dFLC) to assess hematologic response to treatment 6
• Complete response is defined as negative serum and urine immunofixation with normal FLC ratio 6

Diagnostic Algorithm Summary

1. First step: Obtain sFLC with kappa/lambda ratio, SIFE, and UIFE simultaneously 1
2. If monoclonal protein detected: Urgent hematology-oncology referral and consider tissue biopsy (cardiac, abdominal fat, or bone marrow) 1, 6
3. If no monoclonal protein detected: Proceed to bone scintigraphy (99mTc-PYP) to evaluate for ATTR amyloidosis 1, 7
4. Never perform nuclear scintigraphy without first completing the monoclonal protein screen, as >10% of AL amyloidosis cases show cardiac uptake mimicking ATTR amyloidosis 1

Common Pitfalls to Avoid

• Do not rely on SPEP/UPEP alone—these have lower sensitivity than immunofixation and will miss cases 1
• Do not interpret a nuclear scan without concomitant monoclonal protein screening—this is neither appropriate nor valid for distinguishing ATTR from AL amyloidosis 1
• Do not use urinary light chain assays to quantify Bence Jones protein—these are not validated; use urine protein electrophoresis instead 1
• Ensure laboratory consistency—know which sFLC assay your laboratory uses and use the same assay for serial monitoring 1