What is the diagnostic approach for amyloidosis?

Diagnostic Approach for Amyloidosis

The diagnosis of amyloidosis requires tissue biopsy with Congo red staining showing apple-green birefringence under polarized microscopy, followed by mandatory amyloid typing using mass spectrometry (gold standard with 88% sensitivity and 96% specificity), and comprehensive organ assessment to determine extent of involvement. 1, 2

Step 1: Clinical Suspicion and Red Flags

Suspect amyloidosis when patients present with specific clinical constellations:

• Heart failure with preserved ejection fraction (HFpEF) with unexplained left ventricular wall thickness ≥12 mm 1, 3
• Nephrotic-range proteinuria without other clear etiology 1
• Bilateral carpal tunnel syndrome without rheumatoid arthritis or trauma 3
• Macroglossia, periorbital ecchymoses (pathognomonic for AL), or organomegaly (hepatomegaly, enlarged salivary glands) 1, 4
• Peripheral neuropathy with constitutional symptoms (weight loss, fatigue) 1
• Low QRS voltage on ECG despite ventricular wall thickening (voltage-to-mass discordance) 1, 3

Step 2: Initial Laboratory Screening

Perform all three monoclonal protein tests simultaneously—never rely on SPEP/UPEP alone:

• Serum free light chain (sFLC) assay 1, 4
• Serum immunofixation electrophoresis (SIFE) 1, 2
• Urine immunofixation electrophoresis (UIFE) 1, 2
• NT-proBNP or BNP levels (sensitivity 93%, specificity 90% for cardiac involvement; disproportionately elevated relative to degree of heart failure) 1, 3

Step 3: Tissue Biopsy for Definitive Diagnosis

Biopsy is mandatory and cannot be bypassed. 4 Choose the most accessible site:

• Abdominal fat pad aspiration (84% sensitivity for AL amyloidosis, least invasive) 1, 2, 4
• Bone marrow biopsy (required to demonstrate clonal plasma cell proliferation in AL amyloidosis) 1, 2, 4
• Gingival, rectal, or salivary gland biopsy (particularly relevant with facial involvement) 1, 4
• Endomyocardial biopsy (definitive for cardiac amyloidosis when non-invasive methods are equivocal) 1, 3
• Affected organ biopsy (heart, liver, kidney) when other sites are negative 1

Histological confirmation requires:

• Congo red staining showing apple-green birefringence under polarized microscopy 1, 2, 3
• Electron microscopy can provide additional confirmation 1

Step 4: Amyloid Typing (Critical—Do Not Skip)

Mass spectrometry (LC-MS/MS) is the gold standard with 88% sensitivity and 96% specificity 2, 4, 3. Alternative methods include:

• Immunohistochemistry with antibodies against amyloid A, κ- and λ-light chains, and TTR 1
• Immunofluorescence or immunogold electron microscopy (less sensitive/specific than mass spectrometry) 1

Critical distinction: Over 10% of patients with monoclonal gammopathy can have ATTR deposits, not AL amyloidosis 2

Step 5: Subtype Differentiation

For AL Amyloidosis:

• Presence of serum or urine monoclonal gammopathy 1
• Clonal plasma cells in bone marrow 1, 2

For TTR Amyloidosis:

• DNA mutational analysis to differentiate hereditary from wild-type (senile) ATTR 1
• Technetium-99m pyrophosphate (PYP) scintigraphy can distinguish AL from TTR amyloidosis (grade 2-3 cardiac uptake suggests ATTR) 1, 3
• Must confirm absence of monoclonal protein before diagnosing ATTR based on nuclear imaging 3

Step 6: Organ Involvement Assessment

Once amyloidosis is confirmed, identify all affected organs:

Cardiac Evaluation:

• Echocardiography (mandatory in all suspected cardiac amyloidosis or systemic amyloidosis with heart failure) 1, 3
  • Look for: LV wall thickness >12 mm, small cavity size, biatrial enlargement 3
• Cardiac MRI when echocardiography is suggestive but not definitive 3
  • Diagnostic features: LV wall thickness > upper limit of normal, global ECV >0.40, diffuse late gadolinium enhancement 3

Other Organs:

• Renal function tests and urinalysis (proteinuria assessment) 1, 3
• Liver function tests 1, 3
• Neurological evaluation if peripheral neuropathy present 1, 3

Step 7: Risk Stratification

Use cardiac biomarkers for staging (Mayo 2004, Mayo 2012, European 2015, or Boston University models):

• Troponin T (TnT) threshold <0.035 mcg/L 1
• NT-proBNP threshold <332 ng/L 1
• Stage I: both low; Stage II: one elevated; Stage III: both elevated 1

Critical Pitfalls to Avoid

• Do not assume facial droop is stroke—consider soft tissue infiltration with macroglossia or periorbital purpura 4
• Do not diagnose based on monoclonal gammopathy alone—requires tissue confirmation and typing 1, 3
• Do not use SPEP/UPEP alone—requires all three tests (sFLC, SIFE, UIFE) simultaneously 4
• Do not skip amyloid typing—treatment differs completely between AL (chemotherapy) and ATTR (targeted therapy like tafamidis) 1
• Do not assume ATTR based on nuclear imaging alone—must exclude monoclonal protein first 3
• Do not use digoxin or calcium channel blockers in suspected cardiac amyloidosis (bind to amyloid fibrils causing toxicity) 3

Localized vs. Systemic Amyloidosis

For localized amyloidosis, confirm:

• Negative SIFE and UIFE 2
• Absence of clonal plasma cells in bone marrow 2
• Negative abdominal fat aspiration 2
• No clinical/laboratory evidence of other organ involvement 2