Initial Diagnostic Testing for Amyloidosis
The initial diagnostic workup for amyloidosis requires tissue biopsy with Congo red staining to confirm amyloid deposits, followed immediately by mass spectrometry-based typing to identify the precursor protein, combined with screening for monoclonal proteins via serum free light chains and immunofixation electrophoresis of both serum and urine. 1, 2, 3
Core Diagnostic Algorithm
Step 1: Tissue Confirmation of Amyloid Deposits
Obtain tissue biopsy and perform Congo red staining to demonstrate characteristic apple-green birefringence under polarized light, which is the gold standard for confirming amyloidosis. 3
Biopsy site selection depends on suspected amyloid subtype:
For suspected AL (light chain) amyloidosis: Start with less invasive sites first 1, 2
For suspected ATTR (transthyretin) amyloidosis: Fat pad biopsy has unacceptably low sensitivity (15% for wild-type, 45% for hereditary), so consider endomyocardial biopsy or nuclear imaging instead 2, 3
Endomyocardial biopsy has approximately 100% sensitivity and specificity for cardiac amyloid and should be performed when non-invasive sites are negative but clinical suspicion remains high 2, 3
Step 2: Amyloid Typing (Critical for Treatment Selection)
Mass spectrometry (LC-MS/MS) is the gold standard for identifying the precursor protein with 88% sensitivity and 96% specificity. 2, 3 This step is non-negotiable because treatment differs completely between AL and ATTR amyloidosis, and misdiagnosis leads to inappropriate therapy and patient harm. 1, 2
If LC-MS/MS is unavailable locally, transfer Congo red-positive samples to an experienced reference laboratory for definitive typing. 2
Immunohistochemistry can be used in experienced centers but is less reliable than mass spectrometry. 2, 3
Step 3: Monoclonal Protein Screening (Essential for All Patients)
All patients must undergo comprehensive monoclonal protein assessment regardless of suspected subtype, as approximately 5% of patients over age 70 have MGUS, and over 10% of patients with monoclonal gammopathy can have ATTR deposits. 1, 2
Required tests include: 1, 2, 3
- Serum free light chain (sFLC) assay
- Serum immunofixation electrophoresis (SIFE)
- Urine immunofixation electrophoresis (UIFE)
Critical pitfall: Do NOT rely on serum/urine protein electrophoresis (SPEP/UPEP) alone, as these have inadequate sensitivity for AL amyloidosis, which typically produces low levels of monoclonal protein. 1
Step 4: Subtype-Specific Additional Testing
For AL amyloidosis (if monoclonal protein detected):
- Bone marrow biopsy to demonstrate clonal plasma cells 1
- Collaborate with hematology to exclude multiple myeloma or B-cell lymphoproliferative disorders 1, 2
For ATTR amyloidosis:
- DNA mutational analysis (genetic testing) to differentiate hereditary (variant) from wild-type (senile) ATTR 2
- Nuclear scintigraphy with 99mTc-PYP/DPD/HMDP can diagnose ATTR cardiac amyloidosis non-invasively IF no monoclonal protein is detected 1, 2
- If ANY monoclonal protein is present (even MGUS), endomyocardial biopsy is mandatory to definitively distinguish AL from ATTR cardiac amyloidosis, as bone scintigraphy alone is insufficient 2
Step 5: Comprehensive Organ Assessment
Systematically evaluate all potentially affected organs to determine disease extent, treatment strategy, and prognosis: 1, 2
- Cardiac evaluation: Echocardiography in all patients; look for LV wall thickness >12 mm, relative apical sparing (strain ratio >1), and ≥Grade 2 diastolic dysfunction 3
- Renal assessment: Measure proteinuria and renal function 1, 2
- Neurologic examination: Assess for peripheral neuropathy and autonomic dysfunction 1, 3
- Laboratory workup: CBC with differential, BUN, creatinine, electrolytes 1
Clinical Red Flags That Should Trigger Amyloidosis Workup
History and examination clues: 1, 3
- Bilateral carpal tunnel syndrome
- Heart failure with preserved ejection fraction (HFpEF), particularly in men
- Unexplained peripheral neuropathy with autonomic dysfunction (postural hypotension, alternating bowel pattern)
- Macroglossia
- Unexplained proteinuria
- Hepatomegaly with mildly elevated alkaline phosphatase
- Biceps tendon rupture
- Lumbar spinal stenosis
- Periorbital ecchymoses or acquired factor X deficiency with coagulopathy
Imaging clues: 1
- Concentric LV wall thickening with low QRS voltage on ECG
- Restrictive LV filling with RV wall thickening
- Diffuse subendocardial or transmural late gadolinium enhancement on cardiac MRI
- Apical sparing on longitudinal strain imaging
Critical Pitfalls to Avoid
Never assume AL amyloidosis based solely on monoclonal protein presence without tissue typing, as ATTR and AL can coexist, and over 10% of patients with monoclonal gammopathy have ATTR deposits. 1, 2
Do not use fat pad biopsy as the sole diagnostic test for suspected ATTR cardiac amyloidosis due to its poor sensitivity (15% for wild-type). 2
In cases of suspected concomitant AL and ATTR cardiac amyloidosis, cardiac biopsy is the preferred route to definitively establish the cardiac pathology. 1, 2
Avoid gadolinium-based contrast in patients with estimated GFR <30 ml/min/1.73 m² due to risk of nephrogenic systemic fibrosis. 3