How to Diagnose Amyloidosis
The diagnosis of amyloidosis requires tissue biopsy demonstrating Congo red-positive deposits with apple-green birefringence under polarized light, followed by mandatory amyloid typing via immunohistochemistry or mass spectrometry to determine the specific protein type (AL vs ATTR vs other). 1, 2
Initial Clinical Suspicion: Red Flag Symptoms
Suspect amyloidosis when patients present with:
Cardiac manifestations:
- Unexplained heart failure with preserved ejection fraction
- Left ventricular wall thickness >12 mm without other cause
- Elevated NT-proBNP (≥332 ng/L has >99% sensitivity for cardiac involvement in AL)
- Elevated troponins 1
Systemic manifestations:
- Nephrotic syndrome
- Hepatic dysfunction
- Peripheral or autonomic neuropathy
- "Atypical" MGUS or smoldering myeloma presentation 3
Key point: Amyloidosis presents with multisystemic symptoms, and delayed diagnosis is a major obstacle to effective treatment. Early recognition before end-stage organ failure is critical for survival. 3, 4
Diagnostic Algorithm
Step 1: Screen for Plasma Cell Disorder (for AL amyloidosis)
Obtain all three tests simultaneously 2:
- Serum immunofixation
- Urine immunofixation
- Serum free light chains (FLC) with kappa/lambda ratio
- Normal FLC ratio: 0.26-1.65 (Binding Site) or 0.31-1.56 (Siemens) in normal renal function
- Adjusted to 0.31-3.7 in renal failure 1
Step 2: Obtain Tissue Biopsy
For AL amyloidosis: Organ biopsy is NOT required in 85% of patients 3. Surrogate site biopsies are highly effective and preferred 2:
- Abdominal fat pad aspiration (first-line, least invasive)
- Bone marrow biopsy (should be done regardless for plasma cell assessment)
- Minor salivary gland biopsy
Target organ biopsy (kidney, heart, liver, nerve) may be favored when:
- Surrogate biopsies are negative but clinical suspicion remains high
- Specific organ pathology assessment is needed 2
For ATTR-CM: Cardiac scintigraphy with bone tracers can provide non-invasive diagnosis when combined with absence of monoclonal protein (see Step 4) 5
Step 3: Confirm Amyloid Deposits
Tissue must demonstrate:
- Congo red staining with characteristic apple-green birefringence under polarized light
- Rigid, nonbranching 10-12 nm width fibrils on electron microscopy (rarely performed) 1
Step 4: Mandatory Amyloid Typing
This step is absolutely critical and cannot be skipped 2, 3:
Gold standard: Laser capture mass spectrometry 3
Alternative: Immunohistochemistry (less accurate but widely available)
For ATTR-CM specifically: If cardiac scintigraphy shows grade 2-3 uptake AND monoclonal protein screening is negative, ATTR-CM can be diagnosed non-invasively without biopsy 5
Step 5: Determine AL vs ATTR vs Other Types
AL amyloidosis:
- Positive monoclonal protein on screening
- Immunohistochemistry/mass spec confirms light chain composition
- <10% plasma cells in bone marrow (distinguishes from myeloma) 1
ATTR amyloidosis:
- Negative monoclonal protein screening
- Positive cardiac scintigraphy (for ATTR-CM)
- Mandatory TTR genotyping to distinguish:
Step 6: Stage the Disease
For AL amyloidosis - Four-stage system based on 3:
- NT-proBNP (or BNP)
- Troponin T or I
- Difference between involved and uninvolved FLC (dFLC)
5-year survival by stage:
- Stage I: 82%
- Stage II: 62%
- Stage III: 34%
- Stage IV: 20%
Critical Pitfalls to Avoid
Never assume amyloid type without confirmation - Treatment differs dramatically between AL and ATTR, making typing mandatory 2, 3
Don't skip monoclonal protein screening in suspected ATTR - Up to 5% of patients may have concurrent MGUS, which changes management 5
Don't delay biopsy in high-risk populations:
Recognize that surrogate biopsies are sufficient - Avoid unnecessary invasive organ biopsies in most AL cases 2
Early diagnosis is paramount - Patients diagnosed in early stages have 80% 5-year survival vs <30% in advanced disease; treatment is most effective before significant end-organ dysfunction 3, 7