What is the diagnostic approach for a patient suspected of having amyloidosis?

Diagnostic Approach for Amyloidosis

The diagnosis of amyloidosis requires tissue biopsy with Congo red staining demonstrating apple-green birefringence under polarized light microscopy, followed by mandatory amyloid typing via mass spectrometry or immunohistochemistry to guide treatment. 1, 2

Step 1: Establish Clinical Suspicion

Look for specific red flag combinations that should trigger diagnostic workup:

Cardiac Manifestations

• Left ventricular wall thickness ≥12-14 mm with low QRS voltage on ECG (voltage-to-mass discordance) 3
• Heart failure with preserved ejection fraction (HFpEF) with unexplained LV hypertrophy 3
• Bilateral carpal tunnel syndrome without rheumatoid arthritis or trauma 3
• Spontaneous biceps tendon rupture or lumbar spinal stenosis 3

Renal Manifestations

• Nephrotic syndrome with proteinuria >0.5 g/24 hours 4
• Unexplained renal dysfunction with hepatomegaly and elevated alkaline phosphatase 1

Neurologic Manifestations

• Distal sensory loss with reduced/absent reflexes in a length-dependent pattern 5
• Autonomic dysfunction: orthostatic hypotension, alternating diarrhea/constipation, urinary retention 5
• Peripheral neuropathy progressing more rapidly than typical diabetic neuropathy 5

Other Systemic Features

• Macroglossia and periorbital purpura (highly specific but insensitive for AL amyloidosis) 6
• Unintentional weight loss with early satiety 1

Step 2: Initial Laboratory Screening

Before any biopsy, perform monoclonal protein screening to differentiate AL from other types: 1, 3

• Serum and urine immunofixation electrophoresis 3
• Serum free light chain (FLC) assay with kappa/lambda ratio (normal: 0.26-1.65) 2
• Elevated NT-proBNP (>332 ng/L has >99% sensitivity for cardiac involvement) 3
• Troponin T or high-sensitivity troponin for cardiac staging 3

Critical caveat: 89% of AL amyloidosis patients have detectable monoclonal protein in serum or urine, but only 40% show a spike on standard electrophoresis—immunofixation is essential 4. The presence of monoclonal protein suggests AL amyloidosis but does not establish the diagnosis without tissue confirmation 1.

Step 3: Obtain Tissue Diagnosis (Mandatory)

The gold standard is tissue biopsy with Congo red staining showing apple-green birefringence under polarized light microscopy. 1, 2

Biopsy Site Selection (in order of preference):

For suspected systemic amyloidosis:

• Abdominal fat pad aspiration (first-line, safe and convenient) 7
• Bone marrow biopsy (high yield, especially if hematologic workup needed) 2, 4
• Rectal biopsy (good sensitivity for systemic disease) 1, 7

For organ-specific involvement:

• Endomyocardial biopsy provides definitive diagnosis of cardiac amyloidosis 1
• Kidney biopsy for nephrotic syndrome 4
• Bronchoscopy with biopsy for tracheobronchial disease 2
• Endoscopy with duodenal biopsy (highest GI yield) for gastrointestinal symptoms 1
• Skin or nerve biopsy for neuropathy evaluation 5

Important pitfall: Always specify Congo red staining when submitting samples to pathology, as amyloidosis may be missed on standard H&E staining alone 1. Look for eosinophilic (pink) hyaline material on H&E, then confirm with Congo red 2.

Critical warning: Non-clinically involved site biopsies have high false-negative rates, especially in ATTR amyloidosis 5. Biopsy from symptomatic organs when possible.

Step 4: Amyloid Typing (Essential—Treatment Depends on Type)

Once amyloid is confirmed, typing is mandatory because treatment differs fundamentally between types. 2, 8

Gold Standard Method:

• Mass spectrometry (88% sensitivity, 96% specificity) is the most accurate method for identifying the protein subtype 2, 8

Alternative Methods:

• Immunohistochemistry with antibodies against amyloid A, κ- and λ-light chains, and TTR 1
• Immunofluorescence or immunogold electron microscopy 2

Caveat: Immunohistochemistry has limitations in specificity and sensitivity of commercial antibodies 8. When results are equivocal, proceed to mass spectrometry.

For TTR Amyloidosis:

If TTR amyloid is detected, perform DNA mutational analysis (TTR gene sequencing) to differentiate hereditary ATTRv from wild-type ATTRwt. 1, 5 This is crucial even without family history, as ATTRv has variable penetrance 5.

Step 5: Cardiac Imaging for Cardiac Involvement

Echocardiography is the first-line cardiac imaging modality in all suspected cases. 1, 3

Key Echocardiographic Findings:

• LV wall thickness ≥12 mm with small cavity 3
• Biatrial enlargement disproportionate to ventricular dysfunction 3
• Restrictive transmitral Doppler filling pattern 3
• Apical sparing pattern on longitudinal strain with apical-to-basal ratio >2.1 (highly suggestive) 3
• Thickened valves without significant stenosis 3

Cardiac MRI (When Echo is Suggestive or Equivocal):

• Diffuse subendocardial or transmural late gadolinium enhancement (LGE) (88% sensitivity, 100% specificity for AL) 3
• Myocardial nulling before blood pool nulling 3
• Elevated native T1 values (>1020-1044 ms) and extracellular volume (ECV >0.40) 3

Important limitation: Neither echocardiography nor cardiac MRI can distinguish AL from ATTR amyloidosis 3.

Step 6: Nuclear Imaging for ATTR Diagnosis (Non-Invasive Alternative)

For ATTR cardiac amyloidosis, nuclear imaging with 99mTc-PYP/DPD/HMDP can be diagnostic WITHOUT biopsy when: 1, 3

• Grade 2-3 cardiac uptake is present, AND
• Monoclonal protein screening is negative

This approach has revolutionized ATTR diagnosis by avoiding endomyocardial biopsy in appropriate clinical contexts 3.

Step 7: Assess Extent of Organ Involvement

Once amyloidosis is confirmed, systematically evaluate all potentially affected organs, as extent determines treatment strategy and prognosis: 1

Cardiac Assessment:

• ECG (look for low voltage, pseudoinfarct pattern) 3
• NT-proBNP and troponin for staging 3
• Echocardiography with strain imaging 3

Renal Assessment:

• 24-hour urine protein collection 2, 5
• Estimated glomerular filtration rate (eGFR) 2

Neurologic Assessment:

• Nerve conduction studies and electromyography 5
• Quantitative sensory testing for small fiber function 5
• Autonomic function testing (postural BP, heart rate variability) 5

Hepatic Assessment:

• Hepatomegaly with elevated alkaline phosphatase suggests hepatic involvement in AL amyloidosis 1

Nutritional Assessment:

• Modified BMI (mBMI = serum albumin × BMI): mBMI <600 kg/m²·g/L indicates poor nutritional status and poor prognosis 1

Common Diagnostic Pitfalls to Avoid

1. Never rely on imaging alone—tissue diagnosis with Congo red staining is mandatory 2
2. Always perform amyloid typing—treatment differs fundamentally between AL and TTR amyloidosis 2
3. Don't assume elevated SAA alone diagnoses AA amyloidosis—tissue confirmation is always required 5
4. Request Congo red staining specifically—diagnosis may be missed on routine histology 1
5. Screen all MGUS patients routinely with focused history, physical exam, and urine albumin assessment 6

Prognosis Considerations

Cardiac involvement is the major determinant of survival, and earlier detection improves outcomes 6. Median survival with congestive heart failure is only 6 months after symptom onset, with cardiac causes accounting for 40% of deaths 4. Overall median survival for untreated AL amyloidosis is 12 months 4.