What are the diagnostic steps for amyloidosis (a condition characterized by the deposition of amyloid, a particular type of protein, in various tissues of the body)?

Diagnostic Approach to Amyloidosis

The definitive diagnosis of amyloidosis requires histological confirmation through tissue biopsy, with Congo red staining demonstrating characteristic apple-green birefringence under polarized microscopy, followed by typing of the amyloid protein to determine the specific type of amyloidosis. 1

Initial Evaluation and Clinical Suspicion

When suspecting amyloidosis, look for these key clinical presentations:

• Cardiac manifestations: Restrictive cardiomyopathy, heart failure with preserved ejection fraction
• Renal manifestations: Unexplained proteinuria, nephrotic syndrome
• Neurological manifestations: Peripheral neuropathy with autonomic features, bilateral carpal tunnel syndrome
• Gastrointestinal manifestations: Hepatomegaly, macroglossia
• Other signs: Periorbital ecchymoses, acquired factor X deficiency with coagulopathy

Diagnostic Algorithm

Step 1: Tissue Diagnosis

• Biopsy sites (in order of preference):

  • Affected organ (if accessible)
  • Abdominal fat pad aspiration (less invasive, excellent sensitivity: 84% for AL, 45% for ATTRv, 15% for ATTRwt) 1
  • Bone marrow (69% sensitivity in systemic AL amyloidosis) 1
  • Other sites: gingiva, rectum, kidney, liver

• Histological processing:

  • Congo red staining (gold standard) - shows apple-green birefringence under polarized light 1, 2
  • Light microscopy - reveals amorphous pink deposits in the interstitium 1

Step 2: Amyloid Typing

• Mass spectrometry (gold standard): Liquid chromatography with tandem mass spectrometry (LC-MS/MS) - 88% sensitivity, 96% specificity 1
• Immunohistochemistry: Using antibodies against amyloid A, κ- and λ-light chains, and TTR amyloid 1
• Immunogold electron microscopy: For experienced centers 1

Step 3: Identify Amyloid Type-Specific Testing

For suspected AL amyloidosis:

• Monoclonal protein screening:
  • Serum free light chains (sFLC)
  • Serum immunofixation electrophoresis (SIFE)
  • Urine immunofixation electrophoresis (UIFE)
  • Note: SPEP/UPEP alone is inadequate (misses nearly 50% of cases) 1
• Bone marrow biopsy: To demonstrate clonal plasma cell population 1
• Exclude multiple myeloma or B-cell lymphoproliferative disorders 1

For suspected ATTR amyloidosis:

• If TTR amyloid is detected: DNA mutational analysis to differentiate between senile (wild-type) and hereditary amyloidosis 1
• Nuclear imaging: Technetium Tc 99m dicarboxypropane diphosphonate (Tc-PYP) scan to distinguish between AL and TTR amyloidosis 1

Step 4: Organ Involvement Assessment

• Cardiac assessment:

  • Echocardiography (recommended for all suspected cases) 1
  • Cardiac MRI for infiltrative cardiomyopathy features 1
  • BNP/NT-proBNP levels (93% sensitivity, 90% specificity for cardiac involvement) 1
  • ECG (low QRS voltage with ventricular wall thickening suggests cardiac amyloidosis) 1

• Other organ assessments:

  • Renal function tests and urinalysis for proteinuria
  • Liver function tests (elevated alkaline phosphatase common) 3
  • Neurological evaluation for peripheral and autonomic neuropathy

Common Pitfalls and Caveats

1. Misdiagnosis risk: Never rely solely on clinical or DNA studies for typing; always identify the amyloid protein within deposits 2

2. False negatives: A negative biopsy from one site doesn't exclude amyloidosis - consider sampling multiple sites if clinical suspicion is high 1

3. Monoclonal gammopathy interpretation:

   • 10-40% of ATTR-CM patients have evidence of plasma cell dyscrasia without AL amyloidosis 1
   • Monoclonal gammopathy of uncertain significance (MGUS) increases with age (5% in patients >70 years) 1
   • Renal dysfunction can cause spurious abnormal light chain results 1

4. Biopsy site selection: When both AL and ATTR amyloidosis are suspected (e.g., MGUS with abnormal nuclear scintigraphy), cardiac biopsy is preferred to definitively establish cardiac pathology 1

5. Typing accuracy: Immunohistochemistry must be interpreted cautiously; inconclusive results require referral for more sophisticated methods 2

6. Tissue quality: Fresh tissue is preferred for optimal diagnostic yield 2

By following this structured approach to diagnosis, patients with amyloidosis can be identified early, allowing for appropriate type-specific treatment that can significantly improve morbidity, mortality, and quality of life outcomes.