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Difficulty: Very hardOrdering Laboratory Tests

A 68-year-old man is evaluated in the clinic for progressively worsening shortness of breath, fatigue, and lower extremity edema over the past 4 months. He also notes orthostatic dizziness and a tingling sensation in his feet bilaterally. Two years ago, he underwent bilateral carpal tunnel release. His temperature is 37.0°C (98.6°F), blood pressure is 102/64 mm Hg, and pulse is 84/min. Jugular venous distention is present while seated at 45 degrees. Cardiopulmonary examination reveals clear lungs, distant heart sounds, and a prominent S4 gallop. Pitting edema is noted up to the knees bilaterally. Neurologic examination shows decreased sensation to pinprick in a stocking distribution. An electrocardiogram demonstrates low voltage in the limb leads and a pseudoinfarction pattern in the anteroseptal leads. Echocardiography shows severe concentric left ventricular hypertrophy with an interventricular septal thickness of 1.7 cm, biatrial enlargement, and a restrictive filling pattern. The left ventricular ejection fraction is 55%. Which of the following is the most appropriate next step in diagnostic testing?

  1. Serum free light chain assay and serum and urine immunofixationAnswer
  2. B
    Technetium-99m pyrophosphate myocardial scintigraphy
  3. C
    Endomyocardial biopsy
  4. D
    Abdominal fat pad fine-needle aspiration
  5. E
    Transthyretin gene sequencing

Answer

Serum free light chain assay and serum and urine immunofixation
In patients presenting with clinical, electrocardiographic, and echocardiographic features characteristic of cardiac amyloidosis (e.g., heart failure with preserved ejection fraction, severe left ventricular thickening combined with low voltage ECG, neuropathy, and a history of carpal tunnel syndrome), the most critical initial diagnostic step is to evaluate for light chain (AL) amyloidosis. AL amyloidosis is a hematologic emergency requiring prompt chemotherapy. Furthermore, the non-invasive imaging test for transthyretin (ATTR) amyloidosis can yield false-positive results in the presence of AL amyloidosis. Therefore, evaluating for a monoclonal protein using a highly sensitive combination of a serum free light chain (SFLC) assay and both serum and urine immunofixation is the strictly required first step in laboratory testing.

Step-by-Step Solution

1
Analyze the clinical presentation and physical examination findings.
The patient presents with heart failure with preserved ejection fraction (HFpEF), low voltage on ECG despite severe left ventricular hypertrophy on echocardiogram, a restrictive filling pattern, peripheral and autonomic neuropathy, and a history of bilateral carpal tunnel syndrome.
Identifying this classic constellation of systemic and cardiac findings establishes a very high clinical suspicion for cardiac amyloidosis.
2
Differentiate between the primary subtypes of cardiac amyloidosis that dictate the diagnostic pathway.
The two main types affecting the heart are light chain (AL) amyloidosis and transthyretin (ATTR) amyloidosis.
The diagnostic and therapeutic pathways for these two conditions are completely distinct, and AL amyloidosis is a hematologic malignancy equivalent requiring urgent intervention.
3
Determine the necessary test sequencing to safely and accurately establish the diagnosis.
AL amyloidosis must be evaluated and ruled out first before proceeding to advanced non-invasive imaging for ATTR amyloidosis.
Appropriate test sequencing is critical because AL amyloidosis can cause false-positive results on ATTR-specific imaging, potentially leading to misdiagnosis and a fatal delay in chemotherapy.
4
Select the most appropriate, highly sensitive laboratory tests for the initial step.
A combination of serum free light chain (SFLC) assay along with serum and urine immunofixation electrophoresis (SIFE and UIFE) is selected.
These tests combined provide approximately 99% sensitivity for detecting the monoclonal proteins associated with AL amyloidosis.

Key Concept

In patients with suspected cardiac amyloidosis, the initial diagnostic step must include a serum free light chain assay along with serum and urine immunofixation to rule out light chain (AL) amyloidosis before proceeding to advanced imaging or biopsy.
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