What is the most reliable non‑invasive scan to diagnose cardiac amyloidosis in an older adult with unexplained left‑ventricular wall thickening, heart‑failure symptoms, and low‑voltage ECG?

Best Heart Scan to Identify Cardiac Amyloid

Technetium-99m pyrophosphate (99mTc-PYP) bone scintigraphy is the best non-invasive scan to diagnose ATTR cardiac amyloidosis when combined with negative monoclonal protein screening, achieving diagnostic certainty without biopsy in the appropriate clinical context. 1, 2

Diagnostic Algorithm Based on Clinical Presentation

Step 1: Comprehensive Monoclonal Protein Screening (Mandatory First Step)

Before ordering any cardiac imaging, you must simultaneously obtain all three tests: 1, 2, 3

• Serum free light chain (sFLC) assay with κ/λ ratio
• Serum immunofixation electrophoresis (SIFE)
• Urine immunofixation electrophoresis (UIFE)

This combination achieves >99% sensitivity for detecting AL amyloidosis and is essential because approximately 5% of individuals >70 years have MGUS, and >10% of patients with monoclonal gammopathy harbor ATTR rather than AL deposits. 1, 3

Step 2: Pathway Selection Based on Monoclonal Protein Results

Pathway A: Monoclonal Protein ABSENT

Proceed directly to 99mTc-PYP bone scintigraphy (or 99mTc-DPD/HMDP outside the United States). 1, 2, 3

ATTR cardiac amyloidosis is diagnosed non-invasively when ALL of the following criteria are met: 1, 2

1. Grade 2 or 3 myocardial uptake on visual scoring (Grade 2 = uptake equal to bone; Grade 3 = uptake greater than bone) 1, 2
2. Heart-to-contralateral lung ratio >1.5 at 1-hour imaging 4, 2
3. SPECT imaging confirms true myocardial retention (not blood pool or rib uptake) 1
4. Absence of monoclonal protein on comprehensive screening 1, 2
5. Typical cardiac imaging features present: 2, 3
   • LV wall thickness >12 mm without alternative cause
   • Relative apical sparing on longitudinal strain (apical/basal+mid ratio >1)
   • Grade ≥2 diastolic dysfunction

This combination has very high specificity and positive predictive value for ATTR-CM, eliminating the need for endomyocardial biopsy. 1

Pathway B: Monoclonal Protein PRESENT (including MGUS)

Endomyocardial biopsy is mandatory because 99mTc-PYP scans may be positive even in AL amyloidosis, and bone scintigraphy alone cannot distinguish ATTR-CM from AL-CM when any monoclonal protein is detected. 1, 2, 3

• Over 10% of patients with Grade 2-3 PYP uptake and detectable monoclonal protein have AL rather than ATTR amyloidosis 3
• Both AL and ATTR can coexist in patients with monoclonal gammopathy 3

Step 3: Cardiac MRI Role (Complementary, Not Primary)

Cardiac MRI with late gadolinium enhancement (LGE) should be considered when: 5, 2

• Echocardiographic windows are inadequate 5
• Additional tissue characterization is needed 2
• Confirming suspected apical hypertrophy or aneurysm 5

Typical CMR findings in cardiac amyloidosis include: 4, 2, 6

• Diffuse subendocardial or transmural LGE (hallmark pattern) 4, 6
• Global extracellular volume (ECV) >0.40 2
• Abnormal gadolinium kinetics with myocardial nulling prior to blood pool nulling 2
• Elevated native T1 mapping values (>1100 ms in amyloidosis) 6

However, CMR cannot reliably differentiate ATTR from AL amyloidosis, making it complementary rather than definitive for subtyping. 5, 2

Critical Pitfalls to Avoid

Do Not Interpret Bone Scintigraphy Without Monoclonal Protein Screening

This is the most critical error. Ordering 99mTc-PYP without concurrent monoclonal protein assessment can lead to misdiagnosis, as the scan cannot distinguish AL from ATTR when a plasma cell disorder is present. 1, 3

Do Not Assume AL Amyloidosis Based Solely on Monoclonal Protein Presence

Over 10% of patients with monoclonal gammopathy have ATTR deposits rather than AL, so tissue typing is mandatory when monoclonal protein is detected. 1, 3

Do Not Rely on Fat Pad Biopsy for ATTR Cardiac Amyloidosis

Abdominal fat pad biopsy has only 15% sensitivity for wild-type ATTR and 45% for hereditary ATTR, making it unreliable for suspected ATTR-CM. 3 It is appropriate for suspected AL amyloidosis (76-95% sensitivity) but not for ATTR. 3

Do Not Skip SPECT Imaging

Planar imaging alone may show false-positive uptake from blood pool or rib activity. SPECT is required to confirm true myocardial retention of the tracer. 1

Genetic Testing After ATTR Diagnosis

Once ATTR-CM is confirmed, TTR gene sequencing is mandatory to differentiate wild-type (ATTRwt) from hereditary (ATTRv) disease, as this determines family screening needs and eligibility for specific therapies. 1, 3

Summary: The Optimal Diagnostic Sequence

1. Obtain comprehensive monoclonal protein screening first (sFLC, SIFE, UIFE) 1, 2, 3
2. If negative: 99mTc-PYP bone scintigraphy with SPECT provides non-invasive ATTR diagnosis 1, 2
3. If positive (any monoclonal protein): Endomyocardial biopsy with mass spectrometry typing is required 1, 2, 3
4. Cardiac MRI: Complementary for tissue characterization but cannot differentiate subtypes 5, 2, 6
5. TTR gene sequencing: Mandatory after ATTR confirmation to identify hereditary variants 1, 3