A Heart-to-Contralateral Lung Ratio of 0.84:1 is NEGATIVE for Transthyretin Cardiac Amyloidosis
A PYP scan with an H/CL ratio of 0.84:1 is definitively negative for ATTR cardiac amyloidosis and effectively excludes the diagnosis.
Interpretation Framework
The established diagnostic thresholds for PYP imaging are clear and evidence-based:
- H/CL ratio ≥1.5 at 1 hour is diagnostic for ATTR-CA 1
- H/CL ratio ≥1.3 at 3 hours can identify ATTR-CA 1
- Your ratio of 0.84:1 falls well below both thresholds, regardless of imaging timing
Why This is Negative
The consensus guidelines specify that Grade 2 or 3 myocardial uptake (which corresponds to H/CL ratios above these thresholds) is required for diagnosis of ATTR cardiac amyloidosis 2. Your ratio of 0.84:1 would correspond to Grade 0 or Grade 1 uptake at most, which does not meet diagnostic criteria.
Critical Diagnostic Criteria
For non-invasive diagnosis of ATTR-CA, ALL of the following must be present 2:
- Grade 2 or 3 myocardial uptake (H/CL ≥1.3-1.5 depending on timing)
- Absence of monoclonal plasma cell dyscrasia
- Typical cardiac imaging features on echo/CMR
Your ratio fails the first criterion entirely.
Important Caveats
The H/CL Ratio Must Be Interpreted Correctly
The guidelines emphasize that H/CL ratio should not be used in isolation 1. The ratio is only meaningful when:
- Visual myocardial uptake is confirmed on SPECT imaging
- If visual grade is 2 or 3, the diagnosis is confirmed without needing the H/CL ratio
- H/CL ratio is most helpful for equivocal cases (borderline Grade 1 vs 2)
When Grade 0-1 Uptake Matters
While Grade 0-1 uptake excludes ATTR-CA, it may be seen in AL (light chain) cardiac amyloidosis 1. If clinical suspicion for cardiac amyloidosis remains high despite this negative PYP scan, you must still exclude AL amyloidosis through:
- Serum and urine immunofixation
- Serum free light chain assay
- Consider endomyocardial biopsy if suspicion persists
Threshold Considerations in Low Pretest Probability
Recent data suggests that even the ≥1.3 threshold can result in misclassifications in populations with low pretest probability of ATTR-CA 3. Your ratio of 0.84:1 is so far below any diagnostic threshold that this concern is irrelevant—this is unequivocally negative.
Bottom Line
With an H/CL ratio of 0.84:1, this PYP scan does not support a diagnosis of ATTR cardiac amyloidosis. If cardiac amyloidosis remains in your differential diagnosis based on clinical features (heart failure with increased wall thickness, elevated biomarkers, restrictive physiology), you must pursue alternative diagnostic pathways focusing on AL amyloidosis or consider other causes of the patient's cardiac phenotype.