Normal SDS Score in Nuclear Stress Tests
A normal Summed Difference Score (SDS) in nuclear stress tests is typically ≤1, indicating no significant stress-induced myocardial ischemia. 1, 2
Understanding Nuclear Stress Test Scoring
Nuclear stress tests use a standardized scoring system to quantify myocardial perfusion:
- Summed Stress Score (SSS): Quantifies perfusion defects during stress
- Summed Rest Score (SRS): Quantifies perfusion defects at rest
- Summed Difference Score (SDS): The difference between SSS and SRS, representing the extent of reversible ischemia 3, 2
Interpretation of SDS Values
The SDS directly correlates with the extent of stress-induced ischemia:
- SDS ≤1: No significant ischemia (normal) 2
- SDS >1: Presence of ischemia 2
- SDS values reflecting mild ischemia: Affects <10% of the myocardium 1
- SDS values reflecting moderate to severe ischemia: Affects ≥10% of the myocardium, associated with an annual risk of cardiac death or MI ≥5% 1
Clinical Significance of SDS
The SDS has important prognostic implications:
- SDS correlates better with regional Difference in Myocardial Blood Flow (DMBF) than with Coronary Flow Reserve (CFR), as both SDS and DMBF represent the difference between stress and rest measurements 2
- Expert scoring of SDS has significantly greater prognostic ability compared to automated software measurements 3
- Higher SDS values correlate with more severe ischemia and worse clinical outcomes 1
Relationship to Risk Assessment
Nuclear stress test results help stratify patient risk:
- Normal or mildly abnormal nuclear MPI is associated with a low annual risk of cardiac death and AMI (generally <1%) 1
- Moderate to severe abnormalities (including elevated SDS) are associated with an annual risk of cardiovascular death or MI ≥5% 1
- The extent of ischemic myocardium as quantified by SDS forms an effective prognostic score for predicting cardiac mortality 1
Common Pitfalls in SDS Interpretation
- SDS should not be considered only dichotomously (normal vs. abnormal); it represents a continuous variable with increasing risk as values rise 1
- Values above the cutpoint do not necessarily rule out myocardial ischemia 1
- Automated SDS measurements, while helpful, should not replace expert visual interpretation, as expert scoring has superior prognostic value 3
- Patient factors such as high body mass index can affect image quality and potentially impact scoring accuracy 4