When to Start Aspirin Based on CAC Score and ASCVD Risk
Direct Recommendation
For adults aged 40-70 years with 10-20% 10-year ASCVD risk and low bleeding risk, aspirin should be considered ONLY if the coronary artery calcium (CAC) score is ≥100, and even then only after shared decision-making that emphasizes the narrow benefit-to-harm ratio. 1, 2
Age-Based Exclusions (Apply First)
- Do NOT initiate aspirin if age >70 years, regardless of ASCVD risk or CAC score—bleeding risk exceeds cardiovascular benefit in this age group 1, 3
- Do NOT initiate aspirin if age <40 years—insufficient evidence for benefit 1
Bleeding Risk Assessment (Must Be Low to Proceed)
Absolute contraindications—do NOT use aspirin if ANY of the following are present: 1, 4
- History of gastrointestinal bleeding or active peptic ulcer disease
- Thrombocytopenia or known bleeding disorder
- Chronic kidney disease (increases bleeding risk)
- Concurrent use of NSAIDs, anticoagulants (warfarin/DOACs), or corticosteroids
- Uncontrolled hypertension
CAC-Guided Algorithm for Ages 40-70 with 10-20% ASCVD Risk and Low Bleeding Risk
CAC = 0
- Do NOT start aspirin 2, 5
- The 5-year number needed to treat (NNT) is 1,190 versus number needed to harm (NNH) of 567—net harm 2
- CAC = 0 indicates minimal atherosclerotic burden and very low near-term event risk despite calculated ASCVD score 1, 2
CAC = 1-99
- Generally do NOT start aspirin 2, 5
- NNT remains higher than NNH in this range (NNT ~476 vs NNH ~355 overall) 2
- Consider optimizing statin therapy, blood pressure control, and lifestyle modification instead 4, 6
CAC ≥100
- Aspirin MAY be considered after shared decision-making 1, 2
- NNT5 = 140 versus NNH5 = 518—favorable benefit-to-harm ratio 2
- Use low-dose aspirin 75-100 mg daily (81 mg standard U.S. tablet) 4, 6
- The 2019 ACC/AHA guidelines specifically cite "significant elevation in coronary artery calcium score" as a risk-enhancing factor that may justify aspirin consideration 1
CAC ≥400
- Aspirin should be strongly considered (if bleeding risk remains low) 2, 7
- This threshold indicates very high atherosclerotic burden and substantially elevated near-term event risk 7
- NNT is most favorable at this level while NNH remains acceptable 2
- Consider time-limited therapy (e.g., 2 years) while aggressive statin and blood pressure therapy stabilize plaques 7
Why CAC Outperforms ASCVD Risk Score Alone
- The pooled cohort equations (PCE) overestimate actual observed risk in contemporary populations receiving optimal statin and blood pressure therapy 1, 4
- In the MESA study, using ASCVD risk >20% alone would qualify only 5% of participants for aspirin, yet many with lower calculated risk but CAC ≥100 had favorable NNT/NNH ratios 2
- CAC directly visualizes atherosclerotic plaque burden, whereas PCE estimates risk from traditional factors that may be well-controlled with modern therapy 1, 2
- Multiple international guidelines (ACC/AHA, ESC, NLA) endorse CAC as the preferred risk stratifier when management is uncertain after initial risk calculation 1
Essential Shared Decision-Making Points
When discussing aspirin with a patient who has CAC ≥100: 1, 4, 6
- Primary benefit: 12% relative reduction in cardiovascular events, driven mainly by fewer non-fatal myocardial infarctions—minimal effect on cardiovascular mortality 4, 6
- Primary harm: 29% relative increase in major bleeding (predominantly gastrointestinal), occurring in approximately 5 per 1,000 patients per year 4, 6
- Net effect: Treating 100 patients for 5-10 years prevents approximately 1 cardiovascular event while causing approximately 1 major bleeding event 6
- Aspirin is not a substitute for statin therapy, blood pressure control, smoking cessation, or lifestyle modification—these have stronger evidence 4, 6
Common Pitfalls to Avoid
- Do not use the 10% ASCVD risk threshold alone as an automatic indication for aspirin—contemporary trials show unfavorable risk-benefit at this cutoff without CAC stratification 1, 4, 2
- Do not continue aspirin past age 70 without documented established ASCVD (secondary prevention indication)—the ASPREE trial showed increased bleeding and mortality in this age group 4, 3
- Do not overlook bleeding risk factors when calculating net benefit—real-world bleeding rates may reach 5 per 1,000 person-years 4, 6
- Do not use doses >100 mg daily for primary prevention—no additional cardiovascular benefit but higher bleeding risk 1, 4
- Enteric-coated formulations do not reduce bleeding risk compared to regular aspirin 4, 6
Special Population: Diabetes
- Aspirin is not routinely recommended for primary prevention even in patients with diabetes 4, 6
- The ASCEND trial (15,480 diabetic participants) showed 12% cardiovascular event reduction but 29% bleeding increase 4, 6
- For diabetic patients ≥50 years with at least one additional major cardiovascular risk factor, aspirin may be considered only if CAC ≥100 and bleeding risk is low 4, 6
When Aspirin IS Strongly Indicated
Aspirin 75-162 mg daily (typically 81 mg) remains strongly recommended for secondary prevention in all patients with established ASCVD (prior MI, stroke, revascularization)—benefits far exceed bleeding risk in this setting 8, 4, 6