In adults (men < 60 y, women < 70 y) with low‑to‑moderate bleeding risk and a 10‑year atherosclerotic cardiovascular disease risk ≈5%, at what coronary artery calcium score should low‑dose aspirin be started for primary prevention?

Recommended Coronary Artery Calcium Threshold for Starting Aspirin in Primary Prevention

For adults with low-to-moderate bleeding risk and approximately 5% 10-year ASCVD risk, aspirin should be started when the coronary artery calcium (CAC) score is ≥100. 1

CAC-Based Decision Algorithm

CAC ≥100: Consider Aspirin

• Low-dose aspirin (75-100 mg daily, typically 81 mg in the U.S.) is reasonable for patients with CAC ≥100 who have low bleeding risk. 1
• Multiple international guidelines—including the American College of Cardiology/American Heart Association (ACC/AHA), Society of Cardiovascular Computed Tomography (SCCT), National Lipid Association (NLA), Cardiac Society of Australia and New Zealand (CSANZ), and Chinese guidelines—explicitly endorse CAC ≥100 as the threshold where aspirin's cardiovascular benefit exceeds bleeding risk. 1
• At CAC ≥100, the estimated 5-year number-needed-to-treat is 173 for individuals with <10% Framingham Risk Score and 92 for those ≥10%, compared to a number-needed-to-harm of 442 for major bleeding. 2
• Patients with CAC >400 derive even greater net benefit, with aspirin yielding favorable outcomes regardless of traditional risk factors. 1

CAC = 0: Do Not Start Aspirin

• Aspirin is not recommended when CAC = 0, even if calculated ASCVD risk is moderate (≈5%), because the bleeding risk outweighs any cardiovascular benefit. 1, 3
• A CAC score of zero reflects minimal atherosclerotic burden and very low near-term event risk; the estimated 5-year number-needed-to-treat is 2,036 for individuals <10% FRS and 808 for those ≥10% FRS, making the risk-benefit ratio unfavorable. 2
• The only exception is patients with CAC = 0 who have >20% 10-year ASCVD risk, though this scenario is rare in the population described in your question. 1

CAC 1-99: Generally Do Not Start Aspirin

• Evidence is mixed for this intermediate range; aspirin is generally not recommended unless multiple additional high-risk features are present (e.g., strong family history of premature ASCVD, markedly elevated lipoprotein(a), chronic kidney disease with albuminuria). 3

Why CAC Outperforms Traditional Risk Scores

• The pooled cohort equations (PCE) systematically overestimate actual ASCVD risk in contemporary populations receiving optimal statin and blood pressure therapy. 3, 4
• CAC directly visualizes coronary plaque burden, providing more accurate risk stratification than PCE-derived estimates, especially when traditional risk factors are well-controlled. 3
• In patients with ≈5% 10-year ASCVD risk, the number of cardiovascular events prevented by aspirin roughly equals the number of major bleeding episodes induced when using traditional risk scores alone, resulting in near-neutral net benefit. 4
• CAC scoring identifies the subset of intermediate-risk patients who harbor sufficient subclinical atherosclerosis to tip the balance toward net benefit. 5

Absolute Contraindications to Aspirin (Even with CAC ≥100)

Do not prescribe aspirin if any of the following are present:

• Age >70 years (bleeding risk exceeds cardiovascular benefit regardless of CAC score) 3, 6
• History of gastrointestinal bleeding or active peptic ulcer disease 3, 7
• Known bleeding disorder or thrombocytopenia 3, 7
• Concurrent use of oral anticoagulants (warfarin or DOACs), NSAIDs, or systemic corticosteroids 3, 7
• Severe liver disease 3
• Chronic kidney disease (increases bleeding risk substantially) 3, 7
• Uncontrolled hypertension 3

Practical Implementation for Your Patient Population

For adults (men <60 years, women <70 years) with ≈5% 10-year ASCVD risk and low-to-moderate bleeding risk:

1. Obtain CAC scoring to refine risk stratification beyond traditional calculators. 1, 5
2. If CAC ≥100: Initiate aspirin 75-100 mg daily (81 mg standard U.S. tablet) after shared decision-making that emphasizes the modest benefit (≈1 cardiovascular event prevented per 100 patients treated over 5-10 years) balanced against ≈1 major bleeding event. 3, 4
3. If CAC <100: Do not start aspirin; focus on optimizing statin therapy, blood pressure control, smoking cessation, and lifestyle modification, which have stronger evidence for ASCVD risk reduction. 3, 4
4. Reassess bleeding risk regularly; discontinue aspirin if the patient develops new bleeding risk factors or ages beyond 70 years. 4

Common Pitfalls to Avoid

• Do not use the 10% 10-year ASCVD risk threshold alone to initiate aspirin; without CAC stratification, the benefit-to-harm balance is unfavorable in contemporary practice. 3, 8
• Do not exceed 100 mg daily aspirin dose for primary prevention; higher doses increase bleeding risk without additional cardiovascular benefit. 3, 7
• Do not overlook bleeding risk factors when calculating net benefit; real-world major bleeding rates are approximately 5 per 1,000 patients per year. 3, 4
• Do not assume aspirin reduces cardiovascular mortality; the primary benefit is a 12% reduction in non-fatal myocardial infarction, with minimal effect on cardiovascular death (relative risk ≈0.95). 4

Strength of Evidence

The CAC ≥100 threshold is supported by:

• Multiple high-quality observational cohort studies (Multi-Ethnic Study of Atherosclerosis and others) demonstrating graded increases in cardiovascular events corresponding to CAC scores. 2
• Formal endorsement by major international guideline bodies (ACC/AHA, SCCT, NLA, CSANZ, Chinese guidelines) published in 2023, representing the most recent consensus. 1
• Meta-analyses showing that CAC ≥100 identifies patients in whom aspirin's cardiovascular benefit exceeds bleeding risk, whereas CAC = 0 identifies those who would experience net harm. 1, 8