Coronary Artery Calcium Score (CAC = 0 vs. CAC > 0) — ASCVD Risk
Test Characteristics
| Metric | Value |
|---|---|
| False-negative rate | 6% (sensitivity 94%) |
| False-positive rate | 66% (specificity 34%) |
| Bayes factor (positive test) | 1.5× |
| Bayes factor (negative test) | 1/5× |
| Base rate | 7.5% 10-year ASCVD risk (borderline-risk patients) |
Interpreting Results
| Scenario | Prior | + Result | − Result |
|---|---|---|---|
| Borderline risk (5–7.5% 10-yr ASCVD) | 7.5% | 1.5 × 7.5% = 11%7.5%× 1.511% | 1/5 × 7.5% = 1.5%7.5%÷ 51.5% |
| Intermediate risk (7.5–20% 10-yr ASCVD) | 15% | 1.5 × 15% = 22%15%× 1.522% | 1/5 × 15% = 3%15%÷ 53% |
CAC works best as a risk reclassifier — moving patients between risk categories — rather than a traditional diagnostic test. The Bayes factor framework is not the best fit for it. A positive result (CAC > 0) is nearly useless on its own because most adults over 50 have some coronary calcium. The clinical value is almost entirely in the negative result: a CAC of zero is one of the strongest negative risk markers in cardiology, with an annual ASCVD event rate of ~0.03% and a "warranty period" of 5-10 years.
- + result: nearly useless — most adults over 50 have some calcium
- − result: risk drops from borderline (7.5%) to red hair (1.5%) — strong reassurance, may defer statin therapy
Sources:
- Meta-analysis, JAMA Internal Medicine, 2004. JAMA Network.
- MESA cohort, JACC Cardiovascular Imaging, 2020. CAC=0 warranty period.
- AAFP. CAC scoring for prevention.