What does a computed tomography (CT) calcium score indicate in asymptomatic adults with risk factors for cardiovascular disease?

What a CT Calcium Score Shows

A CT calcium score quantifies the amount of calcified atherosclerotic plaque in the coronary arteries, serving as a direct marker of coronary atherosclerosis burden and providing powerful prognostic information about future cardiovascular events beyond traditional risk factors. 1, 2

Direct Measurement of Atherosclerosis

• The test detects and quantifies calcium deposits within coronary artery walls using non-contrast CT imaging, typically calculated using the Agatston method (area of calcification multiplied by density weighting factor in Hounsfield units). 1
• Coronary calcification occurs exclusively in atherosclerotic lesions of the arterial intimal layer—it does not occur in normal vessel walls—so any detectable calcium definitively establishes the presence of atherosclerosis. 2
• The degree of calcification correlates with total atherosclerotic plaque burden, though the relationship between calcium and luminal obstruction is modest. 2

Prognostic Value and Risk Stratification

The calcium score provides incremental cardiovascular risk prediction beyond traditional risk calculators like the Framingham Risk Score, with cardiovascular risk increasing proportionally to the score. 1, 2

Score Interpretation for Risk:

• CAC = 0: Very low risk (<1% annual rate) of cardiac death or myocardial infarction over the next 3-5 years, with only 0.4% event rate based on pooled data from 11,815 patients. 1
• CAC 1-100: Low risk category with modest elevation above zero score. 3
• CAC 100-400: Intermediate risk (10-20% over 3-5 years) with 4.3-fold increased relative risk compared to zero score. 1, 3
• CAC 400-1000: High risk (4.6% event rate over 3-5 years) with 7.2-fold increased relative risk. 1, 4
• CAC >1000: Very high risk (7.1% event rate over 3-5 years) with 10.8-fold increased relative risk. 1, 4

What the Test Does NOT Show

A critical caveat: CT calcium scoring has poor specificity for diagnosing obstructive coronary artery disease because calcification does not reliably predict the degree of luminal stenosis. 2

• In symptomatic patients with CAC = 0, studies show 3.5% still have ≥50% stenosis and 1.4% have ≥70% stenosis. 1, 2
• The test cannot detect non-calcified atherosclerotic plaques, which may be present in younger patients where atherosclerosis has not advanced to calcification stage. 1, 2
• Obstructive CAD without calcium occurs more frequently in younger individuals. 1
• The calcium score should be viewed as a marker of atherosclerotic burden, not stenosis severity. 2

Technical Aspects

• The scan uses prospectively ECG-triggered acquisition with low radiation exposure (typical effective dose 0.9-1.5 mSv, equivalent to 1-2 mammograms per breast). 1, 2
• Images are 2.5-3.0 mm thick axial slices obtained through the heart without contrast injection. 1
• Calcific lesions are defined as having CT density >130 Hounsfield units and area >1 mm². 2

Appropriate Clinical Use

The American College of Cardiology and American Heart Association recommend calcium scoring primarily for risk stratification in asymptomatic adults aged 40-75 years with intermediate (7.5-20%) or borderline (5-7.5%) 10-year ASCVD risk when decisions about preventive therapy are uncertain. 1, 2

• The test should not be performed in men <40 years or women <50 years due to very low prevalence of detectable calcium. 1
• It is not recommended for symptomatic patients as a surrogate for detecting obstructive disease, since 7-38% of symptomatic patients with CAC = 0 have obstructive CAD. 2
• The test is not appropriate for truly low-risk (<5% 10-year ASCVD risk) or high-risk (>20%) asymptomatic patients, as it does not change management in these groups. 1, 5

Impact on Clinical Management

• Visualization of calcium scores motivates both physicians and patients to initiate preventive therapies—studies show aspirin and statin use increased from 44% to 90% in patients with elevated scores. 1
• A zero score in intermediate-risk patients allows downgrading to lower risk category, potentially avoiding statin therapy. 1, 2
• Scores >100-400 (especially >75th percentile for age/gender) warrant aggressive risk factor modification and consideration of statin therapy. 1, 4
• Scores >400 indicate need for high-intensity statin therapy, aspirin consideration, and aggressive management of all cardiovascular risk factors. 4

Comparison to Other Modalities

• Calcium scoring outperforms high-sensitivity C-reactive protein and carotid intima-media thickness for cardiovascular risk prediction in multivariable models. 1
• The test provides better risk discrimination than traditional risk factors alone, with C-statistic improving from 0.79 to 0.83 when calcium score is added to risk factor models. 1
• CT coronary angiography (CCTA) can visualize both calcified and non-calcified plaque and assess stenosis severity, but is not recommended for risk assessment in asymptomatic adults due to higher radiation exposure and cost. 1