The Coronary Artery Calcium (CAC) scan is a non-invasive imaging procedure using Computed Tomography (CT) to assess atherosclerosis in the heart’s arteries. It provides a direct measure of the extent of calcified plaque, which is an early sign of coronary artery disease. This diagnostic tool is increasingly used to refine the estimation of an individual’s future risk for heart-related events. Understanding the accuracy and clinical application of this screening method is important for those considering its use.
What the Scan Measures
The CAC scan measures the amount of calcified, or “hard,” plaque within the coronary arteries. The CT machine identifies lesions with a density greater than 130 Hounsfield Units, which is the standard threshold for calcium detection. It is a rapid, low-radiation procedure that captures cross-sectional images of the heart. The presence of calcium in this context is almost exclusively a marker for atherosclerosis, the underlying cause of most heart attacks and strokes.
The scan results are quantified into a single numerical value known as the Agatston score. This score is calculated by multiplying the area of each calcified lesion by a weighting factor based on its density. Lesion density is assigned a factor between one and four, with four representing the densest calcification, and the total Agatston score is the sum of these weighted values from all calcified areas across the coronary tree.
Reliability of the Measurement
The accuracy of the CAC scan is high. A score of zero (CAC=0) is considered a powerful result, as it has a high negative predictive value, often reported to be near 99%, for ruling out significant obstructive coronary artery disease over the short term. This means the absence of calcification strongly suggests a low likelihood of having a heart attack in the next several years.
However, the scan’s accuracy is limited by its inability to detect soft, non-calcified plaque, which is often the more unstable and rupture-prone type. The scan only measures the hard, stable remnants of long-standing atherosclerosis, not the potentially dangerous early-stage or vulnerable plaques. Therefore, the scan provides an excellent measure of plaque burden but not necessarily plaque vulnerability.
Regarding measurement precision, the Agatston score demonstrates excellent reproducibility when performed on the same scanner by the same operator, with correlation coefficients typically above 0.98. However, when repeated scans are performed, the inter-scan variability can be substantial, sometimes reported to be between 15% and 22%. This variation is more pronounced at lower, non-zero scores than at higher scores. Variability between different CT scanner manufacturers (inter-vendor variability) can also occur.
Despite this numerical variability, the precision is generally sufficient for clinical use because the slight differences rarely lead to a patient being classified into a different clinical risk category. For this reason, the clinical interpretation focuses on the broad risk categories rather than the exact number.
Interpreting the Score
The Agatston score is interpreted using specific categories that correlate with the severity of coronary artery disease and subsequent cardiovascular risk. A score of 0 indicates no detectable calcified plaque, placing the individual in the lowest risk category for a cardiovascular event. Scores between 1 and 100 are considered minimal to mild evidence of calcification, suggesting a low-to-intermediate risk. Scores ranging from 101 to 400 represent a moderate amount of plaque and correspond to a moderately high risk, while any score above 400 is considered extensive calcification and signifies a high risk of future cardiovascular events.
The score is fundamentally a measure of an individual’s total atherosclerotic burden, reflecting the lifetime accumulation of disease. A high score, even in someone with otherwise favorable traditional risk factors, demonstrates that the underlying disease process is advanced. The prognostic value of the score is what makes it a powerful tool for risk stratification.
Clinical Application and Utility
The primary clinical utility of the CAC scan is to refine the risk assessment for asymptomatic individuals who fall into the “intermediate risk” category based on traditional risk calculators. These patients typically have a 10-year risk of atherosclerotic cardiovascular disease between 7.5% and 20%, where the decision to start preventive medication, such as a statin, is often uncertain. The scan provides the objective evidence needed to guide this decision.
If an intermediate-risk patient has a CAC score of 0, the result effectively reclassifies them to a much lower risk status, allowing a safe delay or deferral of statin therapy. Conversely, an intermediate-risk patient with a score greater than 100 is often reclassified to a high-risk status, which supports the initiation of aggressive preventive treatment. This ability to “re-risk” individuals is the main strength of the test.
The scan is generally not recommended for individuals who are already at very low risk or those already at very high risk. It is also not appropriate for patients currently experiencing symptoms like chest pain, as they require more urgent diagnostic tests to assess for acute blockages. In its appropriate setting, the CAC scan serves as a practical decision-aid for determining who would benefit most from aggressive prevention.