A Coronary Artery Calcium (CAC) score test is a non-invasive procedure that uses a Computed Tomography (CT) scan to detect and measure calcified plaque within the coronary arteries. This plaque is a direct sign of atherosclerosis, which is the underlying cause of most heart attacks and strokes. The test yields a single number, the Agatston score, which estimates the total calcium burden in the heart vessels. By quantifying this calcium, the CAC score indicates an individual’s accumulated risk for future cardiovascular events and helps evaluate the presence of silent disease.
Understanding the Calcium Score
The CAC score is calculated using the Agatston method, which combines the area and density of calcified lesions detected in the coronary arteries. For each area of calcification, a density factor, based on Hounsfield units measured by the CT scan, is multiplied by the lesion’s area. These weighted scores are then summed to produce the final Agatston score, which can range from zero to over 1,000 units.
A score of zero indicates no detectable calcified plaque and suggests a very low likelihood of a near-term cardiac event. Scores between 1 and 100 are considered mild evidence of disease, indicating the presence of atherosclerosis. A score in the 101 to 400 range signifies a moderate plaque burden and suggests an elevated risk compared to lower scores.
Scores exceeding 400 represent extensive calcification and are associated with a high burden of coronary artery disease. The test only detects calcified plaque, which is the older, more stable form of plaque. It does not directly visualize soft, non-calcified plaque, which can be more prone to rupture and cause an acute event.
Predictive Accuracy for Cardiac Events
The reliability of the CAC score stems from its strong prognostic power, predicting future cardiac events independently of traditional risk factors. Unlike cholesterol or blood pressure, which measure risk factors, the CAC score measures the actual presence and extent of the disease itself. This assessment is useful because it can reclassify a patient’s risk profile, moving them from an intermediate-risk category to either low-risk or high-risk.
The most powerful finding is a score of zero, which carries a high negative predictive value for a major cardiac event over the next five to ten years. For asymptomatic individuals, a zero score is associated with a very low event rate, often less than one event per 1,000 person-years, even if they possess other traditional risk factors. This finding can reassure patients and potentially lead to withholding or delaying preventative medication like statins.
Conversely, a high score, particularly one greater than 400, reliably indicates a significant increase in the risk for future heart attacks and strokes. This extensive calcification correlates with a substantially higher event rate, sometimes up to 11 times higher than those with a zero score. The CAC score’s predictive accuracy is consistently stronger for coronary heart disease events than for stroke events.
The score’s prognostic value improves upon the risk determined by traditional models, such as the Framingham Risk Score. By providing a tangible, visual measure of disease, the CAC score offers incremental information that helps refine long-term risk assessment. It serves as a biological measure of risk not captured by standard blood tests or clinical measurements alone.
Clinical Utility and Appropriate Screening
The practical application of the CAC score lies in its ability to refine risk stratification and guide therapeutic decisions, especially for people in an ambiguous risk category. Current guidelines suggest the test is most useful for asymptomatic individuals at intermediate risk for a future cardiovascular event. This group includes people aged 40 to 75 who have a calculated 10-year risk between 5% and 20%.
For these intermediate-risk patients, the score helps resolve uncertainty about whether to start preventative therapy, such as statin medication. A score of zero may support deferring statin therapy, provided the patient does not have other high-risk conditions like diabetes. However, a score between 1 and 99 favors the initiation of statin use, especially in older patients.
The test is not recommended for very low-risk individuals, as the result is likely to be zero and would not change the treatment plan. Similarly, patients already at very high risk or experiencing symptoms of heart disease, like chest pain, should proceed directly to more advanced diagnostic tests. In these high-risk cases, the CAC score provides little additional actionable information, as treatment decisions are often already determined by clinical presentation. The CAC score information is designed for use in a shared decision-making discussion, allowing patients and physicians to weigh the benefits of aggressive intervention.