The Coronary Artery Calcium (CAC) score test is a non-invasive medical imaging procedure that assesses the risk for future heart problems. It uses a specialized computed tomography (CT) scan to look for calcified deposits in the walls of the coronary arteries. The presence and quantity of this calcium are directly related to the extent of atherosclerosis. This screening tool provides physicians with direct evidence of underlying coronary artery disease, helping refine risk estimates beyond traditional factors like cholesterol and blood pressure.
Detecting Arterial Plaque
The CAC score test physically measures established mineral deposits within the heart’s arteries using a non-contrast CT scan, which requires no injected dye and takes only a few minutes. The imaging specifically targets dense, bright white specks representing calcified atherosclerotic plaque. This calcium is a marker of plaque that has been present for some time and hardened, confirming the existence of coronary artery disease. The CT machine quantifies these deposits by calculating the area and density of each lesion, generating a single numerical value.
The test is highly effective at detecting this hard, stable plaque but does not visualize soft, non-calcified plaque, which is often an earlier or more unstable form of the disease. Despite this limitation, the CAC test provides a highly reproducible and objective measure of the total burden of established coronary atherosclerosis.
Interpreting the Calcium Score
The standardized result is reported as the Agatston score, calculated by summing the scores of all calcified lesions across the four main coronary arteries. This scoring system provides a quantitative measure of the extent of the disease, ranging from zero to well over 1,000, with higher numbers indicating a greater amount of calcified plaque.
The score is categorized to reflect the burden of atherosclerosis and associated risk:
- A score of zero suggests the absence of detectable calcified plaque and a very low likelihood of a near-term cardiovascular event.
- Scores between 1 and 100 indicate mild calcification, confirming the presence of coronary atherosclerosis and suggesting a low to intermediate risk.
- Scores between 101 and 400 are classified as moderate calcification, pointing to a higher plaque burden and a moderately increased risk.
- An Agatston score exceeding 400 signifies severe calcification and a very high plaque burden, associated with a significantly elevated risk of a major adverse cardiac event.
The Predictive Power of the Score
The primary accuracy of the CAC score lies in its power as an independent prognostic tool for predicting future cardiovascular events. The score offers a measure of risk that is distinct from and adds to the information provided by traditional risk calculators, such as the Atherosclerotic Cardiovascular Disease (ASCVD) risk score. Studies show that when the CAC score is added to traditional risk factors, it significantly improves the classification of an individual’s risk for heart attack and cardiac death.
This improved accuracy is demonstrated by the score’s ability to reclassify individuals who might be incorrectly placed by standard assessment models. For instance, a person categorized as low-risk based on conventional factors but who has a high CAC score is correctly moved into a higher-risk category. Conversely, the score is powerful in providing reassurance, highlighted by the “Power of Zero” concept.
An Agatston score of zero is a strong indicator of low risk. Individuals with a zero score have an exceptionally low chance—less than 1% per year—of experiencing a major cardiac event over the next five to ten years, even if they possess some traditional risk factors. The risk of cardiac events increases significantly as the score rises, with scores over 100 associated with a seven to twelve-fold increase in incidence compared to a score of zero.
Contextual Factors Influencing Utility
The utility and reliability of the CAC test are determined by the clinical context in which it is used. Current guidelines recommend the test primarily for asymptomatic individuals who fall into the intermediate-risk category based on traditional risk assessments. For these individuals, the score helps resolve uncertainty about whether to initiate preventative therapies, such as statin medication.
A limitation is the test’s inability to detect non-calcified, or soft, plaque. A zero score does not guarantee the complete absence of all atherosclerosis, especially in younger individuals who may have early-stage plaque that has not yet calcified. Physicians emphasize that a zero score indicates a very low short-term risk, but not a zero lifetime risk.
Age is another modifying factor, as scores naturally increase over time, requiring interpretation relative to the patient’s age and sex using a percentile rank. The score is generally not recommended for individuals who already have known coronary artery disease or those presenting with acute symptoms. Additionally, the presence of coronary artery stents can compromise accuracy by causing significant artifacts on the CT scan.