The Coronary Artery Calcium (CAC) score test is a non-invasive imaging procedure that uses a specialized computed tomography (CT) scan to look for calcified atherosclerotic plaque in the heart’s arteries. This test measures the amount of calcium deposits, which are a direct marker of coronary artery disease (CAD) burden. The presence and quantity of this calcium provide information that helps assess an individual’s potential for a future heart attack or stroke. Understanding the accuracy of the CAC test requires distinguishing between its ability to physically detect plaque and its power to predict future health events.
Understanding the Agatston Score and Measurement
The CAC test physically measures the calcified portion of atherosclerotic plaque built up on the inner walls of the coronary arteries. These calcium deposits signify long-standing, matured plaque, making the test highly specific for atherosclerosis. The imaging is typically performed using a non-contrast, electrocardiogram-gated CT scan.
The result of the scan is quantified using the Agatston score, a validated system developed in the 1990s. This score is a weighted measure of both the density and the total area of calcification detected across all four major coronary arteries. The total Agatston score, reported in Agatston Units (AU), establishes the patient’s category of calcified plaque burden, with common thresholds being 0 (no detectable calcium), 1–100 (mild), 101–400 (moderate), and over 400 (severe).
Diagnostic Precision Versus Predictive Power
The accuracy of the CAC score is analyzed through two lenses: its diagnostic precision and its predictive power. Diagnostically, the test is highly precise at identifying calcified plaque, serving as a specific marker for atherosclerosis. The presence of any calcium (a score greater than zero) confirms coronary artery disease.
The test is noted for its high negative predictive value. A score of zero is strongly associated with the absence of significant obstructive coronary artery disease in asymptomatic individuals. A CAC score of 0 suggests a very low risk of experiencing a cardiovascular event over the next five to ten years, with event rates typically below 1%. This makes the test a valuable tool for risk stratification.
The score’s primary utility lies in its predictive power—its ability to forecast a future cardiovascular event like a heart attack or stroke. Studies show the CAC score to be an independent predictor of risk, adding substantial information beyond traditional risk factors like cholesterol or blood pressure. The risk of a major cardiac event increases progressively with the score. Scores over 400 carry a risk comparable to that of someone who has already had a heart attack. The score helps reclassify individuals considered intermediate risk by traditional calculations, more accurately reflecting their true biological risk.
Limitations and Factors Affecting Score Reliability
While the CAC score precisely measures calcified plaque, a major limitation is its inability to detect soft, non-calcified plaque. This soft plaque is often more prone to rupture and cause an acute event, yet it is invisible to the calcium scan. Consequently, a score of zero does not guarantee the complete absence of all atherosclerotic disease, particularly in younger individuals, such as those under 45, who may have non-calcified plaque.
Technical factors can also affect the reliability and consistency of the score. The scan requires precise cardiac gating, which synchronizes the CT image acquisition with the patient’s heart rhythm. Patient movement or an irregular heart rhythm can introduce variability or artifacts.
To ensure proper interpretation, the absolute Agatston score is reported alongside a percentile score, which compares the patient’s result to others of the same age and sex. Relying only on the absolute number without considering the age- and sex-specific percentile can lead to a misinterpretation of the true biological risk. This contextualization helps prevent underestimating risk in younger patients or over-treating elderly patients whose calcification may reflect normal aging.
Integrating Calcium Scores into Patient Care
The clinical utility of the CAC score is primarily in its ability to refine cardiovascular risk assessment, especially for asymptomatic patients whose risk is considered borderline or intermediate based on traditional factors. This process is known as risk reclassification, where the score moves a patient into a more accurate risk category, thereby guiding preventative therapy decisions.
A low score (CAC of 0) often leads to a decision to defer or delay the initiation of cholesterol-lowering medications, such as statins, as the risk of a near-term event is exceptionally low. Conversely, a high score often prompts a more aggressive treatment strategy than risk factors alone might suggest. A score of 100 or greater generally warrants the initiation of moderate- to high-intensity statin therapy, even if the patient’s cholesterol levels are not severely elevated.
Patients with very high scores, such as those over 300, may be considered for intensive lipid-lowering therapy and the addition of low-dose aspirin, a regimen typically reserved for individuals who have already had a cardiac event. Regardless of the score, any result above zero strongly reinforces the necessity of aggressive lifestyle modifications, including diet changes, regular exercise, and smoking cessation. The CAC score acts as a personalized, tangible measure of subclinical disease, encouraging commitment to evidence-based risk reduction strategies.