The Coronary Artery Calcium (CAC) score is a measurement derived from a non-contrast computed tomography (CT) scan of the heart, estimating the amount of calcified plaque in the coronary arteries. This score serves as a powerful tool in cardiovascular risk assessment, especially for individuals who do not yet show symptoms of heart disease. While the test reliably predicts future cardiac events, the resulting number is not an absolute certainty. Whether a calcium score can be wrong depends on differentiating between a technical error and a clinical limitation. Technical miscalculation is rare, but the score’s inability to detect all forms of arterial disease requires careful clinical judgment.
What the Calcium Score Measures
The CAC scan quantifies the extent of atherosclerotic plaque that has hardened or calcified within the heart’s arteries. The presence of calcium is a direct sign of coronary atherosclerosis, a disease process that often begins decades before symptoms appear. The resulting number, known as the Agatston score, is calculated by measuring both the area and the density of every calcium deposit found in the coronary arteries.
The Agatston score ranges from zero (no detectable calcified plaque) up to hundreds or thousands, signifying a significant plaque burden. Higher scores correlate to higher long-term risk of a heart attack. This score quantifies the total extent of calcified disease, not the severity of any single blockage, which is a key distinction when evaluating a patient’s risk profile.
Technical Factors Affecting Accuracy
The resulting score can be technically inaccurate due to issues related to image acquisition or analysis. One source of error is patient movement during the brief scanning time, which creates a motion artifact. This blurring effect can either falsely inflate the score by smearing the calcium or falsely deflate it by scattering the signal.
A common source of a false-positive reading is the accidental inclusion of calcification from non-coronary structures located near the heart. For example, calcium deposits in the mitral valve annulus or the aortic valve may be mistakenly counted as coronary calcium, particularly near the left circumflex artery, where the anatomy is complex. An experienced radiologist must carefully review the images to ensure calcification from these adjacent structures is excluded from the final Agatston calculation.
Technical variations in the CT scanner itself can also introduce minor discrepancies. Differences in scanning protocols, such as slice thickness, can affect the calculation through the partial volume effect. Although these technical errors are infrequent, they may prompt a physician to request a review or a repeat scan if the score seems inconsistent with the patient’s other risk factors.
Limitations in Detecting Dangerous Plaque
The most significant way a calcium score can be misleading is due to the test’s fundamental limitations. The CAC scan only identifies calcified, stable plaque, representing a chronic, long-term disease burden. It cannot see non-calcified, or “soft,” plaque, which is often the most dangerous type.
Soft plaque is rich in inflammatory cells and lipids and is much more prone to rupture, typically causing a sudden heart attack. A patient with unstable soft plaque could have a CAC score of zero because the plaque has not yet calcified. This scenario, where a low score provides false reassurance, is known as the “zero paradox.” It is particularly relevant for younger individuals or those with high-risk conditions like familial hypercholesterolemia.
The scan provides a static snapshot of chronic disease and does not assess blood flow or the presence of an acute blockage. Even a high score does not indicate if a specific artery is severely narrowed or if the plaque is about to rupture. The score measures risk over the next decade, not an immediate cardiac event.
Interpreting the Score in Clinical Practice
The CAC score is rarely used as a standalone number and must be integrated with a patient’s complete clinical picture. Physicians use the score to refine risk stratification, especially for patients in an intermediate-risk category based on traditional factors. The score helps determine whether a patient should intensify preventive measures, such as starting statin medication.
For a patient with an intermediate risk profile, a score of zero may lead to de-escalation of treatment, as this result is associated with a very low ten-year cardiac event risk. Conversely, a high score (typically over 400) signals the need for aggressive intervention, including high-intensity statin therapy and lifestyle changes. This is because a high score indicates a substantial total plaque burden, regardless of other metrics.
When a score seems unexpectedly high or low, the physician must consider factors like age, symptoms, and specific risk factors. For instance, a young patient with diabetes who has a zero score may still be considered high-risk due to the likelihood of non-calcified plaque, warranting aggressive risk management. The score is a powerful piece of evidence, but its true value is realized only when interpreted alongside the whole patient history.