Coronary Artery Calcium (CAC)Testing

What is CAC testing?

CAC scoring is a non-invasive CT scan that measures calcium deposits in the walls of the coronary arteries. Since calcium only accumulates where atherosclerotic plaque has formed, the test is a direct measure of subclinical coronary artery disease — even before any symptoms appear.

How it works: A specialized CT scanner takes multiple cross-sectional images of the heart, usually in 10–15 minutes with no contrast dye, no needles, and minimal radiation (~1 mSv, roughly equivalent to a mammogram).

The Agatston score is the standard output — it calculates calcium volume and density. The higher the score, the greater the plaque burden.

Who should get a CAC scan?

It’s most useful for people in a gray zone — those at intermediate cardiovascular risk (10-year ASCVD risk of 7.5–20%) where the result would genuinely change a clinical decision. Guidelines from the ACC/AHA support its use for:

• Adults 40–75 years old with borderline or intermediate risk
• People uncertain about starting statin therapy
• Those with a family history of premature heart disease
• Patients with diabetes or chronic kidney disease where risk may be underestimated

A score of zero is powerful. A CAC = 0 in someone at intermediate risk is associated with a very low short-term event rate, and guidelines now allow clinicians to safely defer statin therapy in those patients — this is one of the most clinically useful applications.

Limitations to know:

• It measures calcified plaque only — soft, non-calcified plaque (which can be equally dangerous) is not captured
• Does not show whether stenosis (blockage) is present, only plaque burden
• Small radiation exposure (~1 mSv) — not appropriate as a screening tool in young, very low-risk individuals
• Results need clinical context; a high score doesn’t mean symptoms are imminent

CAC scoring: deeper clinical context

CAC is not just a screening tool — it’s a powerful risk reclassifier. The fundamental insight is that the Agatston score directly reflects the total atherosclerotic burden a patient has accumulated over their lifetime. It captures what has already happened in the arterial wall, independently of traditional risk factors. A patient can have normal cholesterol, no hypertension, and no diabetes — yet have a CAC score of 400, meaning decades of subclinical disease have been silently progressing.

The concept of “CAC zero” deserves special clinical attention. A score of zero confers what epidemiologists call a “warranty period” — the risk of a major cardiovascular event over the next 5–10 years is so low (~1%) that guidelines now support deferring statin therapy and reassessing in 5 years. This is one of the few tests in cardiology that can actively de-escalate treatment and spare patients unnecessary medication.

Conversely, a CAC percentile score (comparing a patient’s score against age-, sex-, and ethnicity-matched peers) provides even more nuanced risk stratification. A 50-year-old man with a score of 150 might be at the 75th percentile — notably elevated — while the same score in a 75-year-old puts him below average. The MESA (Multi-Ethnic Study of Atherosclerosis) risk calculator incorporates this percentile alongside traditional risk factors for more precise event prediction.

Here’s the score-to-medication framework clinicians use:Statin intensity — what the tiers actually mean in practice

• For CAC 1–99, the ACC/AHA guidelines suggest a shared decision-making conversation. If the patient’s 10-year ASCVD risk is borderline (5–7.5%) and CAC is low (1–49), the clinician and patient may reasonably choose lifestyle modification alone. At CAC 50–99, a low-to-moderate intensity statin (e.g. atorvastatin 10–20 mg or rosuvastatin 5–10 mg) is typically introduced, targeting an LDL-C reduction of 30–50%.
• For CAC 100–399, high-intensity statin therapy (atorvastatin 40–80 mg, or rosuvastatin 20–40 mg) is standard, aiming for >50% LDL-C reduction. Aspirin at 75–100 mg/day is considered when the 10-year ASCVD risk exceeds 10% and bleeding risk is assessed as low.
• For CAC ≥400, the atherosclerotic burden is severe enough that guidelines treat these patients similarly to those with established ASCVD (secondary prevention). High-intensity statins plus ezetimibe (to achieve LDL-C <55–70 mg/dL) are standard. PCSK9 inhibitors (evolocumab, alirocumab) are reserved for patients who cannot reach LDL targets despite maximal oral therapy, or those with familial hypercholesterolaemia.

How to think about the choice between tests

The key clinical distinctions are between anatomical tests (CAC, CCTA, ICA) and functional tests (stress ECG, stress echo, MPI). CAC and CCTA tell you what the arteries look like; functional tests tell you whether the heart is actually ischemic under stress. These answer different questions.

CAC is the right first-line test for an asymptomatic person in a gray-risk zone where you need a binary decision about starting preventive therapy. It is not appropriate for someone presenting with chest pain — that patient needs a functional test or CCTA to ask “is this artery blocking flow right now?”

CCTA has overtaken stress testing in many guidelines for stable chest pain evaluation (notably the 2021 ESC guidelines). It visualizes both obstructive stenosis and non-obstructive plaque and directly guides whether a patient needs invasive intervention. The PROMISE and SCOT-HEART trials demonstrated that CCTA-guided pathways reduce MI risk and avoid unnecessary catheterizations.

Stress echocardiography and nuclear MPI are most valuable when you already know a patient has CAD and want to assess the hemodynamic significance of a lesion — or to evaluate myocardial viability before revascularization. Cardiac MRI is the gold standard for cardiomyopathy evaluation, myocardial fibrosis quantification (late gadolinium enhancement), and pericardial disease — a completely different clinical question from coronary plaque.

Two easily missed clinical details.

First, CAC progression over serial scans carries independent prognostic information. A patient whose score doubles within 3–5 years faces a higher event risk than someone with a stable high score — tracking the rate of change refines risk estimation beyond the absolute number.

Second, CAC can be zero in patients with predominantly soft, lipid-rich plaque — the kind that is most prone to rupture. Young patients (under 45) with acute MI not infrequently have near-zero CAC scores, because their disease is driven by vulnerable non-calcified lesions. This is the fundamental limitation: CAC measures what has hardened, not what is about to break.

As a clinician, the most powerful model is to use CAC early in the risk-stratification cascade — it costs little, exposes patients to minimal radiation, and either definitively de-escalates treatment (score = 0) or escalates it with an objective, hard-to-argue-with number rather than a risk calculation that many patients find abstract.