CAC Score (Coronary Calcium): How to Interpret Your Result

What the CAC Score Actually Measures

The coronary artery calcium score quantifies calcified atherosclerotic plaque inside the walls of your coronary arteries. A non-contrast, ECG-gated CT scan acquires images of the heart in roughly 10 minutes, and software calculates the Agatston score based on the area and density of each calcified lesion 1. Arthur Agatston developed the method in 1990, and it remains the universal standard.

Calcium deposits form within atherosclerotic plaques as they mature. A higher score correlates with a larger total plaque burden, both calcified and non-calcified. The Multi-Ethnic Study of Atherosclerosis (MESA), which enrolled 6,814 adults free of clinical cardiovascular disease, demonstrated that CAC independently predicts coronary heart disease events across all racial and ethnic groups studied 2. After 10 years of follow-up in MESA, participants with CAC scores above 300 had a coronary event rate roughly 7 times that of participants with CAC of zero 3.

Important context: a CAC of zero does not exclude all plaque. Non-calcified ("soft") plaques exist and can cause events, though the negative predictive value of CAC 0 remains exceptionally high. A 15-year follow-up from MESA found that CAC 0 still conferred a very low annualized coronary heart disease event rate of approximately 1.1 per 1,000 person-years 4.

Standard Risk Categories by Score

The 2018 AHA/ACC Cholesterol Guideline and the 2019 ACC/AHA Primary Prevention Guideline organize CAC results into four tiers 5. Each tier carries distinct clinical implications.

CAC 0 (no calcium detected). A score of zero indicates no measurable calcified plaque. For borderline-risk patients, the 2019 ACC/AHA guideline states that CAC 0 supports deferring statin therapy if no other high-risk conditions (diabetes, family history of premature ASCVD, smoking) override that decision 6. This "power of zero" has been validated across more than 10 large cohort studies spanning over 60,000 individuals 7.

CAC 1 to 99 (mild plaque). Detectable calcium is present but limited. Risk is above that of CAC 0 but remains relatively moderate. In MESA, 10-year CHD event rates for this group were approximately 3.9% 3. The ACC/AHA guideline favors statin initiation when additional risk enhancers (LDL-C above 160 mg/dL, metabolic syndrome, chronic kidney disease, family history) coexist 6.

CAC 100 to 399 (moderate plaque). Patients in this range cross a clinically meaningful threshold. A CAC at or above 100, or a percentile at or above the 75th for age and sex, provides strong grounds for statin therapy according to the 2018 guideline 5. The Heinz Nixdorf Recall Study (N=4,129) confirmed that CAC 100+ independently predicted myocardial infarction and cardiac death over 5 years of follow-up, adding prognostic value beyond traditional Framingham risk factors 8.

CAC 400 and above (extensive plaque). This range signals heavy plaque burden. The Dallas Heart Study showed that a CAC above 400 confers a 10-year coronary event risk exceeding 20%, placing these patients in a risk stratum comparable to secondary prevention populations 9. Statin therapy is recommended. Stress testing or coronary CT angiography may follow if symptoms or other risk markers suggest obstructive lesions.

Why Age-Sex Percentile Matters

Raw Agatston scores must be interpreted alongside age and sex percentile data. A 75-year-old man with a CAC of 150 may fall below the 50th percentile for his demographic, while a 45-year-old woman with the same score sits far above the 90th percentile. The clinical urgency differs substantially.

The MESA CAC calculator (available at mesa-nhlbi.org) provides percentile rankings based on the 6,814-person MESA cohort stratified by age, sex, and race/ethnicity 2. The 2018 AHA/ACC guideline specifically identifies a CAC score at or above the 75th percentile for age and sex as a threshold favoring statin initiation in intermediate-risk patients 5.

A practical example: a 55-year-old white man with a CAC of 90 sits near the 50th percentile. A 55-year-old white woman with CAC 90 is above the 90th percentile. Same raw number. Very different risk context.

Dr. Khurram Nasir, a preventive cardiologist at Houston Methodist who has published extensively on CAC-guided prevention, has stated: "The CAC score is the most powerful single test we have for reclassifying cardiovascular risk in asymptomatic adults, but it only works well when interpreted relative to age and sex norms" 10.

When Clinicians Order a CAC Scan

The 2019 ACC/AHA Primary Prevention Guideline recommends CAC scoring as a shared decision-making tool for adults aged 40 to 75 whose 10-year atherosclerotic cardiovascular disease (ASCVD) risk falls between 5% and 20%, the borderline to intermediate range 6. For these patients, the CAC result either upgrades or downgrades the statin decision.

CAC scanning is not recommended for patients already on statins for established indications, patients with known coronary artery disease, or symptomatic patients who need diagnostic angiography instead. The scan is also less useful at extremes of risk: very low-risk patients (<5% ASCVD risk) rarely benefit because their event rate is already low, and very high-risk patients (>20%) should receive statins regardless of calcium burden.

The USPSTF assigned an "I" (insufficient evidence) rating to CAC screening for the general asymptomatic population in 2018, noting that while the test predicts events, direct evidence that CAC-guided therapy improves clinical outcomes was limited at that time 11. The ROBINSCA trial (N=43,637), a Dutch population-based randomized trial, subsequently reported that CAC-based screening did not reduce coronary events compared to usual care over a median 4.4-year follow-up, though critics noted the trial's short duration and low statin uptake in the screening arm 12.

The Society of Cardiovascular Computed Tomography (SCCT) endorses CAC for borderline and intermediate-risk adults and considers it appropriate in select populations including those with a family history of premature coronary disease 13.

What Happens After You Get Your Score

Clinical response follows a stepwise approach based on the result tier.

If CAC is 0: Reassess in 5 to 10 years. Short-term cardiac event risk is low. Lifestyle optimization (exercise, dietary pattern, smoking cessation, blood pressure management) remains the foundation. The 2019 ACC/AHA guideline supports deferring statin therapy in borderline-risk patients with CAC 0 unless diabetes, LDL above 190, or premature family history applies 6.

If CAC is 1 to 99: Lifestyle intervention intensifies. Statin discussion should weigh additional risk enhancers. The MESA Air sub-study showed that even mild calcium (CAC 1 to 100) conferred a 3-fold increase in coronary events compared to CAC 0 over median 11.1 years of follow-up 14.

If CAC is 100 or above: Statin therapy is favored. Aspirin may be considered for select patients aged 40 to 70 who are not at increased bleeding risk, consistent with the 2019 ACC/AHA guideline 6. Additional evaluation (coronary CT angiography, stress testing) may be appropriate if symptoms develop or if the score rises sharply above 400.

If CAC is above 1,000: Coronary CT angiography or functional stress testing is often pursued to evaluate for obstructive disease. These patients carry annualized event rates comparable to patients with established coronary artery disease.

Can You Lower a CAC Score?

This is one of the most common questions patients ask. The short answer: not really. Statin therapy consistently reduces cardiovascular events in patients with elevated CAC, but statins do not lower CAC scores. In fact, several studies show that statins modestly increase the rate of coronary calcium progression while still reducing events, likely because statins promote calcification of unstable lipid-rich plaques into denser, more stable calcified plaques 15.

The St. Francis Heart Study randomized 1,005 asymptomatic individuals with CAC above the 80th percentile to atorvastatin 20 mg versus placebo. After 4.3 years, atorvastatin reduced LDL by 39% and total cardiovascular events by 42% in those with baseline CAC above 400, yet calcium scores progressed in both arms 16. The takeaway: tracking treatment success by re-scanning the CAC is misleading. Plaque stabilization, not calcium reduction, is the goal.

No supplement, diet, or exercise protocol has been shown in randomized trials to reverse established coronary calcification. Vitamin K2 has been hypothesized to redirect calcium from arteries to bone, but the VitaK-CAC trial (N=166) found no statistically significant reduction in CAC progression with menaquinone-7 supplementation at 360 mcg daily over 2 years compared to placebo 17. Aggressive LDL lowering (statin plus ezetimibe, or statin plus PCSK9 inhibitor) reduces event risk but does not erase calcium already deposited.

The 2018 AHA/ACC guideline explicitly discourages serial CAC scanning to monitor treatment response 5.

Special Populations and Considerations

Younger adults (under 40). CAC scanning is not routinely recommended below age 40 because calcium prevalence is low. When it is present in this age group, it carries strong prognostic significance. A study from the CAC Consortium (N=22,346 adults under 50) found that even low CAC (1 to 10) in younger individuals predicted a significantly higher rate of atherosclerotic events compared to age-matched peers with CAC 0 18.

Women. Women develop coronary calcification roughly a decade later than men. The same absolute score may represent a higher percentile, and thus higher relative risk, in women. In MESA, women with CAC 1 to 99 had hazard ratios for coronary events comparable to men with CAC 100 to 300 after adjusting for age and risk factors 2.

Diabetes. Type 2 diabetes already places patients at elevated cardiovascular risk. The 2019 ACC/AHA guideline recommends statins for all diabetic patients aged 40 to 75, and CAC scoring may help refine intensity decisions (moderate vs. high-intensity statin) in this group 6.

South Asian descent. South Asians develop coronary artery disease at younger ages and lower traditional risk factor levels. The Mediators of Atherosclerosis in South Asians Living in America (MASALA) study showed that CAC prevalence and progression rates were significant predictors of subclinical atherosclerosis burden in this population 19.

Chronic kidney disease. CKD accelerates vascular calcification through altered phosphate metabolism. CAC scores in CKD patients tend to be substantially higher than in the general population, and standard Agatston thresholds may underestimate relative risk in this group 20.

Limitations of the CAC Score

The Agatston score detects only calcified plaque. Non-calcified, lipid-rich plaques (the kind most likely to rupture acutely) are invisible on a non-contrast CT. Coronary CT angiography provides more complete plaque characterization but at higher radiation exposure and cost.

A CAC of zero does not mean zero risk. Roughly 5% of acute coronary events occur in patients with CAC 0, driven by non-calcified plaque or coronary spasm 7. The negative predictive value is high but not absolute.

The ACCF/AHA 2010 guideline on assessment of cardiovascular risk in asymptomatic adults gave CAC a Class IIa recommendation for intermediate-risk patients and Class III (no benefit) for low-risk patients 21. Overuse in low-risk populations leads to unnecessary downstream testing, radiation exposure, and patient anxiety without improving outcomes.

As the 2019 ACC/AHA guideline co-author Dr. Michael Blaha of Johns Hopkins has noted: "CAC is the tiebreaker, not the first test. Reserve it for patients in whom the statin decision is genuinely uncertain after standard risk assessment" 10.

Practical Steps Before and After the Scan

No special preparation is required. Caffeine and beta-blockers may be discussed with the ordering clinician, but unlike coronary CT angiography, a non-contrast CAC scan does not require contrast dye or heart rate control. The scan takes 5 to 10 minutes. Radiation exposure is approximately 1 mSv, equivalent to roughly 4 months of natural background radiation 13.

After receiving the result, ask your clinician three questions: What is my age-sex percentile? Does this score change my statin recommendation? Do I need any follow-up imaging? These three data points convert a raw number into a treatment plan.

Patients with CAC above 100 should ensure their LDL-C, blood pressure, HbA1c (if diabetic), and smoking status are optimized to guideline targets per the 2018 AHA/ACC recommendations 5.