A coronary artery calcium score, also called an Agatston score, tells you how much calcified plaque has built up in the arteries that supply your heart. A score of zero means no detectable calcium and a low risk of heart attack, while anything above 300 signals extensive coronary artery disease and a significantly higher risk. But the number alone doesn’t tell the whole story. Your age, sex, ethnicity, and overall cardiovascular risk profile all shape what your score actually means for you.
What the Numbers Mean
Your results come back as a single number, the Agatston score, calculated from the amount and density of calcium deposits found during the CT scan. The standard categories break down like this:
- 0: No calcium detected. Low probability of a future heart attack.
- 1 to 99: Mild plaque buildup. Some coronary artery disease is present, but the amount is relatively small.
- 100 to 299: Moderate plaque buildup. This range is a meaningful threshold in clinical guidelines and often influences treatment decisions.
- 300 to 399: Significant plaque. Heart attack risk is notably elevated.
- 400 and above: Extensive coronary artery disease. Annual cardiac event rates in this group are substantially higher, roughly 5.6% per year in one study of asymptomatic patients with diabetes, compared to 0.7% for those with lower scores.
These raw numbers are useful, but they need context. A score of 150 means something very different in a 45-year-old woman than in a 75-year-old man, because calcium naturally accumulates with age.
Why Your Percentile Matters More Than the Raw Score
Along with the Agatston score, you may receive a percentile ranking. This compares your calcium level to other people of the same age, sex, and ethnicity who don’t have known heart disease. The reference values come from the Multi-Ethnic Study of Atherosclerosis (MESA), a large database that includes White, African-American, Hispanic, and Chinese participants aged 45 to 84.
A percentile at or above the 75th mark means you have more calcium than roughly three-quarters of your demographic peers, and this has been linked to a significantly higher heart attack risk. The 2019 guidelines from the American College of Cardiology and the American Heart Association specifically flag a score at or above 100, or at or above the 75th percentile for your age, sex, and ethnicity, as the point where risk is clearly elevated enough to warrant preventive treatment. Conversely, if your raw score seems high but your percentile is average for your age group, the finding may be less alarming than it first appears.
How a Calcium Score Affects Treatment Decisions
Calcium scoring is most useful for people whose heart disease risk falls in a gray zone. The ACC/AHA guidelines recommend it primarily for adults at borderline risk (5% to under 7.5% ten-year risk of a cardiovascular event) or intermediate risk (7.5% to under 20%). For these groups, the scan can push the assessment clearly in one direction or the other.
If your score is 100 or higher, your actual event rate likely exceeds the threshold where starting a statin makes sense. In practice, this finding often tips the conversation toward medication. One study found that 90% of patients with scores above 400 stuck with their statin therapy over 3.6 years, compared to just 44% of those with a zero score. Seeing the calcium in their arteries made a real psychological difference in how seriously people took the prescription.
A score of zero, on the other hand, can be genuinely reassuring for someone on the fence. In borderline and intermediate risk groups, a zero score generally puts the ten-year event rate below 7.5%, which may justify holding off on a statin. This can be a powerful tool in shared decision-making with your doctor, especially if you’re reluctant to start a lifelong medication.
Aspirin and Calcium Scores
Low-dose aspirin for primary prevention (meaning you haven’t had a heart attack or stroke) has become more controversial in recent years because of bleeding risks. Current evidence suggests a net benefit from aspirin only for people with a calcium score of 100 or higher, a cardiovascular risk of at least 5%, and a low risk of bleeding. For people with low cardiovascular risk, aspirin tends to cause more harm than good regardless of their calcium score.
Tracking Changes Over Time
Calcium scores aren’t static. Coronary calcium typically grows at 10% to 20% of your baseline score per year, even with treatment. What matters clinically is not that the number goes up, but how fast it climbs. A progression rate above 15% per year has been associated with a 17-fold increased risk of a cardiac event.
Treatment can slow this progression significantly. In one study of patients with high cholesterol, the median annual increase in calcium dropped from 25% during an untreated period to about 9% once statin therapy started. Repeat scans, if your doctor orders them, are really about measuring the pace of change rather than expecting the number to go down. Statins stabilize plaque and may actually increase calcification slightly as soft, unstable plaque becomes harder and more stable, which is paradoxically a good thing.
What a Zero Score Can Miss
A zero score is one of the most reassuring results in cardiovascular screening, but it isn’t a guarantee. The scan only detects calcified plaque. Noncalcified, or “soft,” plaque doesn’t show up at all. Case reports have documented patients with a calcium score of zero who had severe artery blockages (70% to 99% narrowing) caused entirely by soft plaque.
This limitation is most relevant for people who already have symptoms like chest pain or shortness of breath. In asymptomatic individuals, a zero score has strong negative predictive value, meaning it’s very reliable at ruling out significant disease. But if you’re experiencing symptoms, a zero score shouldn’t be used as the sole reason to stop investigating. The scan was designed as a screening tool for people without symptoms, and it performs best in that role.
Radiation Exposure From the Scan
A coronary calcium scan delivers a median effective dose of about 2.3 millisieverts (mSv), though it can range from 0.8 to 10.5 mSv depending on the scanner and protocol used. For comparison, a standard chest X-ray is about 0.02 mSv, and natural background radiation in the U.S. gives you roughly 3 mSv per year. The estimated lifetime excess cancer risk from a single scan at age 40 is about 9 per 100,000 men and 28 per 100,000 women at the median dose. These are small numbers, but they’re part of why the test is targeted at people in the borderline-to-intermediate risk range rather than recommended as universal screening for everyone.
If repeat scans every five years were done from ages 45 to 75 for men and 55 to 75 for women, the cumulative lifetime cancer risk would be roughly 42 per 100,000 for men and 62 per 100,000 for women. This is low enough that for most people in the appropriate risk category, the cardiovascular information gained outweighs the radiation cost.