Should a healthy person have a CT scan to look for calcium in the heart arteries?

For most people, that is the wrong first question. Australia’s cardiovascular guideline begins with an estimate of absolute risk using age, blood pressure, cholesterol, smoking, diabetes and other clinical information. A coronary artery calcium score becomes useful later, when that assessment leaves a real treatment decision unresolved.

The distinction prevents two common errors. A score of zero does not mean “no heart disease”. A high score does not show how severely an artery is narrowed. The scan measures one feature—calcified plaque—and uses it to refine the probability of a future cardiovascular event.

What the scan actually measures

Coronary artery calcium, usually shortened to CAC, is measured with a low-dose computed tomography scan. No contrast dye or intravenous line is required. Software identifies calcium in the walls of the coronary arteries and reports an Agatston score, which combines the area and density of the calcified plaque.

CAC is evidence of coronary atherosclerosis, the process in which plaque develops in artery walls. More calcium generally travels with greater total plaque burden and higher future risk. The test does not map blood flow, identify every soft or non-calcified plaque, or tell us whether chest pain is angina. Symptoms need a diagnostic assessment rather than a screening score.

Reports may also include a percentile comparing the result with people of a similar age and sex. The absolute Agatston score and the percentile answer different questions: how much calcified plaque is present, and how unusual that amount is for the comparison group.

The strongest evidence is about prediction

The Multi-Ethnic Study of Atherosclerosis, or MESA, followed 6,814 US adults aged 45 to 84 who had no clinical cardiovascular disease when they entered the study. Just over half were women. Researchers observed them for a median of 10.2 years after measuring CAC.

Cardiovascular event rates rose in steps with the score. Across demographic subgroups, the 10-year rate of heart attack, stroke or related cardiovascular death ranged from 1.3% to 5.6% when CAC was zero, and from 13.1% to 25.6% when it was above 300. Among women, the corresponding rates were 2.8% and 19.0%; among men, 3.0% and 18.2%.

After adjustment for conventional risk factors, people with scores from 1 to 100 had about twice the event rate of those with zero. Scores from 101 to 300 were associated with roughly three times the rate, and scores above 300 with about four-and-a-half times the rate.

Those are strong prognostic associations. MESA was observational, however. It shows that the scan separates groups with different future event rates; it does not show that offering the scan, or choosing treatment from its result, prevents a heart attack or stroke. The cohort was recruited in the United States more than two decades ago, so its subgroup percentages are not personal forecasts for an Australian reader today.

A better risk estimate is not automatically a better outcome

A 2025 systematic review asked the next question: what happens when asymptomatic adults and their clinicians are shown a CAC result and act on it?

The review included seven randomised trials and one prospective controlled cohort, with 51,554 participants in total. The studies differed sharply. Some disclosed the score; others linked particular scores to statin recommendations. Participants’ average ages ranged from 42 to 64, and follow-up and baseline risk also varied.

Some studies reported better cholesterol control or medication adherence. In one of three adherence trials, 63.3% of the CAC group remained adherent to statins compared with 45.6% of controls. Other medication and behaviour results disagreed across studies.

Hard clinical outcomes were too sparse to settle the issue. Three studies involving 6,552 people recorded only 52 cardiovascular events. None demonstrated fewer events or deaths, and the reviewers could not pool the results because the populations and interventions were too different. They judged most studies at moderate or high risk of bias.

This is the central evidence gap. CAC predicts risk well, but evidence that population scanning improves health outcomes remains insufficient. A scan can still help a person choose between reasonable options when the result is likely to move the decision. That is a narrower claim than saying the test saves lives.

Where Australian guidance places the test

The 2023 Australian guideline does not recommend CAC for general population screening. It also advises against measuring it in people with known coronary disease or those already at high cardiovascular risk: preventive treatment is indicated in those groups regardless of the score.

CAC can be considered when calculated risk is low or intermediate and the calculator misses another concern, or when a person and clinician remain uncertain about changing preventive treatment. In that setting, zero can support moving selected people to a lower risk category. A score above 99 Agatston units, or at least the 75th percentile for age and sex, can support moving risk upward.

Age changes the interpretation. Calcification is less common in younger adults, so zero cannot exclude early non-calcified plaque. After about 75, calcium is common and separates risk less cleanly. The result also adds little when treatment would continue unchanged whatever number appears.

The scan carries a low dose of radiation. It may reveal an incidental finding, such as a lung nodule, that leads to more imaging or procedures. It can also create anxiety and an out-of-pocket cost because Medicare does not currently subsidise CAC testing.

The useful question comes before the scan

Before ordering CAC, a clinician and patient can write down what each plausible result would change. Would zero alter the treatment discussion? Would a score over 100 strengthen the case for lipid-lowering therapy? Would treatment already be appropriate because of known disease or high absolute risk? If every answer leads to the same plan, the scan has supplied information without improving the decision.

The memorable question is: “Which decision is uncertain, and how would this score resolve it?” That keeps coronary calcium in its proper role—as an occasional tie-breaker after a full risk assessment, not a verdict on whether the heart is healthy.

Sources

Evidence note: This Insight is general education, not individual medical advice or a recommendation to request imaging or change treatment. Symptoms, cardiovascular risk and test results should be assessed by a qualified clinician.