Learn · Part 2. The tests · 4 min read
Coronary artery calcium score
A quick non-contrast CT that measures calcified plaque, not ischemia, and what a score of zero or a high score can and cannot tell you.
60-second take
- A CAC scan is a quick CT without contrast that measures calcified coronary plaque. It does not test for ischemia.
- A score of zero lowers, but does not remove, the chance of obstructive disease. It does not exclude noncalcified plaque, which is more common in younger people with risk factors.
- The guidelines use it as a first-line option in low-risk symptomatic patients, as an add-on in others, and the AUC rate it by ASCVD risk in people with no symptoms.
- It uses ionizing radiation. The sources used here give no dose for it.
What it is and how it works
A coronary artery calcium (CAC) scan is a quick CT of the heart done without contrast. It detects calcified plaque in the coronary arteries and measures how much there is . The amount is reported as a number, usually the Agatston score, or as a visual estimate, and is sorted into categories under the coronary artery calcium data and reporting system (CAC-DRS) . The appropriate use criteria (AUC) group scores as 0, 1 to 99, 100 to 299, and 300 or more .
You do not always need a dedicated scan. When a chest CT done for another reason shows the coronary arteries, calcium can be estimated by eye .
A CAC scan uses a CT scanner, so it involves ionizing radiation. The sources for this chapter do not give a dose for it. The AUC list radiation as a limitation of SPECT, PET, CCTA and invasive angiography, but not of CAC scoring . The 2021 chest pain guideline places CAC scoring among the least costly options .
What the result tells you
The score is a measure of calcified plaque. It is not a test for ischemia, and it cannot see plaque that has not calcified. That shapes how the guidelines use it.
| Setting | What the sources say |
|---|---|
| Stable chest pain, no known CAD, low risk | CAC testing is reasonable as a first-line test for excluding calcified plaque and identifying a low likelihood of obstructive CAD. A score of zero identifies a low-risk group that may not need more diagnostic testing . |
| Stable chest pain, intermediate-high risk | Adding CAC to stress testing can be useful, and observational data suggest CAC improves risk assessment, helps detect plaque and guides preventive treatment . |
| No symptoms, no known atherosclerotic disease | The AUC rate CAC by ASCVD risk band. The panel concluded that most testing is not likely warranted in these patients, with CAC scoring the exception, with greater support across the spectrum of risk . |
| Established chronic coronary disease | A higher calcium score is listed as a feature linked to a higher risk of major adverse cardiovascular events, alone and in addition to functional imaging . |
In symptomatic patients, a score of zero lowers the likelihood of obstructive disease but does not remove it. In the PROMISE trial, most events (84%) occurred in patients with detectable calcium, which leaves some in those with none . In the CRESCENT I trial, detectable calcium, or a high pretest risk, was what triggered follow-up CCTA (see Go deeper) . The AUC also rate which tests to consider next for each CAC-DRS category in symptomatic patients .
Strengths and limitations
Strengths
- Fast, and no contrast agent is needed .
- Robust prognostic value .
- Among the least costly tests .
- A zero score can help decide who does not need further testing, and a positive score can guide preventive treatment .
Limitations
- Shows calcified plaque only, and says nothing directly about ischemia or how tight a stenosis is.
- A score of zero does not exclude noncalcified plaque, which is more common in younger people with risk factors .
- It uses ionizing radiation, with no dose given in the sources used here.
- Very high calcium can degrade CCTA quality, so a high score may change which test is practical next .
Practical points
- Younger patients. Predominantly noncalcified plaque is more common in people under about 45 to 50 years who have diabetes, HIV, smoking or a strong family history of premature disease. The SCCT consensus says CCTA may be appropriate in selected asymptomatic high-risk people, especially those likely to have a large amount of noncalcified plaque .
- Symptomatic patients. The chest pain guideline supports CAC as a first-line test in symptomatic patients who are categorized as low risk. For others it describes CAC as an addition to stress testing rather than a replacement .
- Radiation and premenopausal women. The AUC suggest considering testing without radiation, or no testing, for low-risk premenopausal women .
- Preparation. There is no contrast agent, so contrast allergy and contrast-related kidney concerns do not arise. The sources give no special preparation for the scan itself.
- Reading the report. Look for the score, its CAC-DRS category, and whether it was measured or estimated by eye. Then ask what question it answers: risk and prevention, or whether more testing is needed.
- Scope. The 2023 chronic coronary disease guideline notes that asymptomatic people with extensive calcium and no prior atherosclerotic event are at high risk, but does not address them and refers readers to prevention guidelines .
The SCCT consensus compares CAC with a positive functional stress test for predicting adverse events. Any nonzero CAC had higher sensitivity (84% versus 43%) and a positive functional test had higher specificity (79% versus 35%). Higher CAC cutoffs raise specificity and lower sensitivity .
In CRESCENT I, 350 symptomatic patients were randomized to CAC scanning or exercise ECG. Only those with detectable CAC, or high pretest risk, went on to CCTA. At 1 year the CAC arm had fewer cardiovascular events (1-year event-free survival 97% versus 90%), and nearly 40% of patients did not need CCTA, with 16% lower costs . The SCCT authors note that longer follow-up is needed before routine implementation of such tiered strategies .
Calcium also feeds pretest probability. The CAD Consortium “extended” model adds CAC to the basic and clinical models, and each added level improves prediction .
An example from the tool
For people with no symptoms, the criteria rate CAC scoring by ASCVD risk. Open the scenario below to see how each test is rated, and why.
Example from the tool
Intermediate ASCVD risk 7.5% to 20% with or without risk-enhancing factors
- Exercise ECGMay Be Appropriate
- Exercise SPECTRarely Appropriate
- Pharmacologic SPECTRarely Appropriate
- Pharmacologic PETRarely Appropriate
- Exercise echoRarely Appropriate
- Dobutamine echoRarely Appropriate
- Stress CMRRarely Appropriate
- CAC scoreAppropriate
- CCTARarely Appropriate
- Invasive angiographyRarely Appropriate
- No testMay Be Appropriate
Try the scenario builder → to see how a calcium score changes what the criteria say about further testing.
Try it in the tool
Sources cited on this page
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