My doctor recently recommended that I get a cardiac CT calcium scan to help stratify my risk of cardiovascular disease (CVD). So of course I was interested in how good this scan is, because diagnostic tests to assess risk, especially in those who are asymptomatic, can be tricky. I was happy to discover that the short answer is – yes. These tests are very helpful in stratifying risk of coronary events, even in young patients, those without a prior cardiac event, and those with otherwise high traditional CVD risk factors. (I had a good test outcome if you are interested – an Agatston score of zero.)
It’s a good opportunity to review the role of diagnostic testing, and other factors, in stratifying risk of disease or medical events, and further the role of stratifying risk in determining the best treatment. Treatments in medicine, as we have discussed many times, is all about risk vs benefit. No treatment is risk-free, and interventions come with cost, inconvenience, and opportunity cost. Some may also be invasive or have significant side effects. Therefore we always want to determine, as much as possible, which population will have the greatest benefit to risk ratio, and where the point is where those lines cross. Only with certain nutrients do we literally or metaphorically “put it in the water”.
Cardiac events are notoriously difficult to predict. Harriet wrote in 2008 about cardiac CT angiograms, and hit most of the salient points. CT angiograms look at stenosis or blockage in the coronary arteries, and questioned at the time whether or not CT angiograms predicted future events or made a significant difference in treatment or outcome. This is because stenosis of the coronary arteries, while it is one piece of relevant information, by itself is not a great predictor of the future risk of heart attacks.
This may seem counterintuitive, but makes sense in the context of how most heart attacks occur. One way is for plaque to build up on the inside wall of a coronary artery until it simply blocks blood flow completely, infarcting coronary tissue. But this is rarely what happens. Such blockages would occur slowly, over years, resulting in collateral blood flow to compensate for the slow decrease in flow through the original artery. Literally new small arteries grow to bypass the blockage.
Much more often the cause of a heart attack is an ulceration of the plaque – the plaque breaks down through inflammation, causing platelets to clump at the site causing a sudden thrombus (blood clot) which blocks flow. This is why antiplatelet drugs, like aspirin, are helpful in reducing the risk for such events. Plaques that are ready to ulcerate have calcium in them (oversimplified but true enough). So cardiologists developed a technique for imaging calcium in the coronary arteries. They then did the hard work of real medical science – studying if and how useful such scans are.
The CT scan for calcium in the coronary arteries was first introduced in 1990. This was followed by 30 years of research, exploring every aspect of this test. In 2018, the cardiovascular society cholesterol guidelines recommended a calcium scan to help determine if someone should go on statin drugs to reduce their cardiovascular risk. Use of this san has increased since then. Also, the technology itself has improved over time. The current standard is multidetector ECG-gated CT, which is fast, does not use any contrast, and uses significantly less radiation exposure than older scans. So the risk and inconvenience has gone down while evidence for benefit has piled up.
That evidence has shows multiple things. First and foremost, the degree of calcium in your coronary arteries is a strong and independent predictor of future risk of coronary events. It is independent of age, of other traditional risk factors (cholesterol, weight, smoking, and family history), and of prior events. It is better than any other single predictive factor, and as good a predictor as having a prior event (which has always been the best predictor of future events).
This does not mean that everyone should get one. One common question for instructors to ask students, to help them learn clinical decision making, is – how will this affect your management? You want to order this test? Will it affect anything that you do? For CT calcium scans the main utility seems to be determining statin therapy. There are different ways to measure calcium, but the dominant method is now the Agatston score, which measures visible calcium deposits of a certain threshold. A score of 0 means very low risk, 1-100 is low risk, 101-300 is moderate risk, and >300 is high risk.
Further, we can use these scores, combined with other risk factors, to make informed decisions about statin use. If the score is 0 then statin use can be deferred, unless someone has very high cholesterol. Low risk you can probably wait and repeat the scan in 3-10 years, with moderate risk it is probably a good idea to go on statins but not absolutely necessary, and for high risk, statin therapy is highly recommended. This is always coupled with lifestyle interventions.
Therefore, cardiac calcium scans are a well-researched diagnostic tool, developed based on our basic understanding of the mechanism of most heart attacks, with high predictive value and high clinical value in individualizing preventive treatment. The technology itself has also evolved to make it a quick, easy, and low risk study.
This history of this diagnostic tool also shows nicely how science-based medicine works. After the scan was developed, it did not immediately go into widespread use. Doctors were skeptical of its utility, and asked every possible question about what it did and did not predict, and eventually developed an evidence-based recommendation for how it can be incorporated into the total care of patients.
This stands in stark contract to many “alternative” or pseudoscientific medical interventions, where a diagnostic or therapeutic intervention is developed, often out of whole cloth based on some hand-wavy justification, and then widely adopted without any real assessment of its safety and utility. Never do you see three decades of careful research before use is cautiously recommended.
