Calcium Scan And Subtractive Medicine

27.07.2017
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Being a radiologist, I rarely speak to patients, but I was asked to counsel Mrs. Patel (not her real name, so calm down HIPAA totalitarians), who was worried about the risks of radiation from cardiac calcium CT scan. Because of her risk factors for atherosclerosis, her cardiologist wanted her to take statins for primary prevention, but she was reluctant to start statins.

They eventually reached a truce. If she had even a speck of calcium in her coronary arteries she would take statins. If her calcium score was zero she wouldn’t. This type of shared decision making is the most frequent reason why cardiologists order calcium scans at my institution.

A calcium scan is a nifty test, not because it improves outcomes – that’s a population-based consideration – but because it changes management, specifically when there is zero calcium. It does this by so lowering the patient’s risk profile that they no longer meet the risk threshold deemed by the AHA [American Heart Association], and endorsed by the USPSTF [U.S. Preventive Services Task Force], for starting statins for primary prevention.

You can quibble about the threshold for recommending statins, but there is no quibbling that [a] zero-calcium scan often reclassifies risk bringing the person to a lower risk than previously thought. Zero calcium portends a happier future. In one study, of those who were eligible for statins because their estimated risk of cardiovascular events over 10 years was > 7.5%, nearly half had zero calcium, which put them at a lower risk profile.

Calcium scans unmask a tremendous amount of risk heterogeneity, even amongst those at high risk for cardiovascular events. A zero calcium even in someone with the highest Framingham Risk Score (FRS) shifts their risk profile to within the safer territories of the lower FRS. The shift is quite dramatic, and affects the intensity of primary prevention or the need to indulge in it. I’m reluctant to use the term ‘precision medicine’ because Vinay Prasad will chew me alive on Twitter, but careful use of calcium scans is an example of ‘preciser medicine.’

The rhetoric with calcium scans can be misleading. For example, some call it a ‘mammogram of the heart’ – aside from the insensitive gender nihilism, this also insinuates, to borrow a slogan from the screening world, that ‘calcium scans save lives.’ This is not strictly correct. First, a zero-calcium scan does not exclude the deadlier, non-calcified plaque, which is often the culprit lesion in young people with fatal myocardial infarctions. But such rhetoric would also need a sufficiently powered randomized controlled trial. The sample size is likely to be immense, perhaps greater than the population of Mumbai, because in an undifferentiated population, many of whom may already be eligible for statins for primary prevention, and thus be optimally medicated, the incremental mortality benefits, at population level, from change in therapy, conditional on knowing that there’s calcium is not likely to be tremendous. Calcium scan is an example of subtractive medicine – it subtracts statins from people. We need more subtractive medicine.

Calcium scans can induce downstream testing, but radiologists are getting better at limiting the field of view to the heart, to avoid seeing the thyroid and adrenal glands – organs which often beg to be over tested. See no thyroid nodule, hear no thyroid nodule. There’s no getting away from the pesky lungs, unfortunately, which means sometimes we will see and follow-up lung nodules, and sometimes we will catch an incidental lung cancer and pretend to have saved the patient’s life. There is also the ascending aorta, a tortuous structure jutting out of the heart, and wandering aimlessly for some distance, which, but for the grace of a radiologist’s caliper-happy fingers, is minding its own business (I tell radiology residents not to measure the ascending aorta unless it’s blitheringly obvious that it’s large).

Carefully used, with the intent of changing management, a calcium scan, specifically when there is zero calcium, is a useful test. It’s like a GPS which tells you to avoid the NJ [New Jersey] Turnpike.

 

This article originally appeared in the Health Care Blog.

Image copyright: LoggaWiggler/pixabay/CC0

Article last time updated on 01.08.2017.

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