I remember two patients who, on the surface, seemed to tell completely opposite stories. The first was a mid 30ish man whose calcium score was around 900 (normal being zero and high being anything more than 400) — a number that immediately sounds alarming. We proceeded to coronary angiography, an invasive test to assess the state of the arteries. There were blockages, but they were relatively minor and did not require stenting. He was, therefore, managed with medication and risk-factor control.Then there was another patient: a 38-year-old man with diabetes who had chest pain on exertion. His calcium score was zero. You might think that meant his coronary arteries were completely clear. They were not. He had a 90 per cent blockage and ultimately required a stent.So how can both things be true? The answer lies in understanding what a coronary calcium score actually measures and, equally importantly, what it does not measure.Calcium is a marker of plaque, not a measurement of blockageWhen cholesterol plaque builds up inside an artery, some plaques, over time, become hardened and develop calcium deposits. This is part of the process of atherosclerosis. A calcium-score CT scan detects these calcified deposits and gives them a numerical score, commonly referred to as the coronary artery calcium (CAC) score.This calcium score tells us about the presence and burden of calcified coronary plaque. It does not tell us what percentage of the artery is blocked. A score of 900, therefore, does not mean that an artery is 90 per cent blocked. It means there is a substantial burden of coronary calcium and, consequently, a higher likelihood of underlying coronary atherosclerosis.In general, coronary calcium is associated with greater cardiovascular risk. But there is no simple one-to-one relationship between the calcium score and the percentage of narrowing inside an individual artery.Why can someone have a calcium score of zero and still have a major blockage?This is where the second patient becomes important. Not every coronary plaque is calcified. Some plaques contain calcium and become hard over time. Other plaques can remain predominantly soft or non-calcified. A calcium-score scan is specifically looking for calcium, so a soft, non-calcified plaque may not contribute to the calcium score. That is why a calcium score of zero does not mean that there is absolutely no plaque or that significant coronary disease is impossible.Story continues below this adThis is particularly important when a person already has concerning symptoms or important risk factors such as diabetes. In the 38-year-old patient, the calcium score was zero, but his symptoms prompted further evaluation. The subsequent assessment demonstrated a 90 per cent stenosis, which required stenting.The lesson is simple: A calcium score is a risk marker; it is not an angiogram.Then what is the role of CT coronary angiography?A calcium-score scan and a CT coronary angiogram (CTCA) answer different questions. A calcium score essentially asks: “Is there calcified plaque in the coronary arteries, and how much calcium is present?” A CT coronary angiogram goes further. It can visualise the coronary arteries and the plaque within them, including non-calcified plaque, and can assess whether there is narrowing of the artery. Therefore, CT coronary angiography can be an important adjunct to calcium scoring when more anatomical information is needed. The calcium score should not be thought of as a substitute for CT coronary angiography when the clinical situation calls for anatomical assessment.And neither test should be interpreted in isolation from the patient’s symptoms and overall risk profile.Story continues below this adWhat does a high calcium score mean?At a population level, higher calcium scores are associated with higher cardiovascular risk. The score can, therefore, be particularly useful for risk stratification and screening in appropriately selected people, helping clinicians decide how aggressively cardiovascular risk factors should be addressed. But we should avoid the common mistake of translating the score directly into a percentage of blockage.Does calcium in the artery come from calcium supplements or vitamin D?The calcium seen on a coronary calcium scan should not simply be interpreted as “calcium that has accumulated because the person ate too much calcium.” Coronary artery calcification is a manifestation of the atherosclerotic disease process. It is not the same thing as saying that dietary calcium or a calcium supplement has directly deposited itself into the coronary arteries. Similarly, the presence of coronary calcium should not automatically be attributed to vitamin D supplementation.And what about aspirin and statins?Finding coronary calcium is a reason to take cardiovascular risk seriously, but it does not automatically mean that every person with any calcium should take aspirin or blood clot busters. Statin (bad cholesterol buster) therapy is often considered when coronary calcium indicates increased cardiovascular risk, but the decision depends on the individual’s overall risk, age, diabetes status, cholesterol levels, other medical factors and the clinician’s assessment.Think of the calcium score as a warning signal rather than a measuring tape.Story continues below this ad(Dr Shetty is the lead cardiologist and medical director, Sparsh Hospital, Bengaluru)