Coronary artery calcium scoring: A valuable tool in primary care
SHOULD WE TEST PATIENTS AT HIGH OR LOW RISK?
Who, then, should we test? For patients at high or low risk according to the Pooled Cohort Equation, 2 questions determine whether calcium scoring is warranted: how much would an extremely high or low score (ie, 0 or > 400) change the risk of an event, and how likely is an extreme score?
The first question relates to the usefulness of the test, the second to its cost-effectiveness. If even an extreme score cannot move a patient’s risk into or out of the treatment range, then testing is unwarranted. At the same time, if few patients have an extreme score, then cost per test that changes practice will be high.
Because calcium scoring is a direct test for disease, it is extremely predictive. When added to risk-factor models, it substantially improves discrimination9 and exhibits excellent calibration.10 This is true whether the outcome is a major cardiovascular event or death from any cause.
But the calcium score is not strong enough to override all other risk factors. A patient with a predicted 10-year risk of 18% according to the Pooled Cohort Equation and a calcium score of 0 could be reclassified as being at low risk, but a patient with a 10-year predicted risk of 35% could not. The same is true for patients at low risk. A patient with a 4% risk and a calcium score higher than 400 would be reclassified as being at high risk, but not a patient with a 1% risk.
Extreme calcium scores are common, especially in patients at high risk. In the Multi-Ethnic Study of Atherosclerosis (MESA) cohort, 45% of patients with a 10-year predicted risk of 7.5% to 20% had a calcium score of 0, reclassifying them into the low-risk category.11 Even if the predicted risk was greater than 20%, 1 in 4 patients had a score of 0. In contrast, if the 10-year predicted risk was below 5%, one-fifth of patients had a calcium score greater than 0, but only 4% had a score greater than 100.
Nevertheless, patients in the low-risk category whose baseline risk is close to 5% may wish to undergo calcium scoring, because a positive test opens the door to a potentially lifesaving treatment. In general, the closer patients are to the treatment threshold, the more likely they are to be reclassified by calcium scoring.
The Society for Cardiovascular Computed Tomography currently recommends coronary artery calcium scoring for patients whose 10-year risk is between 5% and 20%.12 These numbers are easy to remember and a reasonable approximation of the number of patients likely to benefit from testing.
COMBINING CALCIUM SCORING WITH TRADITIONAL RISK FACTORS
Primary care physicians interested in more exact personalized medicine can use a risk calculator derived from the MESA cohort.13 Based on 10-year outcomes for 6,814 participants, Blaha et al8 derived and validated this risk-prediction tool incorporating all the elements of the Pooled Cohort Equation in addition to family history, race, and calcium score.
The tool offered good discrimination and calibration when validated against 2 external cohorts (the Heinz Nixdorf Recall Study and the Dallas Heart Study).10 The C statistics were 0.78 and 0.82, with 10-year risk predicted by the tool within half a percent of the observed event rate in each cohort.
The online calculator displays the 10-year risk based on risk factors alone or including a calcium score, allowing the clinician to gauge the value of testing. For example, a 70-year-old nonsmoking white man with a total cholesterol level of 240 mg/dL, high-density lipoprotein cholesterol 40 mg/dL, and systolic blood pressure 130 mm Hg on amlodipine has a 15.2% 10-year risk (well above the 7.5% threshold for statin therapy). However, if his calcium score is 0, his risk falls to 4.3% (well below the threshold). Sharing such information with patients could help them to decide whether to undergo coronary artery calcium scoring.
Ultimately, the decision to take a statin for primary prevention of coronary artery disease is a personal one. It involves weighing risks, benefits, and preferences. Physicians can facilitate the process by providing information and guidance. Patients are best served by having the most accurate information. In many cases, that information should include calcium scoring.
