How do you interpret a coronary artery calcium (CAC) score?

How to Interpret Coronary Artery Calcium Score

Interpret the CAC score using standardized risk categories, with a score of 0 indicating excellent prognosis (<1% annual cardiac risk), scores 1-100 indicating mild atherosclerosis, scores 101-400 indicating moderate disease, and scores >400 indicating high risk requiring aggressive intervention. 1

Understanding What CAC Score Represents

• CAC score quantifies calcified atherosclerotic plaque burden but represents only approximately 20% of total atherosclerosis burden, since not all plaques contain calcium 1
• The score is a marker of atherosclerosis presence and extent, NOT a measure of stenosis severity or degree of luminal narrowing 1
• CAC is the single best predictor of cardiovascular risk compared to traditional risk factors and other nontraditional markers (superior to carotid intimal thickness, ankle-brachial index, C-reactive protein) 1

Standard Risk Categories and Their Clinical Meaning

CAC Score = 0: "The Power of Zero"

• Annual cardiac death or MI risk <1%, with only 0.47% adverse cardiovascular events at 50-month follow-up in asymptomatic patients 1
• Provides a "warranty period" of excellent prognosis for at least 5 years, even in patients classified as high risk by Framingham score 1
• In diabetic patients, the warranty period is shorter—mortality remains low for 5 years but increases nonlinearly thereafter 1
• Consider repeat scanning in 3-5 years only if diabetes or multiple cardiovascular risk factors are present 1, 2

CAC Score 1-100: Mild Atherosclerosis

• Indicates definite coronary atherosclerosis and low-to-moderate risk (<10% 10-year risk) 3
• For patients ≥55 years with CAC 1-99, moderate-intensity statin therapy is reasonable 2, 4
• Aspirin and statins are generally not recommended for CAC <100 unless other high-risk features present 3

CAC Score 101-400: Moderate Atherosclerosis

• Indicates intermediate risk (10-20% 10-year risk), with relative risk 2.7-4.1 times higher than zero calcium 5
• If score is >75th percentile for age/sex/race, this represents moderately-high risk (15-20%) 3
• Moderate-to-high intensity statin therapy is recommended for CAC ≥100 2, 5

CAC Score >400: Severe Atherosclerosis

• Indicates high risk (>20% 10-year risk) regardless of other factors 1, 3
• ACC/AHA guidelines recommend screening for clinically silent ischemia when CAC >400 1
• Coronary and total cardiovascular risk doubles with CAC progression at this level 2
• One-third of individuals with scores >1000 will have a cardiovascular event within 3 years 6

Age, Sex, and Race Adjustments

• Express CAC score as a percentile adjusted for age, sex, and race (white, Chinese, black, Hispanic) using MESA database 1
• At any given age, females have significantly lower scores than males 6
• A CAC score of 20 corresponds to arterial age ~61 years; CAC of 100 corresponds to arterial age ~73 years 1

Critical Clinical Pitfalls to Avoid

In Asymptomatic Patients

• CAC = 0 does NOT exclude obstructive CAD in symptomatic patients—7-38% of symptomatic patients with CAC = 0 have obstructive disease 1
• CACS should NOT be used to rule out CAD in symptomatic patients and is not routinely recommended in this population 1
• In the CONFIRM study, 13% of symptomatic CAC = 0 patients had nonobstructive disease and 3.5% had ≥50% stenosis 1

Regarding Stenosis Prediction

• CAC score has limited specificity (~50%) for predicting significant coronary stenosis (≥50% narrowing) 5
• High CAC scores indicate atherosclerosis burden but do NOT reliably predict presence of obstructive disease requiring intervention 5
• Location matters: left main coronary artery calcification carries higher prognostic significance than other vessels 2, 5

Regarding Serial Testing

• Statin therapy increases CAC scores as plaques stabilize and calcify—do NOT expect scores to decrease with treatment 1, 4
• Serial CAC testing should NOT be performed sooner than 3-5 years due to measurement variability and slow progression 1, 2, 4
• Follow-up CACS has limited value in patients with baseline CAC >400 who are already at high risk 1

Using CAC Score for Treatment Decisions

When CAC Score Changes Management Most

• CAC scoring is most clinically useful in borderline (5-7.5%) and intermediate-risk (7.5-20%) patients, where it can reclassify >50% of patients 2
• Adding CACS to Framingham data reclassified nearly 85% of intermediate-risk patients and improved prediction (C-statistic 0.78 to 0.82) 1

Specific Treatment Thresholds

• CAC = 0 in borderline/intermediate risk: withhold or delay statins, focus on lifestyle modification 2, 4
• CAC ≥100: initiate moderate-to-high intensity statin therapy with NNT of 30 to prevent one ASCVD event over 5 years 1, 2, 5
• CAC ≥400: aggressive risk factor modification, consider screening for silent ischemia 1

Additional Management Recommendations

• Target blood pressure <130/80 mmHg for patients with elevated CAC 2, 5
• Implement lifestyle modifications: 150 minutes weekly moderate-intensity exercise, Mediterranean or DASH diet, smoking cessation, weight management 2, 5
• For symptomatic patients with high CAC, proceed with functional testing or CT coronary angiography before invasive catheterization 5

Practical Algorithm for Interpretation

1. Obtain absolute CAC score (Agatston units) and percentile for age/sex/race 1
2. Classify into risk category: 0,1-100,101-400, >400 1, 3
3. For asymptomatic patients, use score to guide statin therapy decisions based on thresholds above 2, 4
4. For symptomatic patients, do NOT rely on CAC to exclude obstructive disease—proceed with appropriate functional or anatomic testing 1, 5
5. Initiate aggressive risk factor modification for any CAC >0 2, 5
6. Consider repeat scanning only at 3-5 year intervals for CAC = 0 patients with diabetes or multiple risk factors 1, 2