You probably know your height and weight and may even have your typical blood pressure readings and most recent cholesterol scores memorized. But there's another number that might be even more important for assessing your risk of heart attack or stroke.

The coronary artery calcium (CAC) score measures the amount of calcified plaque you have in those arteries, which is important because coronary plaque is the main underlying cause of — or precursor to — atherosclerotic cardiovascular disease (ASCVD) events such as heart attacks and strokes.

Your calcium score can range from zero to infinity and is proportional with your risk of having such an event up to 15 years. People with no coronary artery calcium — even those with risk factors like diabetes, obesity or advanced age — have low absolute risk of ASCVD events, or the lowest among individuals with similar characteristics. In contrast, people with high CAC scores have an elevated risk of ASCVD, even if they've never had any symptoms of heart disease and appear to be the picture of health.

A calcium test may be the most accurate predictor, or biomarker, of ASCVD risk to date — at least of the safe, noninvasive and relatively inexpensive variety. But not everyone needs one and not everyone who could benefit from one knows about it.

Here's what you need to know about this test, whether you need one and what the results might mean.

Wait, isn't calcium a good thing?

Good question! Calcium in your bones and the food you eat is a good thing, yes. Calcium in your arteries, not so much.

The amount of CAC is a measure of coronary artery plaque, a waxy substance that doesn't directly show up in a CT scan. However, because plaque is a mixture of fat and calcium, over time the calcium begins to leave traces, or deposits, inside the artery. The innermost layer of the artery becomes calcified, and this can be detected by CT scan in the same way that X-rays look through your skin and show your bones.

A buildup of plaque can clog your arteries, which will slow your blood flow and prevent certain areas of your heart from receiving proper levels of oxygen. The plaque deposits also can cause a blood clot which may trigger a heart attack. Because the CAC score gives an indirect measure of the burden of plaque in the coronary arteries, determining your calcium score is a very useful way to predict the likelihood of a future heart attack or stroke.

I already know I have a risk factor for heart attack or stroke. Why do I need this test?

The traditional model of risk assessment looks at your demographic information, family history, health conditions, lifestyle and blood work to help estimate your risk. It doesn't measure the actual burden of plaque in your arteries and isn't highly accurate at predicting who will have calcified plaque and who won't. When you take people in various traditional risk categories and test for CAC, you find a lot of surprising results.

An estimated 15% of individuals assumed to be at low risk have a relatively high amount of calcified plaque and are therefore at higher risk of having an event than anticipated. And about 30%-50% of patients in traditional high-risk categories have no plaque (a CAC score of zero), which puts them at low risk of events.

"With this test, we can stop guessing about the condition of your arteries," says Dr. Miguel Cainzos-Achirica, preventive cardiologist at Houston Methodist Hospital. "Instead, we can see if there's any plaque there, and if so, how much."

So do I need to get a calcium score test?

Currently, the test is recommended for individuals in the intermediate risk range who would like a more precise assessment of their risk to help guide treatment and medication decisions.

Intermediate risk means you have at least one traditional cardiovascular risk factor, such as:

• High blood pressure
• High cholesterol
• Diabetes
• Obesity
  
  

In addition, several recent guidelines from various scientific societies suggest that low-risk individuals with a strong family history of ASCVD, particularly involving events occurring at a premature age, should consider getting the test for further risk assessment.

Who is the test not recommended for?

Because there is some cost to the patient and a small amount of radiation involved in CT scanning, the test is not recommended for those who are unlikely to be able to use the results to make health decisions, such as:

• People under the age of 40, since accumulating coronary artery calcium so early in life is uncommon.
• People who have already had a heart attack, stroke or serious cardiovascular event, which automatically places them at high risk, with extensive management strategies required.
• Pregnant women or anyone who needs to strictly avoid radiation.
• People who have had a recent CAC test, although the test may be repeated at three to five years if the initial score was zero and management is uncertain after that time.
  
  

If you fall outside of the recommended group, but still feel that the results could be valuable, you can discuss it with your provider.

How do I get a calcium score test?

Your primary care provider or preventive cardiologist will order it for you. A good time to bring up this conversation is at your annual physical or regular check-up, when you are already discussing your health stats, risk factors and preventive medications your doctor wants you to take.

What does the test entail?

CT scanners use X-rays, typically in modern multi-detector CT scans. The radiation exposure is usually low — similar to a bilateral mammogram — and no contrast dye is required.

At your appointment, you will change into a hospital gown and lie down on a special scanning table. A technologist will clean three small areas of your chest — men may need to have the areas shaved — and place sticky patches there. The patches contain electrodes that monitor your heart's electrical activity throughout the test.

The table will move into a donut-shaped scanner, and a high-speed camera will take multiple images of your heart within about 10 seconds. After the images are taken, the table will move back, and you get dressed and go home. It should take only a few minutes and be pain and discomfort free.

For most patients with insurance, the out-of-pocket cost of the test ranges from $75-150.

What happens after the test?

The images are combined and analyzed by a computer program and cardiovascular radiologist. If the scan shows specks of calcium embedded in the walls of the arteries, that means there is some calcification present. The computer program will measure it and calculate a score that estimates the amount of coronary artery plaque and the extent of coronary artery disease.

What do the scores mean?

The results will be sent to your doctor, who should go over what they mean in detail. But, in general, the score ranges break down as follows:

Zero: You have no calcified plaque. Your risk of ASCVD event is low — not zero — but the lowest among your group. For instance, if you are a 45-year-old female with diabetes, your risk is not zero — because you still have the risk factor of diabetes — but you have the lowest risk among other 45-year-old females with diabetes. (See the next section for more information about the enhanced flexibility CAC scores of zero can offer.)

1-10: You have a small amount of plaque. Although your overall risk of heart attack is still low, the ASCVD process has started. Prevention strategies should be discussed in order to keep your risk on the lower end, potentially with a focus on lifestyle changes.

11-100: You have some coronary plaque and a moderate chance of heart attack. Your doctor may recommend treatments in addition to lifestyle changes.

101-400: Your chance of having a heart attack is moderate to high. Your health professional will want you to consider intensive healthy lifestyle changes and, possibly, start treatment with a statin to reduce your cholesterol levels.

Over 400: You have a large amount of plaque, and your chance of ASCVD event is high. Your health professional will want you to consider intensive healthy lifestyle changes and pharmacological interventions to reduce your risk of ASCVD, including a statin to reduce your cholesterol levels.

What does a score of zero mean for me?

A CAC score of zero is always good news. But it's particularly good news for people who have one or more traditional risk factors.

"If you expect to find plaque in someone based on their health history, but you don't find any, that means there is something about that person's body that's offered them enhanced protection, or resiliency, from atherosclerotic heart disease," says Dr. Khurram Nasir, preventive cardiologist at Houston Methodist Hospital.

That doesn't mean this protection will last forever, especially if the risk factors persist or get worse. But it does offer a window of opportunity to address health conditions and make lifestyle changes before damage begins to take place in the arteries.

Part of taking the best care of your health is making informed health decisions.

"Low risk means you have flexibility," says Dr. Nasir. "It means you can delay starting a medicine and focus on making lifestyle changes. Or you can start taking the medicine to offer even more protection, if that's what you want. The important thing is to keep aiming to improve your health and reduce your risk factors so that your heart can remain protected for as long as possible."