ApoB is an accurate and low-threshold first step for mapping the number of harmful lipid particles. A CAC score via CT scan actually shows calcified plaque in the coronary arteries, but is most useful at intermediate cardiovascular risk. Together, these two measurements provide a fairly complete picture, but neither fully rules out risk.
A CAC score and ApoB measure two different things that together give a reasonably complete picture of your cardiovascular risk. ApoB counts the number of harmful lipid particles in your blood, similar to LDL cholesterol but more accurate when triglycerides are elevated or your LDL value is borderline. It is a simple blood draw, virtually risk-free, and you can request it from your GP as part of a lipid panel.
The CAC score goes a step further: via a CT scan it shows how much calcified plaque is actually present in your coronary arteries. A higher score means more plaque and more risk; a score of zero provides some reassurance but does not completely rule anything out. The major caveat is that the scan only detects calcified plaque, not the softer variety. The test is most useful when your risk has already been estimated as 'intermediate' and the result has a concrete influence on treatment decisions. At very low or very high risk it adds little.
From a practical standpoint: ApoB is the logical first step, as it is inexpensive, involves no radiation, and is informative for everyone. The CAC scan is a follow-up step if your cardiovascular risk remains unclear after blood tests, and that question is best raised with your GP or cardiologist. The scan does involve a small radiation dose and can sometimes produce incidental findings that in turn lead to further investigation -- something to be aware of before requesting it.
Overview across multiple factors (2 research records, 2 sources). The strength of evidence differs per component -- read the answer for the nuance.