Should You Have Your Apo B Levels Measured?
Updated September 12, 2014.
Written or reviewed by a board-certified physician. See About.com's Medical Review Board.
Maybe your doctor has recommended that you get a blood test called “Apo B” to assess your risk of cardiac disease. Or more likely, maybe you’ve read on-line (probably from a facility that does Apo B testing) that the LDL cholesterol level is an imperfect risk marker, and that you would get a better idea of your cardiac risk if you had an Apo B test. Are they right? Let’s take a look.
What Is Apo B?
Apo B is shorthand for apolipoprotein B, an important protein found on the surface of LDL cholesterol lipoprotein particles. Each LDL particle contains a single Apo B protein. This means that measuring Apo B levels is a way of estimating the number of LDL particles a person has in their blood. And this fact accounts for the significance of the Apo B measurement: The Apo B level is a surrogate measure for the number of LDL lipoprotein particles in the blood.
Why Are LDL Particle Numbers Important?
We have all heard a lot about “LDL cholesterol.” LDL cholesterol is the total amount cholesterol being carried through the bloodstream by LDL lipoproteins. (Other kinds of lipoproteins also carry cholesterol, such as VLDL and chylomicrons).
The reason LDL cholesterol is important is that elevated levels of LDL cholesterol have been strongly associated with an increased risk of cardiac disease. However, it turns out that there are at least two different “kinds” of LDL lipoprotein particles in our blood, and one appears to be more “dangerous” to our health than the other.
Specifically, there are small, dense LDL particles, and big, “fluffy” LDL particles. The size and density of the LDL particle is determined by how much cholesterol it is carrying - the big LDL particles are carrying a lot of cholesterol, and the small ones just a little cholesterol. And the small LDL particles, in several studies, have been correlated with higher cardiac risk.
The reason the small, dense LDL particles are thought to be more dangerous is that they apparently are able to pass more readily through the walls of an artery, and so are more able to deposit their cholesterol loads into an atherosclerotic plaque.
Many experts believe that one reason many patients develop heart disease despite LDL cholesterol levels that are not very high is that their LDL cholesterol is carried in small, dense packages instead of big, fluffy ones. So the LDL cholesterol they do have is more likely to cause damage to their arteries.
These same experts tend to say that, in order to completely characterize our cardiac risk, we should not only have our cholesterol values measured, but also the number of LDL cholesterol particles we have (the higher the number of particles , the more small, dense particles are present). And the Apo B value reflects the number of particles of LDL lipoproteins. This is the rationale for measuring Apo B.
How Important Is It To Measure Apo B?
In general, it is a good idea to have a medical test only when knowing the results of that test may help with medical recommendations, or change our behavior. Knowing our Apo B levels can indeed more fully characterize our cardiac risk. The outstanding question is: how useful is this additional information over and above “standard” blood lipid testing?
In many cases, the presence of small, dense LDL particles can be inferred without actually doing the blood test. LDL particles tend to be small and dense in patients who have elevated triglyceride levels and reduced HDL cholesterol levels, as well as in patients who are sedentary or obese, or who have type 2 diabetes, hypertension, increased abdominal fat, or who are smokers. In other words, if you have a few of these more “traditional” risk factors, you are also likely to have small, dense LDL particles. It is doubtful, in these cases, whether actually measuring Apo B levels adds substantially to an accurate assessment of your cardiac risk.
Furthermore, knowing that Apo B levels are elevated only rarely changes a person’s treatment. Aggressive lifestyle changes (weight loss, smoking cessation, exercise and diet) are thought to be effective in reducing Apo B levels. Statin drugs often (but not always) lower Apo B levels. But people who are at elevated cardiac risk because of their lifestyle choices already know that they should be making lifestyle changes, whether or not they know their Apo B levels. Similarly, the need for statin therapy to reduce cardiac risk can be decided upon in the vast majority of patients without measuring Apo B levels.
The Bottom Line
Measuring Apo B levels can help in assessing a person’s level of cardiac risk. However, in most cases that risk assessment can be made adequately without directly measuring the Apo B level. In addition, knowing the Apo B level usually does not affect the recommendations your doctor will make to help you reduce that risk.
Drugs are currently under development that are aimed specifically at increasing the size of LDL particles (and reducing Apo B levels). Once those drugs are available, and have been shown to improve outcomes over and above currently available therapies, then the measurement of Apo B levels will become extremely important.
For now, however, the measurement of an Apo B level usually is of only marginal benefit. If your doctor is asking you to have an Apo B level done, then (especially if you are paying out-of-pocket for all or some of the test), it is appropriate to ask her how knowing your Apo B value is likely to change your treatment recommendations.
Sources:
Selby JV, Austin MA, Newman B, et al. LDL subclass phenotypes and the insulin resistance syndrome in women. Circulation 1993; 88:381.
Siegel RD, Cupples A, Schaefer EJ, Wilson PW. Lipoproteins, apolipoproteins, and low-density lipoprotein size among diabetics in the Framingham offspring study. Metabolism 1996; 45:1267.
Zambon A, Hokanson JE, Brown BG, Brunzell JD. Evidence for a new pathophysiological mechanism for coronary artery disease regression: hepatic lipase-mediated changes in LDL density. Circulation 1999; 99:1959.
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