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In the clinical laboratory, we routinely measure the cholesterol content of high-density lipoprotein and low-density lipoprotein particles and not the apolipoproteins on the particles or the number of particles. Proprietary detergents and reagents are used in assays for HDL-C and LDL-C to separate lipoproteins, allowing the cholesterol content of specific lipoproteins to be measured. For example, HDL-C is commonly measured using a solution of dextran sulfate and magnesium to selectively precipitate HDL from the other lipoproteins present in the sample. Once isolated, the HDL particles are 'dissolved' and the amount of cholesterol in them is determined photometrically using a color-producing enzyme reaction. LDL-C can be measured directly or can be estimated using the HDL-C, triglycerides and total cholesterol (TC) values. The Friedewald formula is often used to calculate LDL: LDL-C = TC - (HDL-C)+(Triglycerides/5). The important point to consider here is that traditional LDL-C and HDL-C measurements only tell us how much cholesterol is associated with each lipoprotein particle class. We are now learning that the number and size of the particles are important as well. The number of LDL particles appears to be more strongly predictive of cardiovascular disease than the LDL-C content, and small dense LDL are known to be more atherogenic than larger, less dense LDL particles.

Measuring ApoB and ApoA1 can be performed using standard immunoassay techniques. Nephelometry is popular, as are ELISA-based methods that are performed on automated chemistry analyzer platforms. The power of the ApoB/ApoA1 ratio as a cardiovascular risk marker is getting widespread attention. An individual with seemingly normal LDL-C may in fact have high ApoB concentrations. When this individual has his or her ApoB/ApoA1 ratio calculated, the risk is evident. Studies have also shown that patients with metabolic syndrome and type-2 diabetes can also easily be identified with the ApoB/ApoA1 ratio, whereas these patients cannot always be identified by measuring LDL-C and HDL-C.In 2004, the global INTERHEART study of risk factors for acute myocardial infarction concluded that the ApoB/ApoA1 ratio was the most important risk factor in all geographic regions. The ApoB/ApoA1 ratio is easy to use because the risk is integrated into a single number that indicates the balance between atherogenic and antiatherogenic particles.There have been many studies concerning the predictive power of the ApoB/ApoA1 ratio. One study, which involved thousands of patients who were followed for an average of 10 years, showed that the ApoB/ApoA1 ratio was a strong predictor of stroke in addition to other cardiovascular events. Due to the evidence presented in studies like these, the National Academy of Clinical Biochemistry (NACB) has recommended that the ApoB/ApoA1 ratio be used as an alternative to the usual total cholesterol (TC)/HDL cholesterol ratio when determining lipoprotein-related risk for cardiovascular disease. Some believe that ApoB/ApoA1 testing will eventually replace traditional LDL-C and HDL-C measurements.

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Oxidized LDL leads to the release of chemotactic factors from nearby cells; factors which signal leukocytes to migrate to the site. Recall that atherosclerosis is believed to be caused by phagocytic cells such as macrophages, which ingest LDL particles and turn into stationary foam cells. Macrophages have been shown to have increased affinity for oxidized LDL. Thus, oxidation makes LDL more susceptible to phagocytosis and therefore more atherogenic.Since oxidized LDL is more atherogenic than native LDL it makes sense that oxidized LDL may be a cardiovascular risk marker. Indeed, many studies have now correlated increased levels of oxidized LDL with risk of cardiac events.

Although lipoproteins of a particular class are generally within a given size range, there are many biochemical processes that interact with lipoproteins to alter their size, density, and lipid composition. When low-density lipoprotein (LDL) becomes smaller and denser, it is more likely to interact with the arterial wall, leading to deposition of cholesterol and initiating or worsening atherosclerosis. Research has shown that high numbers of smaller, denser LDL are more atherogenic than larger, lighter LDL particles. Small, dense LDL particles are associated with more than a three-fold increase in the risk of coronary heart disease.