Cardio IQ^® Lipoprotein Fractionation, Ion Mobility

Ion mobility lipoprotein fractionation is a technology that uses gas-phase (laminar flow) electrophoresis to separate unmodified lipoproteins on the basis of size. Following the separation, each lipoprotein particle is directly detected and counted as it exits the separation chamber.

Ion mobility fractionation provides direct measurement of lipid subclasses. This approach to advanced lipid testing combines high-resolution separation of the full spectrum of lipoprotein particles with direct quantitation of particles in each lipoprotein subclass.

The sizes of the lipoprotein particles detected and counted are not affected by particle modifications. The particles are unaltered by stains or ultracentrifugation forces. Ionized lipoprotein particles are electrophoretically separated in a gas phase, and lipoprotein particles are distinguished on the basis of size. Size-separated particles are detected and counted by light scattering.

Ionized lipoproteins migrate across a laminar gas-phase flow, based on size and electrical field. Only a single size of lipoprotein will exit the field and be isolated (green line) at any point across the voltage gradient; larger and smaller lipoproteins (dotted black) are not collected. As the voltage ramps across the gradient, all of the lipoproteins are captured.

The high resolution of ion mobility’s lipoprotein subfractionation is comparable to that seen with analytical ultracentrifugation (AnUC) and segmented gradient gel electrophoresis (SGGE) methods.¹ Thus, the extensive literature supporting the clinical use of lipoprotein subclasses derived from these 2 methods can be applied to ion mobility-derived subclasses as well.

Advanced lipid testing with ion mobility is used to help assess the risk for cardiovascular disease (CVD) in patients with intermediate or high risk based on traditional or emerging risk factors, and to assess therapeutic response in patients undergoing lipid-lowering therapy.

Ion mobility technology precisely quantifies lipoprotein fractions across the entire lipoprotein spectrum; this comprises very low-density lipoprotein (VLDL), intermediate-density lipoprotein (IDL), low-density lipoprotein (LDL), and high-density lipoprotein (HDL) particles.¹ However, to facilitate risk assessment, Quest Diagnostics reports only those measurements that were significantly correlated with CVD events in a cohort of men and women from the prospective Malmo Diet and Cancer Study²:

• Small, medium, and total LDL particle numbers (p≤0.004)^2,3
• LDL peak size (p≤0.012) and the associated LDL pattern^2,3
• Large HDL particle number (p<0.001)²

The focus on these measurements, which include increased triglycerides, is consistent with identification of the “atherogenic lipoprotein phenotype” first proposed in 1990.^4,5

An elevated total LDL particle number is associated with a 1.2- to 1.4-fold increase in CVD risk.^2,3  Similarly, elevated small and medium LDL particle numbers have been associated with a 1.2 to 1.4-fold increase.^2,3

Ion mobility identifies 2 main subclasses of HDL: large HDL and small HDL. The antioxidant properties of large HDL may help protect the arterial wall. A decreased large HDL subclass has historically suggested increased CVD risk.

The ion mobility-derived total, small, and medium LDL subclass concentrations (nmol/L) reflect direct detection and quantitation of the number of LDL particles.¹

In contrast, the nuclear magnetic resonance (NMR)-derived LDL particle number (LDL-P, nmol/L) includes the 3 LDL subclasses (small, medium, total LDL) and IDL. The concentration is measured indirectly from NMR signals emanating from terminal methyl groups from all different lipid classes (triglycerides, cholesterol ester, unesterified cholesterol and phospholipids) in the lipoprotein particle shell and core.⁶

As noted above, the ion mobility-derived total LDL particle concentration (nmol/L) reflects direct detection and quantitation of the total number of LDL particles.¹

Apolipoprotein B (apo B) measurements, on the other hand, represent the number of non-HDL particles, including LDL particles. Commercially available apo B assays measure lipoproteins containing apo B-100 and apo B-48. This includes the apo B component of LDL, IDL, VLDL, lipoprotein(a), and chylomicrons. Results are reported as mg/dL.

All results from risk markers should be considered when assessing a person’s risk for CVD, even when they place the person in different risk categories. This is especially important when assessing residual risk.

That being said, it is reasonable that the finer the measuring tool and the more specific the data being generated, the more insightful the conclusions may be. Historically, LDL subclass patterns (ie, phenotype) have been characterized as predominantly small and dense (LDL pattern B) or predominantly large and buoyant (LDL pattern A) in order to characterize populations for analysis in research studies. However, some patients classified as having LDL pattern A can have an unhealthy amount of small LDL.

The LDL particle size is the diameter at the maximum LDL peak height and is a good general measure of the LDL particle distribution. However, it does not always accurately reflect particle distribution.

A high total LDL particle number may be the result of a high number of large LDL particles or a high number of small LDL particles. All LDL particles are atherogenic, but the small LDL particles are considered to be more atherogenic. Thus, knowledge of the individual LDL subclass contribution to the total LDL particle number helps the clinician interpret the total LDL particle number result.

The lipoprotein subfraction reference intervals and cut-points for ion mobility were generated primarily from 2 large, well-known clinical cohort populations of men and women.^7,8 The data derived from these studies were used to represent the population typically tested for CVD risk and to establish the reference intervals for the lipoprotein subfractions. The optimal (O), moderate (M), and high (H) risk categories shown on the report for a select group of lipoprotein subclasses were established at the tertile cut-points of the combined populations.

When assessing a person’s risk for CVD, all results from risk markers should be considered, even when they place the person in different risk categories. This is especially important when assessing residual risk. Below is an example that demonstrates the importance of considering multiple ion mobility-derived risk markers.

Pattern A is classified as optimal, and pattern B is classified as high-risk. This is based on large population studies showing that people without coronary heart disease tend to have an abundance of large, buoyant LDL particles (pattern A), and people with coronary heart disease tend to have an abundance of smaller, dense LDL particles (pattern B).⁵

However, the literature suggests that CVD risk is conferred by a trio of factors that define the atherogenic lipoprotein profile (ALP).⁹ The ALP includes elevated small LDL particles (pattern B), low levels of HDL-cholesterol, and often an elevated fasting triglyceride concentration. Thus, the LDL pattern phenotype is only one aspect of the ALP. Additionally, neither the LDL pattern phenotype nor the ALP reflects risk associated with HDL subclasses or the number of small LDL particles. For an example of how LDL particle number can contribute to the overall CVD risk assessment, consider a patient with an optimal pattern A LDL phenotype and a high total LDL mass. If the number of small LDL particles is also high, the patient may be at increased risk despite the favorable pattern A result. Thus, quantitating lipoprotein subclasses (ie, small, medium) can provide important information when assessing overall CVD risk. Interpreting the favorable pattern A result as indicative of low CVD risk could mistakenly rule out treatment in a patient who could benefit from it.

The ångström (Å) (0.1 nm) is a unit of measure denoting the diameter (size) of a particle. This diameter is used to determine the pattern A vs pattern B LDL phenotype.

The Cardio IQ report shows the particle diameter on the horizontal axis of the lipoprotein fractionation graphical display. The ångström measurement at the apex of the major LDL lipoprotein peak is reported in both the summary results table and the ion mobility detail table, labeled as LDL Peak Size.