Lipoprotein (a): Molar or Mass?

Lipoprotein (a) was first identified in 1963. However, it’s been in the last decade we’ve seen significant advances in our understanding of this ambiguous molecule and its relationship with cardiovascular disease (CVD) risk.

Lp(a) is a macromolecular lipoprotein complex¹ which is thought to display proatherogenic, proinflammatory² and prothrombotic³ potential and is considered an independent causal risk factor for various types of CVD⁴. These properties provide several mechanisms in which elevated Lp(a) levels may contribute to CVD however the true nature of Lp(a)’s relationship to CVD remains largely enigmatic.

Although some of the first studies failed to find a causal relationship, advances in quantification methods soon led to data which showed this relationship did in fact exist. It has been shown that those with the highest levels of Lp(a) are at a 1.5x increased risk of cardiovascular-related death, 1.6x risk of stroke, and up to 4x risk of heart attack when compared with those with the lowest levels⁵.

Accurate measurement of Lp(a) is crucial in determining CVD risk. Quantification methods which account for the size-related variability of Lp(a) molecules are known to produce less bias when compared with those which do not. The Randox Lp(a) reagent is based on the Denka Seiken method which has been shown to produce minimal size-related bias. This assay has FDA 510(K) clearance, illustrating its reliability and safety. Furthermore, the Randox Lp(a) assay is reported in nmol/L, is traceable to the WHO/IFCC reference material, and provides acceptable bias compared with the Northwest Lipid Metabolism Diabetes Research Laboratory (NLMDRKL) gold standard method.

More Info

Physiology and Genetics

Synthesised mainly in the liver, Lp(a), like Low-density lipoprotein (LDL) cholesterol, is composed of a lipid centre made of cholesteryl esters and triacylglycerols, surrounded by a shell of phospholipids, free cholesterol, and an apoB-100 molecule. The major difference between other LDL cholesterol molecules and Lp(a) is the presence of a polymorphic glycoprotein, apo(a), bound to apoB-100 by a single disulphide bond⁵. It is this apo(a) molecule which contributes to Lp(a)’s pathophysiology.

Apo(a) is thought to have evolved from the plasminogen gene (PLG) around 40 million years ago and shares 78-100% sequence homology within the untranslated and coding regions of the fibrinolytic enzyme¹. Like plasminogen, apo(a) contains unique domains named kringles⁴. While plasminogen contains 5 different kringle structures (KIto KV), apo(a) has lost KI through KIII and instead contains several forms of KIV, namely, 1 copy of KIV₁and KIV_3-10, 1-40 copies of KIV₂, 1 copy of KV and an inactive protein domain at the carboxyl terminus of the molecule⁷. These hydrophilic subunits are highly polymorphic due to the variation in KIV₂ repeats.

Individuals may possess two different isoforms of apo(a), one of which will have been passed down from each parent, that are expressed codominantly¹. These isoforms are dependent on the number of KIV₂ repeats they contain². Isoforms with less KIV₂ repeats produce smaller apo(a) isoforms which are found at a higher concentration compared with larger isoforms⁶due to the increased rate at which the smaller molecules can be synthesised⁴. The polymorphisms in KIV₂ repeats account for up to 70% of the variation seen in concentration between individuals, with the remainder being attributed to differences in protein folding, transport, and single nucleotide polymorphisms (SNPs)⁴. SNPs are central in the heterogeneity of apo(a), effecting RNA splicing, nonsense mutations and 5’ untranslated region of the LPA gene resulting in shorter gene translation^4,6.

Mass versus Molar?

The quantification of Lp(a) levels is essential in evaluating CVD risk, yet the units of measurement—mass (mg/dL) versus molar (nmol/L) – play a critical role in the accuracy and reliability of these assessments. Historically, Lp(a) levels have been expressed in mass units (mg/dL), but recent advances advocate for the use of molar units (nmol/L) due to their ability to account for molecular variability⁷.

Mass measurement of Lp(a) quantifies the total mass of Lp(a) particles in a given volume of blood, expressed in milligrams per decilitre (mg/dL). This method has been widely used and aligns with other lipid measurements such as cholesterol and triglycerides⁸. However, it does not account for the significant variability in the size and composition of Lp(a) particles. This can result in an overestimation of Lp(a) concentration in those with large apo(a) isoforms, and conversely underestimation of concentrations in patients with small apo(a) isoforms⁹. Consequently, two individuals with the same mass concentration of Lp(a) may have vastly different particle numbers and sizes, leading to potential discrepancies in risk assessment¹⁰.

In contrast, molar measurement expresses the concentration of Lp(a) particles in terms of their molar quantity, measured in nanomoles per litre (nmol/L). This approach provides a more accurate reflection of the number of Lp(a) particles present, irrespective of their size¹¹. By focusing on particle count rather than mass, molar measurement offers a standardised and minimally biased method that better accounts for the heterogeneity of Lp(a) particles¹².

The conversion between mass and molar units is not straightforward due to the variability in the molecular weight of Lp(a) particles. A commonly used conversion factor is approximately 2.5, meaning 1 mg/dL of Lp(a) is roughly equivalent to 2.5 nmol/L¹³. However, this factor can vary depending on the specific characteristics of the Lp(a) particles in a given sample. For this reason, converting between units is discouraged by various relevant organisations including the European Atherosclerosis (EAS)⁶.

Clinical guidelines and risk assessments have traditionally been based on mass concentrations, but the shift towards molar units is gaining traction. The Randox Lp(a) assay, which reports in nmol/L and is traceable to the WHO/IFCC reference material, exemplifies this trend. This assay not only provides a more accurate measurement but also aligns with the NLMDRKL gold standard method, ensuring minimal size-related bias¹⁴.

The choice between mass and molar measurements has significant clinical implications. Accurate assessment of Lp(a) levels is crucial for identifying individuals at risk of CVD and implementing appropriate interventions. As the understanding of Lp(a) continues to evolve, the adoption of molar measurement is expected to enhance the precision and reliability of Lp(a) testing, ultimately improving patient outcomes.

Randox Lp(a) Reagent

For the reasons above, the European Atherosclerosis Society (EAS) recommends that Lp(a) measurement is of the particles (molar) rather than the total mass, to provide a result with minimal size-related bias.

The Randox Lp(a) assay has FDA 510(K) clearance and is one of the only methodologies on the market that detects the non-variable part of the Lp(a) molecule and therefore suffers minimal size related bias providing more accurate and consistent results. The Randox Lp(a) kit is standardised to the WHO/IFCC reference material, SRM 2B, and is the closest in terms of agreement to the ELISA reference method. We also provide a five-point calibrator with accuracy-based assigned target values which accurately reflects the heterogeneity of isoforms present in the general population. Applications are available for a wide range of biochemistry analysers which details instrument-specific settings for the convenient use of the Randox Lp(a) assay on a variety of systems. Measuring units in nmol/L are available upon request.

Features

Excellent Correlation and Precision: The Randox Lp(a) assay demonstrates excellent performance, evidenced by a correlation coefficient of r=0.995 when compared with other commercially available methods and a within-run precision of less than 2.54%.
510(K) Cleared: Randox’s Lp(a) assay has received FDA 510(k) clearance, signifying its safety and effectiveness and ensuring healthcare professionals can trust its accurate and reliable cardiovascular risk assessments.
Dedicated Five-Point Calibrator Available: Five-point calibrator with accuracy-based assigned target values (in nmol/l) is available, accurately reflecting the heterogeneity of the apo(a) isoforms. Dedicated Lp(a) control is available offering a complete testing package.
WHO/IFCC Reference Material: The Randox Lp(a) assay is reported in nmol/l and traceable to the WHO/IFCC reference material (IFCC SRM 2B) and provides an acceptable bias compared with the Northwest Lipid Metabolism Diabetes Research Laboratory (NLMDRKL) gold standard method.
Applications Available: Applications are available detailing instrument-specific settings for the convenient use of the Randox Lp(a) assay on a wide range of clinical chemistry analysers.
Liquid Ready-To-Use: The Randox Lp(a) assay is available in a liquid ready-to-use format for convenience and ease-of-use.

One study¹⁵ compared 5 commercially available Lp(a) assays on an automated clinical chemistry analyser. The assays tested were manufactured by Diazyme, Kamiya, MedTest, Roche, and Randox. The authors show that all the assays tested met the manufacturers claims for sensitivity, linearity, and precision. However, significant bias was observed in 4 out of 5 assays. The only assay which did not display significant bias was the Randox Lp(a) Assay which is traceable to WHO/IFCC reference material. This report highlights the importance of measuring and reporting Lp(a) in molar concentration rather than in mass units to facilitate standardisation and harmonisation in Lp(a) testing¹⁵.

Conclusions

In conclusion, understanding and accurately measuring Lp(a) is crucial for assessing CVD risk. Despite its enigmatic nature, recent advancements have clarified Lp(a)’s role as a significant independent risk factor for CVD. The shift from mass to molar measurement units is enhancing the precision and reliability of Lp(a) assessments, with the Randox Lp(a) assay leading the way in providing minimal size-related bias and accurate results.

To ensure the most accurate and reliable assessment of your patients’ cardiovascular risk, consider integrating the Randox Lp(a) assay into your diagnostic toolkit. With its FDA 510(k) clearance, traceability to WHO/IFCC reference material, and high precision, the Randox Lp(a) assay is an essential component for any modern clinical laboratory.

Integrate the Randox Lp(a) assay into your practice today to enhance the precision of your cardiovascular risk evaluations.

References

Posted on 5th August 2024 at 4:29 pm.

Written by Jason Armstrong