Why do we need to test for lipoprotein(a)?
Lipoprotein(a) [Lp(a)] is composed of a single molecule of apo(a), apoB, and a low-density lipoprotein (LDL)-like particle. A number of medical societies have recommended identifying high Lp(a) levels for cardiovascular disease (CVD) prevention, and in June 2018 the Centers for Disease Control and Prevention approved the first ICD-10 codes for the diagnosis of elevated Lp(a) levels.
Currently, no pharmaceutical agent targeting Lp(a) is Food and Drug Administration-approved. Nevertheless, Lp(a) testing identifies individuals with increased CVD risk who would not be recognized otherwise and enables them to benefit from rigorous treatment for other CVD risk factors.
What are the limitations of different Lp(a) assays?
The most prevalent methods for measuring Lp(a) are immunoassays, most of which use a variety of polyclonal antibodies against apo(a), react with the highly polymorphic kringle IV type 2 domain. Given the high degree of size heterogeneity in apo(a), an Lp(a) polyclonal immunoassay may over- or under-estimate the Lp(a) level in human blood samples depending on the sizes of apo(a) in assay calibrators.
Another layer of inaccuracy stems from the expression of Lp(a) concentrations in mg/dL. Since Lp(a) particles are extremely variable not only in apo(a) sizes, but also in their lipid contents and glycosylation modifications, it is prohibitively difficult to quantify the accurate mass of Lp(a) in a primary standard, and impossible to apply this determination to secondary reference materials, assay calibrators, and ultimately human samples.
The first World Health Organization/International Federation of Clinical Chemistry and Laboratory Medicine (WHO/IFCC) International Reference Reagent for Lp(a) immunoassays was approved in 2004 with an accuracy-based value assigned in molar concentration (nmol/L) of Lp(a) particles. A monoclonal enzyme-linked immunosorbent assay (ELISA) method that is not affected by apo(a) size variation was designated as the reference method and studies have clearly demonstrated the feasibility of harmonizing Lp(a) assays using the reference material (Clin Chem 2000;46:1956-67).
However, most commercially available immunoassays lack the optimization to minimize impact from variation in apo(a) size, with the exception of one latex-enhanced turbidimetric method that uses a mixture of apo(a) isoforms in its calibrators.
There are other analytical approaches to Lp(a) measurement that are less affected by apo(a) heterogeneity, though, such as ELISA methods that use capture antibody recognizing apo(a) and detection antibody against apoB, as well as methods based on electrophoretic separation of lipoprotein particles followed by apoB or cholesterol staining and quantification.
How should labs prepare for implementing an Lp(a) assay?
Before adopting an Lp(a) assay, labs should first gauge healthcare providers’ expectations regarding the assay’s targeted patient population and guidelines defining desirable Lp(a) levels. Both clinicians and laboratorians should be aware of the racial/ethnic differences in Lp(a) levels. In particular, African-Americans usually have higher levels of this particle than other racial/ethnic groups.
Labs should also ask the following questions about an Lp(a) assay’s analytical features before making a selection: 1) Is the system certified for traceability of Lp(a) values to the WHO/IFCC reference material? 2) Does the assay report Lp(a) in nmol/L instead of mg/dL? Calculation through a conversion factor is not recommended. 3) Are the reagents insensitive to apo(a) size variability? Even if they are, bias may still be present in patients with extreme apo(a) sizes. 4) And, will any patients be managed by Lp(a) testing on different methods at multiple laboratories?
Jing Cao, PhD, DABCC, FADLM, is an assistant professor of pathology and immunology at Baylor College of Medicine and associate director of clinical chemistry at Texas Children’s Hospital in Houston. +Email: [email protected]