What is lipoprotein-associated phospholipase A2 (Lp-PLA2) and why is it useful?
Measuring low-density lipoprotein cholesterol (LDL-C) or non-high-density lipoprotein cholesterol (non-HDL-C) is currently the mainstay when assessing the risk of atherosclerosis leading to cardiovascular disease (CVD). However, not everyone with CVD has elevated LDL-C or non-HDL-C, and even if these markers are elevated and treatment is aggressive, there can still be residual risk that they fail to explain. Therefore, a need exists for other biomarkers of risk.
An important component of atherothrombogenesis is inflammation, and elevated circulating biomarkers of inflammation can be used to identify individuals at higher risk of CVD. One of those markers is Lp-PLA2, an enzyme expressed by the macrophages inside atherosclerotic plaques that circulate bound to lipoproteins. Multivariate analysis in many population studies has shown that individuals with elevated circulating Lp-PLA2 are at higher risk for cardiovascular and stroke events. Also, unlike some biomarkers of inflammation, Lp-PLA2 is not an acute phase reactant, which makes it more specific for the vascular inflammation associated with CVD.
What is the difference between the two available assays for Lp-PLA2?
There are two Food and Drug Administration-approved assays developed by Diadexus for Lp-PLA2 that carry the trademark name PLAC test. One measures the concentration of the protein and the other measures the enzymatic activity of the protein. The concentration assay is an enzyme-linked immunosorbent assay (ELISA) method, while the activity assay is a spectrophotometric assay run on automated chemistry analyzers.
What are the pros and cons of each assay?
While the two assays measure the same protein, the correlation coefficients between them, somewhat surprisingly, range from 0.3–0.6. Laboratorians and clinicians should also be aware of several other differences between the assays. The first is evident from their respective approved patient populations: the concentration assay is cleared for coronary heart disease and stroke while the activity assay is cleared only for coronary heart disease (note, however, that most published studies comparing the assays in the same cohort report similar risk prediction results (The Lancet 2010;375:1536–44).
Second, the manual ELISA format of the concentration assay requires time-consuming manual steps, which may make it difficult to offer routinely in some hospital laboratories. In contrast, the liquid reagents of the activity assay are easily incorporated in the test menu run by almost any automated chemistry analyzer.
Last and perhaps most important, results from the ELISA test can be dramatically inaccurate as a result of pre-analytical specimen handling conditions. Specifically, samples that are either refrigerated or stored at -20°C for 3–4 days before testing can exhibit up to 50% higher concentrations compared to results obtained immediately post collection (Clin Biochem 2011;44:1247–52). This has obvious implications for send-out ordering, add-on patient testing, and testing of previously collected study cohorts.
The activity assay does not display this pre-analytical effect. The only analytical vulnerability of the activity assay that we have discovered is a small but progressive decrease in activity which results over the life of each lot. The laboratory director can easily monitor for this loss in signal using laboratory quality control metrics.
Given the pre-analytical variability that exists with the concentration assay, we have chosen to use the activity assay when measuring Lp-PLA2.
As an important note, during the writing of this article, Diadexus filed for bankruptcy. However, understanding the differences between these two assays will still be useful in the likely event that these tests are marketed in the future, and for interpreting previously published results from the two tests.
To learn more, attend Dr. Donato’s brown bag session at the 68th AACC Annual Scientific Meeting & Clinical Lab Expo, “Lipoprotein-Associated Phospholipase A2: Concentration or Activity – Which to Measure?” on Monday, August 1, 12:30–1:30 p.m. in the Pennsylvania Convention Center, Philadelphia.
Leslie J. Donato, PhD, DABCC, is co-director of cardiovascular laboratory medicine and co-director of the hospital clinical laboratory and point-of-care at Mayo Clinic in Rochester, Minnesota. Email: [email protected]