Lipoprotein(a) Testing and Standardization 

• Some studies have shown no relation between Lipoprotein(a) (Lp(a)) levels and CVD (cardiovascular disease) that led many experts to question the wisdom of measuring Lp(a) in clinical practice.^1-4 However, prospective studies showing a strong positive relationship between Lp(a) and premature CVD far outnumber those showing no relationship.¹

• The possible reasons for these discrepancies are Lp(a) isoforms, degradation resulting from storage, lack of standard methods for measuring Lp(a), and issues related to reagents used.¹ Many of these issues have now been resolved and most recent meta-analysis and genetic studies suggest that Lp(a) is indeed a putative risk factor for atherosclerosis and most of its complications.¹

• Addressing Lp(a)-related risk begins with accurate determination of Lp(a) levels, which requires assays insensitive to the size of apo(a)─the plasminogen like protein bound to apo B-100.⁵

• An accurate standardized measurement of Lp(a), using an International Reference Material has been developed by International Federation of Clinical Chemistry and Laboratory Medicine (IFCC) and  approved by National Heart, Lung, and Blood Institute (NHLBI) and the World Health Organization (WHO). ^5-8  This IFCC reference material uses monoclonal antibody specifically directed towards an epitope present in KIV type 9; only one copy of which is present on every apo(a) molecule, regardless of size. This allows accurate measurement of Lp(a) levels irrespective of isoform size and has become the gold standard for measurement of Lp(a).^5-8

• Although the reference material for the accurate measurement of Lp(a) [regardless of apo(a) size] has been available for many years, many commercial laboratories have not changed their reagents and testing methods and continue to use old reagents and methods resulting in inaccurate results.^5-8 Accordingly,   results of Lp(a) measurements by different labs are not comparable and some of them are clearly inaccurate. ^5-8

• The fourth report from the Physicians’ Health Study has given great insight into the importance of using accurate Lp(a) measurement.⁹ The three initial reports, that showed no relation between Lp(a) and CVD, measured Lp(a) using a method now found to be inaccurate,^2-4 (used monoclonal antibodies for KIV type 2, which underestimated the concentration of Lp(a) with small isoforms).⁹ The latest study measured Lp(a) in duplicate using both the new IFCC reference material as well as the reagent used in the three earlier studies.⁹ The results were again negative when using the old reagent, but strongly positive when the IFCC reference material was used.⁹

• The physicians who had Lp(a) above the 80^th percentile had a two-fold risk of severe CAD (coronary artery disease) and those above the 95th percentile had a four-fold risk of severe CAD.⁹ Furthermore, men with Lp(a) concentrations in the highest quintile and LDL-cholesterol concentrations >160 mg/dL had a 12-fold increased risk.⁹ Other studies have shown that the combination of high Lp(a) and high LDL confer a 14-fold risk compared to a 3-fold risk with isolated LDL elevation.¹⁰

• Unfortunately, many laboratories still use an antibody directed at the KIV type 2 repeat sequence (the kringle sequence), and report results in mg/dL. Such assays and reports can significantly under or overestimate Lp(a) concentration based on the isoform of apolipoprotein(a) used to calibrate the machine.

• Quest Diagnostics measures Lp(a) levels  using an  immunoturbidometric assay distributed by Polymedco shown to be independent of apolipoprotein(a) size; this lab also report Lp(a) level values in nmol/L as recommended by the NHLBI consensus conference.⁵ An Lp(a) value >75 nmol/L is  considered high and is equal to the 30 mg/dl used in earlier reports. All labs that use the NHLBI standardized reagent/method would volunteer the information upon request.

• Lp(a) levels as measured by Wako assay overestimate up to 130% in people with predominantly large isoforms and underestimate up to 37% in subjects with small isoform sizes.¹¹  On the other hand, cholesterol content of Lp(a) is not significantly associated with CAD.

Sources

1. Enas EA, Chacko V, Senthilkumar A, Puthumana N, Mohan V. Elevated lipoprotein(a)–a genetic risk factor for premature vascular disease in people with and without standard risk factors: a review. Dis Mon. Jan 2006;52(1):5-50.

2. Ridker PM, Stampfer MJ, Hennekens CH. Plasma concentration of lipoprotein(a) and the risk of future stroke. Jama. 1995;273(16):1269-1273.

3. Ridker PM, Hennekens CH, Stampfer MJ. A prospective study of lipoprotein(a) and the risk of myocardial infarction. Jama. 1993;270(18):2195-2199.

4. Ridker PM, Stampfer MJ, Rifai N. Novel risk factors for systemic atherosclerosis: a comparison of C-reactive protein, fibrinogen, homocysteine, lipoprotein(a), and standard cholesterol screening as predictors of peripheral arterial disease. Jama. May 16 2001;285(19):2481-2485.

5. Marcovina SM, Koschinsky ML, Albers JJ, Skarlatos S. Report of the National Heart, Lung, and Blood Institute Workshop on Lipoprotein(a) and Cardiovascular Disease: recent advances and future directions. Clinical chemistry. Nov 2003;49(11):1785-1796.

6. Marcovina SM, Albers JJ, Gabel B, Koschinsky ML, Gaur VP. Effect of the number of apolipoprotein(a) kringle 4 domains on immunochemical measurements of lipoprotein(a). Clinical chemistry. 1995;41(2):246-255.

7. Dati F, Tate JR, Marcovina SM, Steinmetz A. First WHO/IFCC International Reference Reagent for Lipoprotein(a) for Immunoassay–Lp(a) SRM 2B. Clin Chem Lab Med. 2004;42(6):670-676.

8. Marcovina SM, Albers JJ, Scanu AM, et al. Use of a reference material proposed by the International Federation of Clinical Chemistry and Laboratory Medicine to evaluate analytical methods for the determination of plasma lipoprotein(a). Clinical chemistry. 2000;46(12):1956-1967.

9. Rifai N, Ma J, Sacks FM, et al. Apolipoprotein(a) size and lipoprotein(a) concentration and future risk of angina pectoris with evidence of severe coronary atherosclerosis in men: The Physicians’ Health Study. Clinical chemistry. Aug 2004;50(8):1364-1371.

10. Hopkins PN, Hunt SC, Schreiner PJ, et al. Lipoprotein(a) interactions with lipid and non-lipid risk factors in patients with early onset coronary artery disease: results from the NHLBI Family Heart Study. Atherosclerosis. 1998;141(2):333-345.

11. Lamon-Fava S, Marcovina SM, Albers JJ, et al. Lipoprotein(a) levels, apo(a) isoform size, and coronary heart disease risk in the Framingham Offspring Study. Journal of lipid research. Jun 2011;52(6):1181-1187.

12. Lamon-Fava S, Marcovina SM, Albers JJ, et al. Lipoprotein(a) levels, apo(a) isoform size, and coronary heart disease risk in the Framingham Offspring Study. J Lipid Res. Jun 2011;52(6):1181-1187.

13. Tzou WS, Douglas PS, Srinivasan SR, Chen W, Berenson G, Stein JH. Advanced lipoprotein testing does not improve identification of subclinical atherosclerosis in young adults: the Bogalusa Heart Study. Ann Intern Med. May 3 2005;142(9):742-750

14. Qasim AN, Martin SS, Mehta NN, et al. Lipoprotein(a) is strongly associated with coronary artery calcification in type-2 diabetic women. Int J Cardiol. Jul 1 2011;150(1):17-21.