Immunoquantification of lipoprotein(a): comparison of nephelometry with electroimmunodiffusion

Lipoprotein(a) and coronary thrombosis and restenosis after stent placement

OBJECTIVES

The objective of this prospective study was to evaluate the relation between high lipoprotein(a) levels and thrombotic and restenotic events after coronary stent implantation.

BACKGROUND

Lipoprotein(a) may promote atherogenesis, coronary thrombosis and restenosis after balloon angioplasty, but the clinical significance remains unclear.

METHODS

The study included 2,223 consecutive patients with successful coronary stent placement. According to the serum level of lipoprotein(a), patients were divided in two groups: 457 patients of the highest quintile formed the high lipoprotein(a) group, and 1,766 patients of the lower four quintiles formed the low lipoprotein(a) group. Primary end points were the incidence of angiographic restenosis at six months and the event-free survival at one year. Secondary end point was the incidence of angiographic stent occlusion.

RESULTS

Early stent occlusion occurred in four of the 457 patients (0.9%) with high and 37 of the 1,766 patients (2.1%) with low lipoprotein(a) levels, odds ratio of 0.41 (95% confidence interval, 0.15 to 1.16). Angiographic restenosis occurred in 173 of the 523 lesions (33.2%) in the high lipoprotein(a) group and 636 of the 1,943 lesions (32.7%) in the low lipoprotein(a) group, odds ratio of 1.02 (0.83 to 1.25). The probability of event-free survival was 73.0% in the high lipoprotein(a) group and 74.8% in the low lipoprotein(a) group (p = 0.45). On the basis of the findings in the low lipoprotein(a) group, the power of this study to detect a 25% increase in the incidence of restenosis and adverse events in the group with elevated lipoprotein(a) was 90% and 75%, respectively.

CONCLUSIONS

Elevated lipoprotein(a) levels did not influence the one-year clinical and angiographic outcome after stent placement. Thrombotic events and measures of restenosis were not adversely affected by the presence of high lipoprotein(a) levels.

Plasma lipoprotein(a) is not a predictor for restenosis after elective high-pressure coronary stenting

BACKGROUND

Lipoprotein(a) is a risk factor for coronary artery disease. Although it has been implicated in restenosis after balloon angioplasty, its role in restenosis within coronary stents is unknown. The aim of the study was to assess the role of plasma lipoprotein(a) as a predictor for restenosis after elective coronary stenting.

METHODS AND RESULTS

Elective, high-pressure stenting of de novo lesions in native coronary arteries with Palmaz-Schatz stents was performed in 325 consecutive patients. Clinical, angiographic, and biochemical data were analyzed prospectively. Angiographic follow-up was performed at 6 months. Lipoprotein(a) levels were compared in patients with and without restenosis. Angiographic follow-up was obtained in 312 patients (96%); recurrence was observed in 67 patients (21.5%). No clinical or biochemical variable was associated with restenosis. Lipoprotein(a) level was 37.81+/-49. 01 mg/dL (median, 22 mg/dL; range, 3 to 262 mg/dL) in restenotic patients and 36.95+/-40.65 mg/dL (median, 22 mg/dL; range, 0 to 244 mg/dL) in nonrestenotic patients (P=NS). The correlations between percent diameter stenosis, minimum luminal diameter, and late loss at follow-up angiography and basal lipoprotein(a) plasma level after logarithmic transformation were 0.006, 0.002, and 0.0017, respectively. Multiple stents were associated with a higher incidence of restenosis (P=0.006), but biochemical data in these patients were similar to those treated with single stents.

CONCLUSIONS

The basal plasma level of lipoprotein(a) measured before the procedure is not a predictor for restenosis after elective high-pressure coronary stenting.

Evaluation of the analytical performances of a new fully automated commercial immunonephelometric assay for lipoprotein(a)

Several commercial methods have been proposed for lipoprotein(a) (Lp(a)) measurements over the past decades. However, only a few of them appear completely suitable in terms of analytical performance, costs and practicability. We evaluated the analytical performance of a new commercial fully automated immunonephelometric assay for Lp(a) measurements on the IMMAGE Immunochemistry System. Mean within- and between-run coefficients of variation were 2.7% (range 1.2-4.7%) and 3.8% (range 1.8-7.9%), respectively. The linearity of the assay was confirmed up to 102 mg/dl and the deviation from the expected values did not exceed 4% (mean deviation: 1.9%). Moreover, the relative non-linearity was acceptable, ranging from 1.4% to 1.6% and hence constantly lower than the 2.5% upper limit. Since there is no reference method for Lp(a) measurements, 100 routine random serum samples measured by the IMMAGE Immunochemistry System immunonephelometric assay were further compared with two other commercial immunonephelometric assays (Array LPA immunonephelometric assay and BNA Latex-Enhanced Lp(a) nephelometric assay). Non-parametric regression and relative Spearman's correlation coefficients were satisfactory, (y=1.009x - 1.38; r=0.998 IMMAGE Immunochemistry System vs. Array LPA and y=0.922x - 0.40; r=0.989 IMMAGE Immunochemistry System vs. Behring Nephelometer Analyzer (BNA) Latex Lp(a) assay). On the basis of the results of the present evaluation we conclude that the analytical performance and the main technical features of the IMMAGE Immunochemistry System immunonephelometric assay make it a suitable method for Lp(a) measurement in clinical laboratories.

Elevated plasma lipoprotein(a) is associated with coronary artery disease in patients with chronic stable angina pectoris

OBJECTIVES

We sought to assess the relation between plasma lipoprotein(a) [Lp(a)] levels, clinical variables and angiographic coronary artery disease (CAD) in patients with chronic stable angina.

BACKGROUND

The relation between plasma Lp(a) levels and the severity and extent of angiographic CAD has not been studied in well characterized patients with stable angina pectoris.

METHODS

We investigated clinical variables, lipid variables and angiographic scores in 129 consecutive white patients (43 women) undergoing coronary angiography for chronic stable angina.

RESULTS

Plasma Lp(a) levels were significantly higher in patients with than in those without significant angiographic stenoses (> or =70%) (372 mg/liter [interquartile range 87 to 884] vs. 105 mg/liter [interquartile range 56 to 366], respectively, p=0.002). This difference remained significant when patients with mild or severe angiographic disease were compared with those with completely normal coronary arteries (312 mg/liter [interquartile range 64 to 864] vs. 116 mg/liter [interquartile range 63 to 366], respectively, p=0.02). However, subset analysis indicated that this difference achieved statistical significance only in women. Multiple logistic regression analysis indicated that Lp(a) concentration was independently predictive of significant angiographic stenoses (adjusted odds ratio [OR] 9.1, 95% confidence interval [CI] 2.0 to 42.1, p=0.006) and remained true even after exclusion of patients receiving lipid-lowering treatment (n=27) (OR 10.4, 95% CI 1.1 to 102.9, p=0.05). Lp(a) also had independent predictive value in a similar analysis using mild or severe angiographic disease as the outcome variable (OR 11.8, 95% CI 1.5 to 90.8, p=0.02).

CONCLUSIONS

Our results indicate that elevated plasma Lp(a) is an independent risk factor for angiographic CAD in chronic stable angina and may have particular significance in women.

Quantification of lipoprotein(a): comparison of an automated latex-enhanced nephelometric assay with an immunoenzymometric method

Several studies indicate the relevance of lipoprotein(a) (Lp(a)) in the genesis of premature coronary artery disease. A simple method for determining the concentration of Lp(a) is therefore of great interest for assessing the risk of coronary artery disease in patients. We compared a new latex-enhanced immunonephelometric assay (Behringwerke AG, Marburg, Germany), using the Behring Nephelometer System 100, with an established immunoenzymometric assay (Immuno, Heidelberg, Germany). A total of 163 patients was studied. Intra- and inter-assay coefficients of variation were between 2.2% and 7.1%, and between 3.4% and 8.6%, depending on the concentration of Lp(a). The correlation between the studied assays was excellent (r = 0.93, y = 0.98x -1.57, Spearman rank, Passing & Bablok). When values above 1000 mg/l for Lp(a) were excluded, the correlation was even higher. Increased light scattering with particle size, which hitherto has been a disadvantage of the nephelometric technique, seems to be negligible using the improved latex-enhanced approach. In patients with triacylglycerol values above 4.5 mmol/l (n = 19) there was no interference with the Behring system, i.e. the results of the nephelometric method were not increasing, and they agreed with those of the immunoenzymometric assay. In conclusion, this new latex-enhanced nephelometric immunoassay represents a rapid and precise method for the quantification of Lp(a).

Lipoprotein(a) in health and disease

Lipoprotein(a) [Lp(a)] represents an LDL-like particle to which the Lp(a)-specific apolipoprotein(a) is linked via a disulfide bridge. It has gained considerable interest as a genetically determined risk factor for atherosclerotic vascular disease. Several studies have described a correlation between elevated Lp(a) plasma levels and coronary heart disease, stroke, and peripheral atherosclerosis. In healthy individuals, Lp(a) plasma concentrations are almost exclusively controlled by the apo(a) gene locus on chromosome 6q2.6-q2.7. More than 30 alleles at this highly polymorphic gene locus determine a size polymorphism of apo(a). There exists an inverse correlation between the size (molecular weight) of apo(a) isoforms and Lp(a) plasma concentrations. The standardization of Lp(a) quantification is still an unresolved task due to the large particle size of Lp(a), the presence of two different apoproteins [apoB and apo(a)], and the large size polymorphism of apo(a) and its homology with plasminogen. A working group sponsored by the IFCC is currently establishing a stable reference standard for Lp(a) as well as a reference method for quantitative analysis. Aside from genetic reasons, abnormal Lp(a) plasma concentrations are observed as secondary to various diseases. Lp(a) plasma levels are elevated over controls in patients with nephrotic syndrome and patients with end-stage renal disease. Following renal transplantation, Lp(a) concentrations decrease to values observed in controls matched for apo(a) type. Controversial data on Lp(a) in diabetes mellitus result mainly from insufficient sample sizes of numerous studies. Large studies and those including apo(a) phenotype analysis came to the conclusion that Lp(a) levels are not or only moderately elevated in insulin-dependent patients. In noninsulin-dependent diabetics, Lp(a) is not elevated. Conflicting data also exist from studies in patients with familial hypercholesterolemia. Several case-control studies reported elevated Lp(a) levels in those patients, suggesting a role of the LDL-receptor pathway for degradation of Lp(a). However, recent turnover studies rejected that concept. Moreover, family studies also revealed data arguing against an influence of the LDL receptor for Lp(a) concentrations. Several rare diseases or disorders, such as LCAT- and LPL-deficiency as well as liver diseases, are associated with low plasma levels or lack of Lp(a).

Comparison of a new immunoturbidometric assay of human serum lipoprotein (a) to the ELISA and the IRMA methods

In the present work we have tested a new immunoturbidometric (IT) lipoprotein(a) (Lp(a)) assay and compared it with two other Lp(a) assay systems (IRMA and ELISA) commonly used in clinical chemistry laboratories. In addition, we have examined the effects of long-term storage and plasminogen on the results of Lp(a). Intra-assay and inter-assay coefficients of variation of the IT method were from 1.6 to 5.5% and from 2.8 to 10.7% depending on the concentration of Lp(a). The correlations between methods were 0.957 (IRMA vs. IT), 0.969 (IRMA vs. ELISA) and 0.956 (IT vs. ELISA) and the regression curves were IRMA = 1.07*IT + 11, IRMA = 1.84*ELISA + 2 and IT = 1.62*ELISA + 17, respectively. Storage of the samples for 5 years at -70 degrees C did not affect serum Lp(a) levels. There was a slight increasing effect of high concentrations of plasminogen on the Lp(a) results, but on physiological serum levels of plasminogen the effect was not significant. We conclude that the IT method provides a simple way to screen serum Lp(a) levels.

Immunoturbidimetric determination of lipoprotein(a): improvement in the measurement of turbid and triglyceride-rich samples

Immunoturbidimetry (IT), a widely used method in clinical chemical laboratories, was checked for its suitability for lipoprotein(a) (Lp(a)) quantification. When the conventional sample diluents were used, turbidimetry gave false results particularly with frozen or lipemic sera which correlated poorly with electroimmunodiffusion (EID). L-Proline which is known to dissociate Lp(a) from other apo B-containing lipoproteins improved the results considerably. One hundred frozen sera were investigated in IT with and without the addition of L-proline to the sample diluent. EID served as a comparison method. In a method comparison (IT vs. EID) linear regression analysis improved from r = 0.793: y = 0.89x - 9.4 (without L-proline) to r = 0.949: y = 0.98x + 4.8 (with L-proline). The improvement of the correlation of the two methods was most pronounced in sera with triglyceride values exceeding 5.5 mmol/l. The IT assay described here was linear between 50 and 1100 mg/l. Total imprecision (coefficient of variation) was below 10%. The assay was not affected by the addition of LDL or plasminogen to the samples. The Lp(a) concentration of the calibrator, i.e. a secondary standard serum, was compared with that of a purified primary Lp(a) standard which consisted of a mixture of four apo(a) isoforms. Total Lp(a) mass (lipids, protein, carbohydrates) was determined chemically and was compared with the Lp(a) mass determined immunochemically by IT and EID. Recovery of the purified Lp(a) was 106% (range 90-116%) in IT and 102% (range 91-115%) in EID. Dose response curves from pure single isoforms (S1 and S4), calibrator and serum samples were parallel. We consider IT to be a simple and rapid method for Lp(a) quantification which is not biased by different apo(a) isoforms.