Determination of lipoprotein(a): enzyme immunoassay and immunoradiometric assay compared

Association of tobacco smoke exposure with metabolic profile from childhood to early adulthood: the Special Turku Coronary Risk Factor Intervention Project

AIMS

To investigate the associations between passive tobacco smoke exposure and daily smoking with a comprehensive metabolic profile, measured repeatedly from childhood to adulthood.

METHODS AND RESULTS

Study cohort was derived from the Special Turku Coronary Risk Factor Intervention Project (STRIP). Smoking status was obtained by questionnaire, while serum cotinine concentrations were measured using gas chromatography. Metabolic measures were quantified by nuclear magnetic resonance metabolomics at 9 (n = 539), 11 (n = 536), 13 (n = 525), 15 (n = 488), 17 (n = 455), and 19 (n = 409) years. Association of passive tobacco smoke exposure with metabolic profile compared participants who reported less-than-weekly smoking and had serum cotinine concentration <1 ng/mL (no exposure) with those whose cotinine concentration was ≥10 ng/mL (passive tobacco smoke exposure). Associations of daily smoking with metabolic profile in adolescence were analysed by comparing participants reporting daily smoking with those reporting no tobacco use and having serum cotinine concentrations <1 ng/mL. Passive tobacco smoke exposure was directly associated with the serum ratio of monounsaturated fatty acids to total fatty acids [β = 0.34 standard deviation (SD), (0.17-0.51), P < 0.0001] and inversely associated with the serum ratios of polyunsaturated fatty acids. Exposure to passive tobacco smoke was directly associated with very-low-density lipoprotein particle size [β = 0.28 SD, (0.12-0.45), P = 0.001] and inversely associated with HDL particle size {β = -0.21 SD, [-0.34 to -0.07], P = 0.003}. Daily smokers exhibited a similar metabolic profile to those exposed to passive tobacco smoke. These results persisted after adjusting for body mass index, STRIP study group allocation, dietary target score, pubertal status, and parental socio-economic status.

CONCLUSION

Both passive and active tobacco smoke exposures during childhood and adolescence are detrimentally associated with circulating metabolic measures indicative of increased cardio-metabolic risk.

High lipoprotein(a) concentrations are associated with impaired endothelial function in children

OBJECTIVE

To examine the association between familial high lipoprotein(a), or Lp(a), concentrations and endothelial function in children participating in the Special Turku Coronary Risk Factor Intervention Project study.

STUDY DESIGN

Seven-month-old children (n = 1062) with their families were randomized to a risk intervention group or to a control group. The intervention group received individualized dietary counseling to reduce the total cholesterol concentration. Children's Lp(a) and lipid values were measured repeatedly. At age 11 years, children were recruited to an ultrasound study of the flow-mediated dilation (FMD) of the brachial artery. The association between relative peak FMD and Lp(a) concentration was examined in 198 control and 193 intervention group children by linear regression analyses adjusted for sex, total cholesterol concentration, and basal artery diameter. The analyses were made in both the control and intervention groups and in the familial risk children who had a parent with Lp(a) concentration greater than 250 mg/l.

RESULTS

Lp(a) concentrations were similar at age 11 years in the intervention and control groups. In all control children, FMD (%) associated inversely with Lp(a) concentration: (β [%/1000 mg/L] = -3.74, 95% CI [-6.43, -1.45]; P = .007) and in 68 familial risk children (β = -4.92, 95% CI [-8.18, -1.66]; P = .0037). In the intervention group the associations were lacking (P > .5), and FMD in the children with high Lp(a) concentrations (>500 mg/L, n = 12) had no attenuation (P = .027).

CONCLUSIONS

Familial high Lp(a) concentration is associated with attenuated endothelial function. This association may be mitigated by an early lifestyle intervention.

TRIAL REGISTRATION

ClinicalTrials.gov: NCT00223600.

Severe coronary disease in an adult considered at low cardiovascular disease risk with a healthy lifestyle

Lipoprotein(a) [Lp(a)] is a lipoprotein subclass well-known among the lipid community to accelerate atherosclerosis and promote thrombosis through incompletely understood mechanism. We report a case of a young man with a healthy lifestyle and no major coronary or vascular risk factors who presented to the emergency department with an acute coronary syndrome and was ultimately found to have severe coronary artery disease. A diagnostic workup revealed elevated Lp(a). He was treated with consequent reduction in Lp(a) concentration. This case highlights the need to better understand atypical lipoproteins, how they relate to cardiovascular disease, the implications for screening family members, and the need to standardize patient management guidelines for the purpose of mortality risk reduction.

Migration at early age from a high to a lower coronary heart disease risk country lowers the risk of subclinical atherosclerosis in middle-aged men

BACKGROUND

Study of migrants offers a natural model to assess environmental risk of coronary heart disease (CHD) in countries differing in CHD occurrence. In Sweden, CHD risk has been markedly lower than in Finland from where a large migration occurred in the 1970s.

OBJECTIVES

To study the structural and functional markers of subclinical atherosclerosis in twin pairs discordant for migration with the main focus on age at migration, length of residence and integration into Swedish society after migration from a high to a lower CHD risk country.

METHODS

Carotid intima-media thickness (IMT) and brachial artery endothelial function (EF) were assessed with high-resolution ultrasound and a set of cardiovascular, socio-economic and psychosocial risk factors were estimated in 76 middle-aged male twin pairs discordant for migration from Finland to Sweden.

RESULTS

Men who had migrated in adolescence had lower IMT values compared with their co-twins living in Finland (0.665 +/- 0.114 vs. 0.802 +/- 0.167 mm, P = 0.009). Also men who integrated well to Swedish society had lower (0.720 +/- 0.154 vs. 0.799 +/- 0.207 mm, P = 0.013) IMT values than their twin brothers living in Finland. Associations between IMT and migration age and between IMT and integration remained significant in multivariate analyses of several CHD risk factors. The intrapair difference in IMT was significantly associated with immigration age and integration (ANOVA, P = 0.0082), the difference being greatest among pairs where the brother living in Sweden had migrated at early age and integrated well to Swedish society. EF was better in men who had migrated to Sweden before the age of 21 years, but not later, compared with their co-twins in Finland (6.4 +/- 4.6% vs. 3.8 +/- 3.6%, P = 0.025).

CONCLUSIONS

Migration at an early age and good integration are beneficial to vascular health associated with moving from a high to a lower CHD risk country, suggesting that an environment-sensitive period influences atherogenesis before adulthood.

Dose-dependent inverse relationship between alcohol consumption and serum Lp(a) levels in black African males

Serum or plasma levels of Lp(a) vary widely between individuals and are higher in Africans and their descendants compared with white persons. In whites, high serum levels of Lp(a) are associated with the premature development of atherosclerosis. In both ethnic groups, serum Lp(a) levels are highly genetically determined and only a few environmental or physiological factors, like testosterone or estrogen, have been shown to lower serum Lp(a) levels. In whites, alcohol consumption is associated with lower serum Lp(a) levels. However, the mechanism underlying this association and whether it holds true for blacks is not known. To address these questions, we analyzed serum Lp(a) levels in 333 middle-aged males of African descent from the Seychelles Islands (Indian Ocean). In addition, we analyzed the size of the apo(a) isoforms and the serum levels of albumin and sex hormones in a subset of 279 subjects. Serum Lp(a) levels were similar in teetotalers (median, 32.5 mg/dL; n=42) and occasional drinkers (median, 34.1 mg/dL; n=112). In contrast, individuals consuming 10 to 80 g of ethanol/d (n=83) and heavy drinkers (>80 g of ethanol/d, n=96) had a 9% and 32% lower median Lp(a) level than teetotalers, respectively (P=0.01). The size distribution of the apo(a) isoforms and the mean serum levels of albumin, estradiol, and luteinizing hormone were similar in teetotalers and occasional drinkers compared with moderate and heavy drinkers. These latter 2 groups had lower serum levels of testosterone and sex hormone-binding globulin. These data indicate that alcohol intake is associated in a dose-dependent manner with lower serum Lp(a) levels in males of African descent and that this association is not related to the size of the apo(a) isoforms, to the synthetic function of the liver, or to sex hormone biochemical status.

Serum lipids, lipoproteins, and lipid metabolizing enzymes in identical twins discordant for obesity

Obesity is associated with adverse changes in plasma lipoprotein metabolism, but it is not known completely how this association is modified by genetic factors. We assessed the contribution of obesity to serum lipid and lipoprotein levels and lipid metabolizing enzyme activities by examining 23 identical twin pairs (9 male, 14 female) who had, on the average, an 18-kg intrapair difference in BW. Compared with lean co-twins, obese co-twins had approximately 20% higher low-density lipoprotein (LDL) cholesterol (P < 0.01), 20% lower high-density lipoprotein2 cholesterol (P = 0.010), and 90% (men) or 35% (women) higher (P < or = 0.06) total, very-low-density lipoprotein and LDL triglycerides. The pairs were divided into subgroups by the gender-specific median value of abdominal visceral fat (AVF) area in the obese co-twin and by apolipoprotein E 4 phenotype. The intrapair differences in serum cholesterol fractions were similar in twin pairs with high or low AVF, whereas only high AVF pairs showed significant differences in triglyceride fractions. The greatest intrapair differences in total, very-low-density lipoprotein and LDL triglycerides were observed in apolipoprotein E 4-positive pairs expressing high AVF. Compared with lean co-twins, lecithin cholesterol acyltransferase activity was 18% higher (P < 0.001) and hepatic lipase activity was 38% higher (P = 0.016) in obese co-twins with high AVF. When genetic factors are identical, obesity is associated with an atherogenic lipid profile, especially in subjects with high visceral fat accumulation.

Evaluation of the new automated ELISA Vidas Lp(a) assay. Comparison with an immunonephelemetric method

Lipoprotein (a) [Lp(a)] is a lipoprotein which has a plasma concentration that is highly correlated with cardiovascular disease. In this study, the new Lp(a) assay for the Vitek Immuno-Diagnostic Assay System (VIDAS) developed by bioMérieux was evaluated. This method uses an enzyme linked fluorescent immunoassay (ELFIA) technique. Within-run and between-run reproducibility of the Vidass Lp(a) assay are characterized by coefficients of variation (CV) of 3 to 5.9% and 3.9 to 5.9%, respectively. Using analysis of variance, no statistical difference was shown between ELFIA and immunonephelometric assay (INA). When comparing results of the Vidas Lp(a) test with the INA, a highly significant correlation of r = 0.9708 and regression line equation y = 0.963x-0.037 was found. Interference of lipemia was studied: no influence was observed up to 12.3 mmol l(-1) triglycerides. No interference of haemoglobin was noted for Lp(a) > 0.20 g l(-1). Hyperbilirubinemia ( > 120 micromol l(-1)) led to results being underestimated for concentrations of Lp(a) which were < 0.20 g l(-1). No pre-analytical interference of citrate was measured but pre-analytical interference of EDTA was found. In conclusion, this new fully automated immunofluorimetric Lp(a) assay enables to the rapid, accurate and reliable determination of Lp(a) in blood samples.

Standardization and clinical management of lipoprotein(a) measurements

The present article proposes personal suggestions to improve determinations and clinical interpretation of results of lipoprotein(a) assays. Methods and procedures for sampling and quantification of the various isoforms of lipoprotein(a) in serum, plasma and urine are reviewed with the aim of improving the reliability and reproducibility of results and reinforcing the clinical utility of lipoprotein(a) measurements.

Tracking of serum lipoprotein (a) concentration and its contribution to serum cholesterol values in children from 7 to 36 months of age in the STRIP Baby Study. Special Turku Coronary Risk Factor Intervention Project for Babies

We investigated the tracking phenomenon of serum lipoprotein (a) concentrations and assessed the impact of serum concentration of lipoprotein (a) cholesterol on total cholesterol concentrations in children from 7 to 36 months of age. Serum samples for lipoprotein (a) and cholesterol determinations at 7, 13, 24 and 36 months were prospectively obtained from 430 children. Serum lipoprotein (a) was determined using immunoradiometric assay. A strong correlation was observed between lipoprotein (a) concentrations at 7 and 36 months of age (r = 0.88, P < 0.001). Seventy-eight per cent to 86% of the children in the lowest and highest lipoprotein (a) quintiles at 13 months remained in the respective quintiles at 36 months. The average contribution of lipoprotein (a) cholesterol to total cholesterol varied from 0.5% to 3.2% (individual variation 0.13-32.39%) depending on the type of milk received and the age of the children. At 7 months the contribution was 0.44% in breast-fed and 0.93% in formula-fed infants (P < 0.0001). The tracking phenomenon of serum lipoprotein (a) concentrations is strong already in early childhood. The contribution of lipoprotein (a) cholesterol to serum total cholesterol concentration should be taken into account when the changes in serum cholesterol levels are interpreted in the first year of life.

Significant reduction of the bias among commercial immunoassays for lipoprotein(a) after use of a uniform calibrator

Despite the increasing interest in the measurements of lipoprotein(a) (Lp(a) in serum or plasma, at present there is no effective standardization for Lp(a) assays; the main problems to solve are represented either by the lack of a suitable primary standard or by the absence of a reliable and widely available reference method. A first step is hence the uniformity of calibration of different immunoassays. We calibrated three commercial immunoassays for Lp(a) (enzyme linked immunosorbent assay (ELISA), latex-enhanced immunonephelometric assay (LINA), and immunonephelometric assay (INA) with either proprietary standards or purified Lp(a) material obtained with a rapid and simple procedure. Final results of purified Lp(a) calibration were reported in terms of protein Lp(a) mass whereas we were able to quantify the exact protein concentration of our purified lipoprotein. The uniformity of the calibration of the different assays led to a significant improvement of regression slopes (from 1.88 to 0.90 ELISA vs. LINA, from 1.45 to 0.95 ELISA vs. INA and from 1.27 to 0.96 INA vs. LINA) and correlation coefficients (from 0.990 to 0.994 ELISA vs. LINA, from 0.987 to 0.990 ELISA vs. INA and from 0.985 to 0.987 INA vs. LINA). Furthermore, the significant differences among Lp(a) values obtained after calibration with proprietary standards were minimized, becoming non-significant in two out of three cases. In conclusion, we demonstrated that a better agreement of Lp(a) values obtained with different commercial assays could be simply reached by uniformity of calibration and by employing standards with values accurately measured.

Correlation of toddlers' serum lipoprotein(a) concentration with parental values and grandparents' coronary heart disease: the STRIP baby study

The correlation between lipoprotein(a) (Lp(a)) concentrations in children aged 7-24 months and their family members was determined and the association between the Lp(a) values of the children and a family history of coronary heart disease (CHD) was assessed. The Lp(a) values of the children correlated strongly with midparent Lp(a) values as early as at 7 months of age (r = 0.54 to 0.59, p < 0.0001). This correlation was stronger than the correlation of serum total cholesterol and total cholesterol corrected for Lp(a)-cholesterol between children and parents. None of the parents had CHD. The median Lp(a) concentration of the parents with a family history of CHD was significantly higher than that of parents with no such history (111 vs 87 mg/1, p = 0.024). However, the children's Lp(a) levels were not associated with CHD in their grandparents. The genetic dependence of the Lp(a) concentration is already evident in infancy. The Lp(a) concentration in young parents, but not in their 24-month-old children, is associated with CHD in grandparents. This may be explained by a dilution of the genetic influence on Lp(a) over two generations.

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).

A sandwich ELISA based on anti-apo(a) and anti-apo B monoclonal antibodies for lipoprotein(a) measurement

Lipoprotein(a) (Lp(a)) is one of the most important independent risk factors for the prediction of premature atherosclerosis. Lp(a) is a low-density lipoprotein (LDL)-like particle which contains a glycoprotein (apoprotein(a)) disulfide linked to apo B-100. We describe a sandwich ELISA based on an anti-apo(a) monoclonal antibody (MAb) and an anti-apo B MAb for the quantitative determination of Lp(a) in human serum. The assay is sensitive, precise and specific. Samples with different apo(a) isoforms had a linear response in a range of 3-70 mg/dl of Lp(a). Correlations between the ELISA and a commercial ELISA, an immunoradiometric assay and electroimmunodiffusion were 0.92, 0.96 and 0.98, respectively. The frequency distribution of Lp(a) concentration in blood donors showed the skew toward the right reported in other populations. Patients with angiographically assessed coronary atherosclerosis had three times higher levels of Lp(a) than those with no signs of coronary atherosclerosis.

Influence of apolipoprotein(a) phenotype on lipoprotein(a) quantification: evaluation of three methods

Three commercially available assays (an enzyme-linked immunosorbent assay ELISA, an immunoradiometric assay, IRMA, and a nephelometric assay) for the determination of lipoprotein(a) [Lp(a)] were compared with respect to the dependency of these assays on the various apolipoprotein(a) [apo(a)] isoforms. Although there was a strong correlation between the three methods, a significant difference between the absolute values (mg/L) was observed (p < 0.001). Using purified Lp(a) preparations, we showed that the ELISA assay quantifies the Lp(a) concentration on a molar basis, independently of the apo(a) isoform size. The IRMA and the nephelometric assay however are apo(a) isoform size dependent and overestimate the Lp(a) concentration of large apo(a) isoforms whereas the amount of small apo(a) isoforms is underestimated. In general, the isoform dependency of the Lp(a) quantification is of limited clinical relevance. In this study, inconsistent risk assignments are made in approximately 3% of the cases, when the Lp(a) concentrations obtained with the apo(a) isoform dependent assays are compared with the isoform independent ELISA.

Fully mechanized latex immunoassay for serum lipoprotein(a)

We have developed a fully automated system to quantify lipoprotein(a) (Lp(a)) in human serum, based on the latex-enhanced turbidimetric immunoassay by application of the Immuno Chemistry Analyzer 501X. This assay was carried out with undiluted serum and was able to detect at Lp(a) levels higher than 4.0 mg/l. When judged to be out of range of the calibration (> 600 mg/l), the sample was automatically re-tested after automatic 10-fold dilution. Within-run C.V.s ranged from 1.9 to 2.1% and between-run C.V.s from 2.7 to 3.9%. Results by the present method were in good agreement with those by the in-house ELISA (r = 0.978) and the commercial ELISA (r = 0.990). The distribution of Lp(a) levels in sera from 508 healthy donors was highly skewed; the mean and median were 158 mg/l and 105 mg/l, respectively.