Abstract
Background
Lipoprotein(a) is an independent predictor of cardiovascular disease and its variability after myocardial infarction was assessed in this study.
Methods
Lipoprotein(a) was analysed by a size insensitive latex immunoturbidimetric end point assay in samples from days 0 to 7 in 31 patients admitted with myocardial infarction.
Results
Median lipoprotein(a) changed by −0.9%, −0.1% and 9.6% on days 1, 2–3 and 4–7, respectively, and was not statistically significant. Median total cholesterol reduced by 8.7%, 9.1%, 14.5% and C-reactive protein increased by 68.4%, 510%, 502% over days 1, 2–3, 4–7, respectively.
Conclusions
Unlike total cholesterol and C-reactive protein, lipoprotein(a) does not demonstrate significant variability for up to seven days after myocardial infarction and measurements made during this period after myocardial infarction are physiologically meaningful.
Introduction
Lipoprotein(a), Lp(a), has been shown to be an independent and robust risk factor for cardiovascular disease (CVD). The association between the increased Lp(a) and CVD is continuous and curvilinear. The European Atherosclerosis Society Panel has recommended that Lp(a) should be measured in patients with CVD or those at high risk of developing the condition. 1
Lp(a) consists of LDL like particles that are bound to apolipoprotein(a), apo(a). 2 Apo(a) consists of two tri-looped structural units called kringles (Kringle IV and Kringle V). Kringle IV has one copy of each type except for Kringle IV Type 2 (Kringle IV-2). The number of Kringle IV-2 repeats varies from 2 to 40 in different individuals and has a major effect on the size of apo(a). This in turn in the past has led to variability in Lp(a) by different assays. It has recently been recommended that Lp(a) should be measured by methods that are insensitive to different apo(a) isoforms. 3
While it is known that the total cholesterol (TC) and C-reactive protein (CRP) are acute-phase reactants (APR) and their concentrations vary following myocardial infarction (MI), little is known about the variability of Lp(a) as an acute-phase reactant. Most of the literature is at least 20 years old, when the Lp(a) concentrations were measured by using methods which were size sensitive. This study was designed to assess the variability of serum Lp(a) in the first week after MI.
Methods
Samples
Thirty-one patients consecutively admitted with a diagnosis of MI were selected. The diagnosis of MI was based on clinical details obtained from request cards, pathology Information Technology (IT) system and troponin results. Samples taken from each of these patients between days 0 (baseline) and 7 following MI were retrieved within 24 h of collection and immediately stored at −70℃ for a week before analysis. The samples were grouped into time points of days 1, 2–3 and 4–7. There were 31, 22, 26 and 20 serum samples on days 0, 1, 2–3 and 4–7, respectively, and Lp(a), TC and CRP were measured on them.
Analytical methods
Serum Lp(a) was measured by a size insensitive latex immunoturbidimetric end point assay (Randox Laboratories). TC and CRP were measured by routine methods (SIEMENS). All analyses were performed on Advia 2400 (SIEMENS), and the intra-assay coefficients of variation (CVs) across the measured range for Lp(a), TC and CRP were between 1.2–2.1%, 1.2–4.3% and 2.0–5.0%, respectively.
Statistical analysis
The baseline value for each patient at day 0 was considered as 100% and the median percentage change at time points was calculated. As each patient’s sample on day 0 acted as a control for subsequent samples for that patient, the power calculation was based on analytical SD of Lp(a) (10.75 mg/L based on the precision experiments, data not shown). Nineteen samples were needed to obtain the power of 80% to detect the change in Lp(a) by 10 mg/L. Boot strapping of the median percentages of each analyte at each time point (10,000 replicates) was performed to generate non-parametric 95% confidence intervals (CI).
Results
The median age of patients was 79 years (minimum 41, maximum 97), and 41% of the patients were females. Lp(a) concentrations ranged from 30 to 2294 mg/L (median 138 mg/L). The median percentage Lp(a) reduced by 0.9% (95% CI −7.9 to 6.3) and 0.1% (95% CI −8.1 to 8) on days 1 and 2–3, respectively as compared to baseline and was not statistically significant. From days 4 to 7, it rose by 9.6% (95% CI −5.8 to 27.3) from baseline but did not reach the statistical significance (Figure 1(a) and (b)).
(a) Absolute variation of Lp(a) after acute MI (day 0 sample of patient number 2 is likely to be an outlier). (b–d) Median percentage variation in Lp(a), TC and CRP, respectively.
On day 1, median percentage TC dropped by 8.7% (95% CI 4.5 to 14.0) as compared to baseline. It dropped further by 9.1% (95% CI 3.8 to 14.0) and 14.5% (95% CI 8.6 to 19.8) by days 2–3 and 4–7, respectively (Figure 1(c)). The median percentage CRP concentrations increased by 68.4% (95% CI 13.5 to 144), 510% (95% CI 110 to 1285) and 502% (95% CI 59.1 to 1290) on days 1, 2–3 and 4–7, respectively (Figure 1(d)).
Discussion and conclusion
European Atherosclerosis Society (EAS) recommended that desirable concentrations of Lp(a) should be <500 mg/L (80th centile). 4 There were nine patients in this subgroup whose Lp(a) concentrations were >500 mg/L. Apart from patient 2 (initial Lp(a) 2294 mg/L), none of the other patients Lp(a) demonstrated the behaviour of an acute initial reduction. This change is unlikely to be due to in vitro degradation as Lp(a) is known to be analytically stable when stored frozen for a week. 5 The wide range of Lp(a) in this study may be due to the varying severity of MI, as it has been shown to correlate with the size of the necrotic area.6,7 Due to the limitation of available clinical details, this correlation could not be analysed in this study.
Our results show that TC reduced and CRP increased significantly immediately over seven days after MI. In the first three days after MI, Lp(a) changed little and thereafter there was a tendency for it to rise between days 4 and 7, but this was not significant as the 95% CI crossed the baseline. A previous study which analysed Lp(a) at two time points (3rd and 28th days post MI) showed a 12% difference between the first and second samples and concluded that Lp(a) increases immediately post MI and return to baseline after four weeks. 8 But Andreassen et al. 9 and Noma et al. 10 showed that increment in Lp(a) was delayed and rose only after seven days after MI.
The findings in this study, where Lp(a) concentrations were measured serially by apo(a) size insensitive method, show that unlike CRP and TC, Lp(a) does not behave like an APR. To the authors’ knowledge, no studies have been carried out to assess its variability in the first week of following MI. Lp(a) measurements are likely to become routine based on the recent recommendations, standardisation of the assay and the likelihood of emerging therapies to treat raised Lp(a). Therefore, this study is important in that it will help in the practical application and interpretation of Lp(a) measurement, as there is no specific guidance or recommendations for the measurement of Lp(a) following MI.
It is concluded that in patients where there is an indication to measure Lp(a), it can be reliably measured for up to seven days after MI.
Footnotes
Acknowledgements
The authors thank Mark Asher for helping in sample analyses.
Declaration of conflicting interests
None declared.
Funding
This research received no specific grant from any funding agency in the public, commercial, or not-for-profit sectors.
Ethical approval
Not required.
Guarantor
PG.
Contributorship
PP researched the literature, conceived the study, and collected the samples. TD researched the literature, helped with the study design and performed the statistical analyses. PG researched the literature, helped with the design of the study and wrote the first draft. WM helped conceive and supervise the study. All authors contributed to the data, reviewed and edited the manuscript and approved the final version.