Evaluation of recombinant enzyme calibration to harmonize lipoprotein-associated phospholipase A 2 activity results between instruments

Performance Characteristics and Clinical Value of the Lipoprotein-Associated Phospholipase A2 by an Enzymatic Kinetic Method

OBJECTIVE

To analyze the performance characteristics, stability, and clinical value of lipoprotein-associated phospholipase A2 (Lp-PLA2) using an enzymatic kinetic method.

METHODS

The performance characteristics included reference intervals, precision, and accuracy. We assessed Lp-PLA2 stability by comparing Lp-PLA2 changes under different conditions. Lp-PLA2 was determined in the following groups: control individuals, patients with coronary heart disease (CHD), patients of different lipid subgroups within CHD, and patients with high total cholesterol (TC). Also, correlations between Lp-PLA2 and traditional cardiovascular risk factors were assessed.

RESULTS

The mean (SD) reference interval of serum Lp-PLA2 activity was 451 (113) U per L with sex differences. Inter- and intra-assay precision revealed coefficients of variance (CVs) of 1.81% to 2.63% and 1.43% to 1.77%. The average bias was 0.33%. Lp-PLA2 activity was stable. In the CHD group, high-lipid subgroups, and high-TC group, Lp-PLA2 was elevated, and correlation was observed between Lp-PLA2 and traditional risk factors.

CONCLUSION

Lp-PLA2 activity has important clinical value in CHD.

Characterization of PLAC® tests in the quantization of lipoprotein associated phospholipase A2 for assessment of cardiovascular diseases

BACKGROUND

PLAC® mass test (diaDexus, Inc.) does not detect all Lp-PLA₂ proteins in the circulation. The total circulating Lp-PLA₂ mass can be quantized by using the CHAPS modified PLAC® mass test. To compare the difference of the PLAC® mass, CHAPS modified PLAC® mass and PLAC® activity tests in risk assessment of CVD, the 3 Lp-PLA₂ quantization methods were characterized using a collection of serum and plasma from CVD patents and matched non-symptomatic controls. Improvement on risk assessment for ischemic stroke by Lp-PLA₂ and lipids were also investigated.

METHODS

Ninety one human sera and plasma from elderly patients with first CVD incidents and 78 matched controls were collected at clinics. Lp-PLA₂ was assessed by PLAC® mass, CHAPS modified PLAC® mass and PLAC® activity tests and data were subjected to statistical analyses. Correlation with lipid cholesterols or Apo proteins was compared for all formats of PLAC® tests. Ratios of Lp-PLA₂ by different PLAC® tests to different lipids were assessed for synergistic enhancement in the indication of ischemic stroke.

RESULTS

The PLAC® mass test was superior to other formats of PLAC® tests in the assessment of CVD and is independent of lipids. The Lp-PLA₂ by the CHAPS modified PLAC® mass test has no separation between the CVD and control groups.

CONCLUSIONS

Both PLAC® mass and PLAC® activity tests are effective but the CHAPS modified PLAC® mass test has no or less utility in the risk assessment of CVD. The ratio of Lp-PLA₂ by either PLAC® mass or PLAC® activity over ApoA1 or (Apo A1 + Apo B) synergistically enhance the risk assessment power for ischemic stroke.

Performance characteristics of lipoprotein-associated phospholipase A2 activity assay on the Dimension Vista analyser and preliminary study of a healthy Italian population

Introduction

Lipoprotein-associated phospholipase A2 (Lp-PLA₂) is a marker of vascular inflammation associated with coronary heart disease and stroke. We evaluated analytical performance of the PLAC® Activity Test on Siemens Dimension Vista® 1500 analyzer and measured Lp-PLA₂ activity in Italian adults to establish reference intervals (RIs) and evaluate correlation with circulating lipids and age.

Materials and methods

The evaluation protocol consisted of precision, linearity, sensitivity, method comparison, substrate depletion (“hook”) effect and interference assessment. Inhibitor (Darapladib) effect was also evaluated. Lp-PLA₂ activity was measured in 250 healthy donors (123 males, 127 females, aged 18-70 years). Central 95% RIs were established using nonparametric statistics.

Results

Intra-assay and inter-assay precision showed CVs of 0.6% - 1.4% and 0.9% - 2.0%, respectively. Linearity replicates showed R² > 0.98. Limit of quantitation was 5.8 U/L (CV = 9.4%). Bland Altman plot showed bias - 0.9, 95% limits of agreement -6.5 - 4.72. Passing-Bablok regression showed excellent correlation (Slope = 1.02, 95% CI: 1.01 to 1.03; Intercept = - 1.86, 95% CI: - 3.08 to - 1.26; R² = 0.999). No “hook effect” was observed at Lp-PLA₂ activities ≤ 1000 U/L. Average Lp-PLA₂ activity in 250 healthy donors was 182 ± 44 U/L (mean ± SD). Males showed statistically significant higher activities than females (P < 0.001). RIs were 107 - 265 U/L for males and 84 - 225 U/L for females. Moderate significant correlation (r = 0.29, P < 0.001) was found between Lp-PLA₂ activity and total cholesterol.

Conclusions

The PLAC® Activity Test shows very good performance characteristics on Dimension Vista® 1500.

Biological variability of lipoprotein-associated phospholipase A2 activity in healthy individuals

OBJECTIVES

Measuring lipoprotein-associated phospholipase A₂ (Lp-PLA₂) activity can aid in identifying individuals at higher risk of coronary heart disease. However, the biological variation of Lp-PLA₂ activity and corresponding reference change value (RCV) is unknown which limits interpretation of results. In this study we aim to define the intra- and inter-individual variability of Lp-PLA₂ activity in a healthy reference population.

METHODS

A total of 24 healthy individuals (22-47years of age) were prospectively collected at several time points: daily for five days (after overnight fast), daily for three days (while non-fasting), weekly for four weeks (after overnight fast), and monthly for 6months (after overnight fast). Intra-individual and inter-individual variability was determined. The index of individuality (IoI) and reference change value (RCV) were calculated for each time period.

RESULTS

Variability in Lp-PLA₂ activity was not different in fasting versus non-fasting states and also did not change in daily versus monthly testing. The RCV for monthly Lp-PLA₂ activity was found to be 17%. More than 90% of the variability was attributable to between person differences while analytic variability comprised <9% of the variability and within-person variability was low at <0.7%. The index of individuality for monthly testing was 0.30 CONCLUSIONS: In a healthy population, Lp-PLA₂ activity displays low analytical and within-person variability and higher inter-individual variability. The change required to differentiate a true change in patient status was determined to be 17% for monthly measurements. The between individual variability and corresponding RCV for the activity assay are lower than previously reported results for the Lp-PLA₂ mass assay.