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.

Advantages of the lipoprotein-associated phospholipase A2 activity assay

OBJECTIVES

Lipoprotein-associated phospholipase A2 (Lp-PLA2) is increased in circulation in patients at higher risk of coronary heart disease (CHD) events and stroke. Therefore, measurement of Lp-PLA2 can be used as an adjunct to traditional cardiovascular risk factors for identifying individuals at higher risk of cardiovascular events. Recently, a reagent for measuring Lp-PLA2 activity (diaDexus, San Francisco, CA) received FDA approval. Here we evaluate the assay performance of the Lp-PLA2 activity assay.

METHODS

Lp-PLA2 activity assay reagent performance was evaluated on an open user-defined channel on a Cobas 6000/c501 (Roche Diagnostics, Indianapolis, IN) using a 5-point calibration curve (0-400nmol/min/mL). Analytical performance was established for the following parameters: precision, linearity, accuracy, analytical sensitivity, analytical specificity, reference interval, reagent lot-to-lot comparison, specimen type, on-board reagent stability, and sample stability.

RESULTS

Assay limit of detection was determined to be 7.8nmol/min/mL with an average %CV of 2.8%. Precision studies revealed a coefficient of variation ≤1.6% between 79 and 307nmol/min/mL and accuracy was demonstrated between 4.8-368.7nmol/min/mL. Comparable results were generated in paired SST serum and EDTA plasma. No age association was found with Lp-PLA2 activity at the 95th percentile however a gender association was identified resulting in gender-specific 95th percentile limits in a healthy reference population. No bias was found when comparing results from several different lots of assay reagent. Lp-PLA2 activity results are extremely stable in both serum and EDTA plasma under refrigerate and frozen storage conditions up to 31days.

CONCLUSIONS

Lp-PLA2 activity assay displays accurate and precise performance characteristics on the Cobas c501 platform. The assay performance is significantly improved over the predecessor immunoassay allowing for adoption of Lp-PLA2 activity in clinical practice.