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.

Assessment of clinical performance without adequate analytical validation: A prescription for confusion.

OBJECTIVES: Lp-PLA2 is a biomarker with promise for predicting cardiac risk. The lack of reproducible results has limited its use. In evaluating a new reagent kit, we investigated conditions for optimal reproducibility. METHODS: The Auto-PLAC reagents were evaluated on the Cobas instrument. Performance characteristics, stability, and population ranges were determined. RESULTS: Analytical performance characteristics replicated manufacturer's claims. The stability profile of the analyte was unusual, with increasing results observed with storage at 4°C or -20°C. Only storage at -70°C gave acceptable stability. Population median values with properly preserved samples were much lower than the cut off previously validated for increased risk. CONCLUSIONS: It is postulated that variability in specimen handling was a major contributor to the lack of traceability of the current reagents to the earlier clinical studies demonstrating its utility. We are now unsure how to identify reliable criteria for result interpretation.