Evaluation of a new Apolipoprotein(a) isoform-independent assay for serum Lipoprotein(a)

The risk factor, Lipoprotein(a), [(Lp(a)], has been measured in numerous cl inical studies by a variety of immunochemical assay methods. It is becoming apparent that for many of these assays antibody specificity towards the ap olipoprotein(a) [apo(a)] repetitive component [the kringle 4 - type 2 repea ts] and apo(a) size heterogeneity can significantly affect the accuracy of serum Lp(a) measurements. To address this issue, we investigated whether ou r current in house Lp(a) [Mercodia] assay showed such bias compared to a re cently available assay [Apo-Tek], claiming to possess superior capability f or isoform-independent measurement of Lp(a). Levels of Lipoprotein(a) by bo th Apo-Tek and Mercodia assays correlated inversely with apo(a) isoform siz es. No significant differences were observed between assays in ranges of Lp (a) concentration within each isoform group. The Mercodia assay exhibited s imilar isoform-independent behaviour to that of Apo-Tek for the quantitatio n of serum Lipoprotein(a). Essentially identical results were obtained by t he two methods, suggesting that Mercodia assay's capture monoclonal antibod y also (as is the case for Apo-Tek) does not recognize the kringle 4-type 2 repetitive domain of apo(a). Correlation of Lp(a) concentrations in patien t specimens between Apo-Tek and Mercodia assays showed good agreement, alth ough an overall higher degree of imprecision and non-linearity was noted fo r the Apo-Tek procedure. A change-over to the Apo-Tek assay would therefore not improve on our current assessment of risk contribution from Lp(a) for atherosclerotic vascular disease in individuals with measurable levels of c irculating Lipoprotein(a).