Abstract Body (Do not enter title and authors here): Background and aims: Different assays have been used to analyze lipoprotein(a) over time, and the need for standardization of Lp(a) assays is well-known. Nonetheless, the impact of the use of different assays for clinical decision making and risk stratification of individuals is understudied. We aimed to investigate how transitioning between Lp(a) assays at a large central laboratory affected the proportion of individuals with Lp(a) result above clinical thresholds.

Methods: We studied nationwide clinical laboratory data from Norway including 185,493 individuals between 18-50 years of age with 272,463 Lp(a) measurements between 2000-2019. Lp(a) was analyzed in non-frozen serum samples using immunoturbidimetric methods from Roche Tina-quant assay (generation 1) in 2000–2009 (N individuals = 75,221) and Siemens LPA assay (N individuals = 123,824) in 2009-2019 in mg/dL units. Because some lipid-lowering drugs can modestly alter plasma Lp(a), the data material was restricted to individuals <50 years due to a low use of medication in this age group.

Results: The Roche assay detected 35% more individuals with Lp(a) > 50 mg/dL than the Siemens assay in the total population. In individuals that had measurements analyzed by both Roche and Siemens assays (N = 13,494), the Roche assay detected 40% more individuals with Lp(a) > 50 mg/dL than the Siemens assay (25%, N = 3,373 vs. 18%, N = 2,475), and 70% more individuals with Lp(a) >85 mg/dL than the Siemens assay (11%, N = 1,478 vs. 6.5%, N = 881).

Conclusion: The choice of Lp(a) immunoassay can influence interpretation of Lp(a) results and decisions, particularly in individuals approaching clinically relevant Lp(a) thresholds.