Illustration of the principle of assay calibration. A shows lipoprotein(a) concentration distributions in a population using three different lipoprotein(a) assays, all with minimal apolipoprotein(a) isoform size measurement bias but lacking traceability to a common calibrator. The lack of uniform assay calibration prevents direct comparison of measurement values across assays, illustrated by assay-specific cut points for a commonly used threshold for increased risk (e.g. levels above the 80th percentile). In B, the three assays have been uniformly calibrated resulting in comparable measurement values across assays, illustrated by the common threshold for increased risk (e.g. 80th percentile, marked by the grey arrow).