Instability of lipoprotein(a) in plasma stored at-70 degrees C: Effects ofconcentration, apolipoprotein(a) genotype, and donor cardiovascular disease

Background: There is considerable evidence to suggest that plasma lipoprote in(a) [Lp(a)] concentration is a cardiovascular risk factor. Confusing resu lts in epidemiologic studies, however, suggest that the effects of storage should be further investigated. The influence of the assay method, the init ial plasma Lp(a) concentration, and the apolipoprotein(a) [apo(a)] genotype are all factors that should be considered. Methods: Blood was obtained from 65 survivors of premature myocardial infar ction and 95 age-matched controls. The plasma samples were stored in steril e conditions at -70 degreesC for 5 years in the presence of antioxidant and antiproteolytic substances. Plasma Lp(a) was measured by immunoturbidimetr y, and apo(a) alleles were determined by pulsed-field gel electrophoresis a nd Southern blotting. Results: Plasma Lp(a) was significantly higher in patients. The mean kringl e number for the smallest isoform was also lower in patients than in contro ls, but no differences were found in the distribution of the largest isofor m. All patients and controls were heterozygotes. During storage, mean Lp(a) decreased significantly in samples from patients (-23%; P < 0.001) but not in samples from controls (-9%; P, not significant). This was not related t o the kringle number and was limited to samples with initial plasma Lp(a) c oncentrations between 41 and 345 mg/L. Conclusions: Plasma Lp(a) from patients is less stable than Lp(a) from cont rols, and the difference is not related to distribution of apo(a) genotypes but may be concentration-dependent. Differential sample stability may comp licate the interpretation of several studies. (C) 2001 American Association for Clinical Chemistry.