Effect of long-term storage on the concentration of albumin and free fatty acids in human sera

Abstract

Stored sera from healthy persons can be used to study relationships between blood variables at the time of sampling and disease appearing several years later but storage may influence the variables. In this work we measured the concentration of albumin and free fatty acids (FFA) in samples from the JANUS serum bank of Norway. Sera from blood donors and persons participating in health screening programs have been added to the bank since 1973. The concentration of albumin and FFA was measured in 443 JANUS bank sera. The material was divided into quartiles according to the length of storage: < 3 years (n = 110), 3-6 years (n = 110), 6-12 years (n = 115) and > 12 years (n = 108). Albumin was measured colorometrically using the bromcresol green method and FFA was determined enzymatically. The serum albumin concentrations (mean - SEM) in the four groups were 55.8 - 0.6, 56.2 - 0.5, 59.9 - 0.6 and 59.5 - 0.6 g/L. The values of groups 3 and 4 were significantly higher than those of groups 1 and 2 (p< 0.001). The serum FFA concentrations in the four groups were 0.56 - 0.03, 0.64 - 0.03, 0.77 - 0.03 and 0.85 - 0.04 mmol/L, i.e. a significant storage effect. The Scheffe´multiple comparison test showed that FFA values in groups 3 and 4 were significantly higher than those in groups 1 and 2 (p< 0.001 for group 4 vs. 1 and 2, and 3 vs. 1; p< 0.04 for group 3 vs. 2). Serum FFA and albumin levels were positively associated (r = 0.489, p< 0.001). Using linear regression analysis, it was estimated that serum albumin values increased by 0.28 g/L per year (i.e. 0.5%) and FFA by 0.02 mmol/L (i.e. 3.8%). Thus, measured by standard methods, serum FFA and albumin could increase in response to several years of storage at ° - 25 C. It is suggested that the storage time dependent increase in FFA is due to FFA liberation from lipoprotein triglycerides, whereas the apparent increase in albumin concentration possibly could be attributed to an unfolding of the protein, allowing more bromcresol green to be bound.