Effect of Storage Time up to Nine Years at -25°C on 15 Selected Biochemical Serum Components

Biorepositories facilitate research and clinical studies in many settings. Modern biobanks use state-of-the art storage methods and low temperatures, while many older collections of biospecimens have been stored at less optimal temperatures. The Janus Serum Bank Cohort in Norway holds over 700,000 serum samples collected decades ago and stored at -25°C. To obtain insights in the stability of serum components at -25°C over prolonged times, we performed 7 measurements for increasing storage time up to 108 months for a panel of 15 serum components.

A selection of analytes (proteins, an enzyme, electrolytes, small molecules, hormones, lipids, and a vitamin) were measured in serum from 40 anonymous donors. The serum components were measured in fresh samples and after 3, 6, 12, 24, 36, 72, and 108 months in storage at -25°C. We tested for variations using analysis of variance and paired sample t-tests and performed trend analyses for these serum component levels against time.

All measured serum components showed differences in values for at least one of the timepoints. Trend analyses identified significantly decreasing levels for nine components, whereas four components showed significantly increasing levels. Two components did not show significant trends.

Storage of serum at -25°C may result in changes in serum analyte levels over time. We cannot exclude that batch effects of assaying kits; laboratory instrument changes and standards contributed to the observed differences. To mitigate the influence of increasing storage time, storage time should be used as matching criteria for control samples included in research projects.