Introduction Long-term storage of blood samples in large prospective epidemiological studies enables future biochemical measurements. However, samples may be stored for more than 10 years before analysis, and information is scarce on the long-term stability of biomarkers in frozen plasma. We have investigated the stability of six lipid analytes and creatinine in samples frozen at −20°C, −40°C, −80°C and, in nitrogen vapour, at −150°C for 13 years.
Methods Multiple 1 ml EDTA plasma aliquots were prepared from blood samples from 21 healthy individuals aged 25–60 years. One aliquot from each subject was analysed immediately for apolipoprotein A1, apolipoprotein B, directly-measured HDL-C and LDL-C, total cholesterol, triglycerides and creatinine. Remaining aliquots were stored at −20°C, −40°C, −80°C or −150°C. An aliquot from each individual and each storage temperature was analysed at intervals over 13 years to provide direct comparisons between temperatures.
Results Compared with storage in nitrogen vapour, storage at -80°C for up to 13 years resulted in a mean plasma concentration change of <6% for all analytes except LDL which changed <10%. However, at higher storage temperatures several analytes changed markedly. For example, after 4 years of storage at −20°C, the mean plasma concentration of HDL-C decreased by more than 14% and LDL-C by more than 35%.
Conclusion This study provides some of the most extensive evidence to date on long-term stability of important lipid analytes and creatinine in frozen plasma, and demonstrates the need to store plasma at −80°C or below for long-term prospective studies.
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