Preanalytical factors in human plasma ascorbate assay

Estimation of vitamin C (ascorbic acid) in various samples of fresh and frozen human plasma has shown that freezing in a regular freezer at -25 degrees C causes an approximately 14% loss of ascorbate in the sample. Freezing the same samples in a deep freezer at -75 degrees C causes less of an ascorbate loss amounting to about 9%. On the other hand, using the dry ice alcohol bath freezing, which shortens the freezing process to a fraction of seconds produced loss of ascorbate by 3.5% only. The storage time of previously frozen samples at -25 degrees C or -75 degrees C, from 2 to 14 days does not produce noticeable differences in the sample ascorbate concentration. Loss of ascorbate in samples frozen in the dry ice alcohol bath may be acceptable assuming analytical variability of ascorbate assay amounting to about 4% and broad biological variability of ascorbate concentration in various clinical conditions. Use of frozen plasma samples for ascorbate assays may essentially facilitate running this analysis in clinical laboratories.