Plasma ascorbic acid preparation and storage for epidemiological studies using TCA precipitation.

Objectives: To introduce a procedure to validate an ascorbic acid method using trichloroacetic acid (TCA) for plasma stabilization at different storage temperatures.

Methods: EDTA and heparin plasma were precipitated with TCA (1:5) containing 0.54 mol/L EDTA, or without. Samples were stored at -20 degrees C and -70 degrees C and their stability was tested at room temperature for 24 h.

Results: A significant 40% loss (p<0.001) of plasma ascorbic acid was found when EDTA samples with added EDTA were stored at -20 degrees C for 2-4 weeks compared with storage at -70 degrees C. Ascorbic acid in heparin plasma without added EDTA was most unstable and samples left at room temperature for 24 h lead to almost a total loss of ascorbic acid. Addition of EDTA to the TCA solution improved stability of samples of both plasma types at room temperature.

Conclusion: The recommended procedure for ascorbic acid determination in plasma stabilized with TCA is immediate storage at -70 degrees C and inclusion of EDTA into the TCA solution.