A Simplified Sample Preparation Method for the Assessment of Plasma Ascorbic Acid in Clinical Settings

Vitamin C deficiency is difficult to diagnose on the basis of clinical presentation alone and requires plasma levels for confirmation. Reference laboratories typically specify shipment of plasma on dry ice. This requirement may complicate clinic work flow and delay vitamin C measurement. Additionally, patients with vitamin C deficiency may experience unnecessary testing and increased health-care costs, as other diagnoses are often considered first. We examined an alternative, more practical shipping method.

Plasma was collected from 17 healthy volunteers by use of heparin tubes with gel separators, and all tubes were centrifuged immediately to separate the plasma layer from the cells. Baseline vitamin C was measured in plasma obtained immediately after specimen collection. Remaining sample tubes were held in Styrofoam containers with cold packs for 30 h or 48 h, followed by vitamin C measurement. Additional samples were exposed to conditions that simulated harsher shipping conditions.

Mean plasma vitamin C was 69.6 μmol/L (SD = 21.5 μmol/L). Vitamin C losses were 5.4% at 30 h (SD = 5.55%, P < 0.05) and 7.6% at 48 h (SD = 5.56%, P < 0.05), which is slightly more than freeze-and-thaw treatment (average loss of 1.4%, SD = 6.9%, NS). The vitamin C method had an intraday variation of 1.88%. Vigorous shaking of 2 samples for 24 h resulted in a -1.9% change in 1 sample, and a +4.1% change in another sample. Exposure of the shipping container to elevated temperature (35 °C for 30 h) did not change the internal temperature of the container.

The shipping procedure uses routine sample handling, standard vacutainers, and can be replicated by health-care centers seeking to evaluate patient vitamin C status.