Case Study#2

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Clinical Assay of Vitamin C Status

• Originally measured photometrically after reaction with either dinitrophenylhydrazine or o-phenylenediamine.
• HPLC is currently the predominant laboratory method.
• However, there remains a large interlaboratory variability with an average CV of 15% across multiple studies. This variability is most likely related to differences in the differential detection of the many ascorbic acid isomers/metabolites.
• Another significant source of artifact is the limited stability of ascorbic acid in plasma and serum (better in whole blood). After only one day at room temperature, serum ascorbic acid measurements are significantly diminished and are undetectable by the second day. Similarly, serum stored at 4 ºC shows significant decreases in ascorbic acid levels after 3-5 days.
• Most preservatives are either reducing or oxidizing agents affecting the relative levels of ascorbic acid, dehydroascorbic acid and other forms. In fact, some methods use high amounts of DTT to allow a measurement of "total ascorbic acid".
• Lastly, serum and plasma measurements do not correlate very well with tissue vitamin C levels. Platelet or leukocyte levels are preferred (need to account for the WBC differential), but these methods are not routinely available through commercial sources.
• Generally, plasma levels diminish EARLIER than tissue levels in deficiency and replete SOONER

This clinical conference was presented by the Chief Resident in Clinical Pathology, Michael Hodsdon, MD, PhD. Web cases are edited for this site by Henry Rinder, MD, Associate Professor of Laboratory Medicine, Yale University School of Medicine. Your comments and suggestions are welcome. Please email us at feedback@lab.med.yale.edu