Measuring vitamin C in critically ill patients: clinical importance and practical difficulties—Is it time for a surrogate marker?

This chapter underlines the potential clinical relevance and practical difficulties of the currently available measurement of vitamin C. In recent years, there has been increasing interest in the effects of intravenous vitamin C in critically ill patients, but results of multiple clinical trials are not consistent. A possible explanation is the heterogeneity in study designs and included patients. Patients with vitamin C deficiency may benefit more from vitamin C therapy than non-deficient patients. Rapid plasma vitamin C measurements could help identify this subgroup, but the ready oxidation of vitamin C ex-vivo leads to several practical difficulties. Vitamin C measurement is therefore cumbersome, time consuming and not available for routine care. To obtain correct samples for vitamin C analysis it is recommended that heparin-anticoagulated tubes be used, that samples be processed within less than 1 h at low temperature, and that the sample be stabilized by acidification and deproteinization with metaphosphoric acid. A potential surrogate marker is the point-of-care static oxidation-reduction potential (sORP), which can estimate vitamin C status within 20 min using unprocessed plasma samples. sORP could be used to screen for low vitamin C status for research and maybe eventually also for clinical purposes.