BackgroundSupra-physiological concentrations of ascorbate, vitamin C, in blood, greater than 1 mM, achieved through intravenous administration (IV), are being tested in clinical trials to treat human disease, e.g. cancer. These trials need information on the high levels of ascorbate achieved in blood upon IV administration of pharmacological ascorbate so appropriate clinical decisions can be made.MethodsHere we demonstrate that in the complex matrix of human blood plasma supra-physiological levels of ascorbate can be quantified by direct UV spectroscopy with use of a microvolume UV–vis spectrophotometer.ResultsDirect quantitation of ascorbate in plasma in the range of 2.9 mM, lower limit of detection, up to at least 35 mM can be achieved without any sample processing, other than centrifugation.ConclusionsThis approach is rapid, economical, and can be used to quantify supraphysiological blood levels of ascorbate associated with the use of IV administration of pharmacological ascorbate to treat disease.