The impact of high-dose vitamin C on blood glucose testing in ¹⁸F-FDG PET imaging

Complementary and alternative therapies in addition to standard oncology protocols are commonly sought by cancer patients; however, few patients disclose their complementary treatments to their cancer care team. A lack of communication may result in unforeseen side effects and the potential for some alternative therapies to interfere with or inhibit conventional treatment. High-dose vitamin C therapy, in particular, may lead to an inability to measure a patient's blood glucose level before (18)F-FDG injection for PET/CT scanning. We report a case of a 52-y-old woman referred for (18)F-FDG PET/CT to evaluate the extent of recurrent colorectal cancer. The PET/CT scan immediately followed a single intravenous dose of 25 g of ascorbic acid from her naturopath. A glucometer that applies the glucose oxidase method for measuring fasting blood glucose was used, for which high doses of vitamin C are listed as a contraindication. The high concentration of ascorbic acid in the patient's blood sample interfered with the chemical reaction on the glucose strip, and therefore no blood glucose measurement could be attained. With more patients receiving alternative and complementary cancer therapies, it is important to know what the implications of orthomolecular therapy might be on routine blood glucose testing for (18)F-FDG PET scans. (18)F-FDG is in direct competition with glucose; therefore, elevated blood glucose levels will cause a decrease in (18)F-FDG absorption and may lead to a false-negative scan.