Which vitamins have antioxidant properties?
A: Vitamins A, C, and E, as well as their precursors, all have antioxidant properties that reduce free radical-mediated cellular damage. Although vitamins A and E at high concentrations have the potential to cause serious health consequences, vitamin C has low toxicity potential. Its serum values in healthy persons range between 23 and 114 µmol/L. However, excess supplementation might result in concentrations as high as 30,000 µmol/L. These supraphysiological concentrations can interfere with a variety of lab assays.
Which lab tests are affected by vitamin C?
Redox reaction-dependent tests, including those for cholesterol, triglycerides, and enzymatic creatinine, are susceptible to interference by high-dose vitamin C. Because of its powerful reducing potential, vitamin C might artificially lower the chromogen signal dependent on peroxide generation. High vitamin C concentrations may also impact potentiometric assays, such as those for electrolytes.
Furthermore, the vast majority of vitamin C is eliminated in urine, and due to renal reabsorption of water, very high urinary vitamin C concentrations can be observed. Urinary vitamin C has been shown to affect dipstick testing for nitrites, bilirubin, glucose, and hemoglobin.
What indicators point to vitamin C interference?
A few telltale signs of antioxidant interference may include delta flags without obvious clinical or technical explanation, nonphysiological results (e.g., negative values), and nonlinear dilutions. Also, clinician inquiries calling results into question may raise suspicion of interference.
In such cases, laboratories may decide to measure vitamin C concentrations in the specimen. However, vitamin C concentration assays are expensive and often only available at reference laboratories. One potential alternative to these assays is to use test strips to detect vitamin C in a patient sample, an approach whose utility was demonstrated by our group in a recent case report (Clin Biochem 2021; doi: 10.1016/j.clinbiochem.2021.07.001). Although test strips are a straightforward and low-cost alternative to high-performance liquid chromatography-based assays, labs must still validate them before using them for clinical testing.
What can labs do to mitigate this interference?
Working in collaboration with providers, laboratories can take a few steps to prevent vitamin C interference from affecting results:
Precollection questionnaires. Patients can complete these before their appointment at the clinic or collection site. The questions may ask about the type, amount, and frequency of any dietary supplements the patient may be taking. If the supplement intake appears to be excessive, the patient would then need to speak with a provider before specimen collection.
Clinician and patient education. Labs can identify the impacted tests and compile a list, which may then be shared with clinicians. To facilitate patient awareness, educational pamphlets explaining test interference in lay terms may also be posted at clinics and collection sites.
Chartable notes. Labs can determine the direction of change caused by interference in each of the affected tests. If the change in the result creates a delta flag, it may be reported with a chartable comment such as, “Antioxidants may interfere with this assay. Interpret the results in the clinical context.”
Finally, labs may employ a few basic measures such as requesting a fresh specimen, performing serial dilutions, and testing on a different platform. When such approaches are not feasible due to limited volumes or the irretrievable nature of the specimen, storing the specimen at room temperature is another possibility, because vitamin C is temperature sensitive and its effectiveness declines with time.
The recent study performed by our group also showed the utility of a vitamin C-neutralizing enzyme called ascorbate oxidase (AO). Labs can treat patient specimens suspected of interference with AO prior to testing. Since this involves specimen manipulation, labs must perform a thorough validation taking into account regulatory aspects before trying this approach.
Vrajesh “Raj” Pandya, PhD, DABCC, is a medical director of clinical chemistry and toxicology at the University of Utah and ARUP Laboratories in Salt Lake City. +Email: [email protected]
A letter to the editor was written in response to this article. View the letter here.