Influence of vitamin C on urine dipstick test results

Background. Vitamin C is a strong reducing agent found at high levels in various foods, and it may influence the results of urine strip tests even at an ordinary consumption levels. After oral administration, we measured urine vitamin C levels using urine strips and evaluated whether vitamin C interfered with various test items. The utility of a urine strip with a vitamin C indicator was assessed. Methods. Thirty-three healthy volunteers each ingested 1,000 mg of vitamin C. Their urine samples were tested for vitamin C using a URiSCAN 11 strip (YD Diagnostics, Korea) before and after administration of vitamin C. Standard materials were added to normal pooled urine to generate urine samples with various concentrations of the analytes tested (blood, bilirubin, nitrite, leukocytes, and glucose), and vitamin C was spiked to predetermined levels. These samples were then tested using two urine strips - URiSCAN and Chemstrip test strip (Roche Diagnostics, Germany) - to evaluate interference from vitamin C. In clinical samples with positive vitamin C results, microscopic and chemical analyses were also conducted to examine the differences. Results. Thirteen urine samples from the 33 volunteers were positive for vitamin C before ingestion, and all subjects were positive after ingestion. Vitamin C spiking of urine demonstrated false-negative results at various concentrations. Of 159 specimens with positive results for vitamin C, 14 showed discrepant results after additional confirmatory tests. Conclusions. Vitamin C in urine can cause significant interference with urine strip tests. A urine strip with a vitamin C indicator is useful to reduce the risk of incorrect results in regard to disease states.