Vitamin C was frequently found in clinical urine samples, and its concentration was higher in individuals undergoing medical check-ups. Urinary vitamin C can interfere with the urine dipstick results. This study gives useful information for predicting false-negative rates of urine dipstick tests caused by vitamin C.

Background: To understand causes of abnormal reaction for the urinalysis, we analyze the interfering substances of clinical urine samples. We focused the effect of urinary vitamin C and fluorescein sodium to the urine chemistry especially glucose, hemoglobin, and leukocyte esterase. Methods: Incidence of urinary vitamin C was determined for patients and people underwent a medical checkup. We decided dipstick results of glucose, hemoglobin, and leukocyte esterase as false negative based on urine sediment and serum glucose results. Dipstick urinalysis was tested by URiSCAN Pro III with URiSCAN 11 strip (YD Diagnostics, Korea). Urine sediments tests were performed by manual microscopic analysis or Sysmex UF-1000i (Sysmex Co., Japan). Results: The incidence of vitamin C was 20.4% for all subjects. The positive rate of the medical checkup group (34.6%) was higher than others. When vitamin C was detected in clinical urine samples, 42.3%, 10.6%, and 8.2% were defined as false negative for glucose, hemoglobin, and leukocyte esterase dipstick tests, respectively. Fluorescein sodium also interfered on the results of hemoglobin and leukocyte esterase of the dipstick reagents. Conclusions: Vitamin C was frequently found in the clinical urine samples, and its incidence was higher in the people who underwent medical checkup. The urinary vitamin C and fluorescein sodium can cause interferences in urine dipstick results. Thus, it is expected that present study will give useful information to predict false negative rates of urine dipstick tests by vitamin C and fluorescein sodium.