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Influence of Vitamin C on Urine Dipstick Test Results

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Annals of Clinical & Laboratory Science, vol. 45, no. 4, 2015 101
Influence of Vitamin C on Urine Dipstick Test Results
Dae-Hyun Ko1, Tae-Dong Jeong1, Sollip Kim2, Hee-Jung Chung3, Woochang Lee1, Sail Chun1, and Won-Ki Min1
1Department of Laboratory Medicine, Asan Medical Center and University of Ulsan College of Medicine, Seoul,
2Department of Laboratory Medicine, Ilsan Paik Hospital, Inje University College of Medicine, Goyang, and
3Department of Laboratory Medicine, Cheil General Hospital and Women's Healthcare Centre, Kwandong University
School of Medicine, Seoul, Korea

Abstract. 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 administra-
tion, 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 concen-
trations 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 differ-
ences. 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 re-
sults 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.

Key words: vitamin C, urine strip test, interference

Introduction UA strips are coated with reagents for the measure-

ment of each analyte, and chemical reactions pro-
Vitamin C is a strong reducing agent present in duce color gradations to be measured semi-quanti-
many foods. Its antioxidant properties are thought tatively. Many of these reactions are oxidation
to help prevent cardiovascular disease and aging. reactions, and the degree of oxidation is propor-
For that reason, the consumption of vitamin C has tional to the concentration of the analyte tested.
become popular. Vitamin C is also involved in can-
cer prevention and improving the quality of life of The presence of vitamin C, an antioxidant, in urine
cancer patients [1]. may therefore cause false-negative results for some
test items. This is especially problematic for blood
Urinalysis (UA) is noninvasive, simple, and one of and glucose, which are detected via the peroxidase
the most important screening tests in clinical prac- reaction. False-negative results may also occur for
tice. UA provides information that can lead to the nitrite, bilirubin, and leukocytes [2,3].
early detection of diseases in many organs. Urine
strip products are available from many diagnostic Many diagnostic companies have sought to develop
companies worldwide, and common test items in- UA strips resistant to such interference [4].
clude blood, bilirubin, urobilinogen, ketones, Alternatively, one company (YD Diagnostics,
protein, nitrite, glucose, pH, specific gravity (SG), Korea) has launched a UA strip with a vitamin C
and leukocytes. test to additionally measure the vitamin C concen-
tration in urine. This study had two objectives: to
Address correspondence to Won-Ki Min, M.D., Ph.D.; Department evaluate the utility of this vitamin C strip by testing
of Laboratory Medicine, University of Ulsan College of Medicine
and Asan Medical Center, 88, Olympic-ro 43-gil, Songpa-gu, Seoul, urine samples after vitamin C ingestion and to in-
138-736, Korea; phone: 82 2 3010 4503; fax: 82 2 478 0884; e mail: vestigate the effects of vitamin C on other test
wkmin@ amc.seoul.kr
results.
0091-7370/15/0400-256. © 2015 by the Association of Clinical Scientists, Inc.
102 Annals of Clinical & Laboratory Science, vol. 45, no. 4, 2015

Table 1. Results using the vitamin C stick, before and after consumption of a vitamin C supplement.

After/Before Negative 1+ 2+ 3+ Total

Negative 0 1 12 7 20
1+ 0 0 3 8 11
2+ 0 0 0 0 0
3+ 0 0 0 2 2

Total 0 1 15 17 33

Table 2. Interference by ascorbic acid using the URiSCAN 11 strip.

Ascorbic acid (mg/dL)

Titrated concentration 0 10 50 100 500

Glucose(mg/dL) 0 Neg. Neg. Neg. Neg. Neg.

100 ± ± ± ± Neg.
250 2+ 1+ 1+ 1+ Neg.
500 3+ 3+ 2+ 3+ Neg.
2000 4+ 4+ 4+ 4+ 3+

Blood(RBC/µL) 0 Neg. Neg. Neg. Neg. Neg.

5 ± ± ± ± Neg.
10 1+ 1+ 1+ 2+ 1+
250 3+ 3+ 3+ 3+ 3+

Leucocytes 0 Neg. Neg. Neg. Neg. Neg.

(WBC/µL) 10 ± ± Neg. Neg. Neg.
25 1+ 1+ Neg. Neg. Neg.
500 3+ 3+ Neg. Neg. Neg.

Nitrite (mg/dL) 0 Neg. Neg. Neg. Neg. Neg.

0.05 Pos. Pos. Neg. Neg. Neg.

Bilirubin (mg/dL) 0 Neg. Neg. Neg. Neg. Neg.

0.5 1+ 1+ Neg. Neg. Neg.
1 2+ 2+ 2+ 1+ Neg.
3 3+ 3+ 3+ 2+ 1+

Materials and Methods Interference test for pooled samples. Pooled urine sam-
ples were prepared from healthy volunteers whose urine
This study was conducted in accordance with the samples were normal on all urine strip tests. Standard
Declaration of Helsinki. It was approved by the Asan materials were then added to the pooled urine to create
Medical Center Institutional Review Board (Project predetermined concentrations of the analytes tested
#2008-0291). (blood: 0, 5, 10, and 250 RBC/µL, bilirubin: 0, 0.5, 1.0,
and 3.0 mg/dL, nitrite: 0 and 0.05 mg/dL, WBCs: 0, 10,
Test for vitamin C in urine before and after oral ad- 25, and 500 WBC/µL, glucose: 0, 100, 250, 500, and
ministration. We recruited 33 healthy volunteers for the 2,000 mg/dL).
experiment. Their urine samples were tested for vitamin
C using a urine strip (URiSCAN 11 strip, YD The samples were spiked with varying amounts of vita-
Diagnostics, Korea) before and 4-8 hours after oral ad- min C, adjusted to final concentrations of 10, 50, 100,
ministration of vitamin C (1,000 mg). The results were and 500 mg/dL. A random urine sample could have vi-
scored as ‘0’ for 0-9 mg/dL, ‘1+’ for 10-16 mg/dL, ‘2+’ tamin C up to 19350 μmol/L (about 340 mg/dL) [2].
for 17-35 mg/dL, and ‘3+’ for 35 mg/dL or more of vi- Some of the urine collected after vitamin C administra-
tamin C. tion consistently showed 3+ after several-fold dilution.
 103
Influence of vitamin C on urine dip stick test results
Therefore, the range was extended to 500 mg/dL, al- Interference test using clinical samples. In total,
though the cut-off for ‘3+’ was 35 mg/dL, to provide evi- 159 specimens (24 %) were positive for vitamin C
dence for the relation between vitamin C and UA strip at reaction intensities of ‘1+’ or more. The positive
results. The samples were then tested for blood, biliru- samples were tested for discrepancy between the
bin, nitrite, glucose, and leukocytes. These tests were
URiSCAN strip and reference methods. Of the
conducted using the URiSCAN 11 strip, which is the
most widely used UA strip in Korea [5], and the
specimens, 14 (8.8%) had discrepancies between
Chemstrip Test Strip (Roche Diagnostics, Germany), the strip results and additional confirmatory tests,
which is relatively resistant to interference by vitamin C as shown in Table 4. Of the specimens, five were
[4]. falsely negative for blood cells and eight were false-
ly negative for leukocytes when using the strip.
Interference tests using clinical samples. To detect vita- One specimen was negative for glucose according
min C using a URiSCAN 11 strip, we tested urine sam- to the URiSCAN strip, contrary to the results ob-
ples requested for UA from July 2008 to August 2008 at tained using the automatic chemistry analyzer
Asan Medical Center, Korea. In total, 676 samples were (122.5 mg/dL).
tested, and 159 samples (24%) were positive for vitamin
C, with a reaction intensity of ‘1’ or greater. All of these
samples were retested using reference methods – micro-
Discussion
scopic analysis of urine sediment to quantify RBC and
WBC, and chemical analysis of glucose using an auto- After ingestion of a typical dose of vitamin C in
matic chemistry analyzer – to compare the results with dietary supplement form, all participant urine sam-
those from the UA strip. ples were positive for vitamin C. High concentra-
tions of vitamin C in urine can cause false decreases
Results in dipstick reactions, with a variable degree of in-
terference, depending on the analytes and their
Test for vitamin C in urine before and after oral concentrations. We confirmed the results again in
administration. Of the 33 volunteers, 39.4% clinical samples that were positive for vitamin C.
(13/33) were positive and the others (60.6%,
20/33) were negative for vitamin C before inges- In the 1980s, the antioxidant properties of vitamin
tion of vitamin C. All subjects were positive after C were shown to be related to its anticancer effects
vitamin C ingestion. One specimen (3.0%) showed [6]. In some studies, intravenous administration of
a 1+ reaction intensity, 15 (42.4%) showed 2+, and high-dose vitamin C to terminal cancer patients
the remaining 17 (51.5%) showed 3+ (Table 1). was relatively tolerable and safe [7,8]. In three ter-
minal cancer patients, high-dose vitamin C, ad-
Interference test with pooled samples. The test re- ministered intravenously, showed long-term tu-
sults using the URiSCAN are shown in Table 2. mor-regressing effects [9]. Moreover, in a study
The samples with 100, 250, or 500 mg/dL glucose evaluating the risk for colorectal cancer in 715
showed false-negatives with 500 mg/dL vitamin C, colorectal cancer patients and 727 age- and gender-
and the blood cell tests were also negative at the matched community controls, vitamin C ingestion
same concentration of vitamin C for the urine sam- was effective in reducing the risk of colorectal can-
ples with 5 RBC/µL. Vitamin C at 50 mg/dL or cer in a dose-dependent manner [10]. Vitamin C
greater concentrations caused the test results to administration may also improve the quality of life
change to negative for nitrite and WBC amounts of terminal cancer patients [11].
over the ranges tested. Samples with 0.5 mg/dL
bilirubin showed negative results with 50 mg/dL or It is well known that vitamin C in urine can cause
greater vitamin C, and the urine samples with high- false-negative results for blood cells and glucose,
er bilirubin levels (1 and 3 mg/dL) were generally the tests for which specifically use oxidation reac-
not affected by vitamin C. tions. In this study, vitamin C interfered with these
reactions as expected when using the URiSCAN
Table 3 shows the results of using the Chemstrip. test, but interference was also seen with higher con-
Interestingly, 500 mg/dL vitamin C increased the centrations of vitamin C using the Chemstrip test.
reaction strength for glucose and blood cells. The Residues of strongly oxidizing cleaning agents in
nitrite and leukocyte results were similar to those of the urine container may also cause false-positive re-
URiSCAN, except for the nitrite reaction at 10 mg/ sults when using the Chemstrip (according to the
dL of vitamin C.
104 Annals of Clinical & Laboratory Science, vol. 45, no. 4, 2015

Table 3. Interference by ascorbic acid using the Chemstrip.

Ascorbic acid (mg/dL)

Titrated concentration 0 10 50 100 500

Glucose(mg/dL)
0 Neg. Neg. Neg. Neg. 3+
100 ± 1+ 1+ 1+ 3+
250 2+ 2+ 2+ 2+ 3+
500 3+ 3+ 3+ 3+ 3+
2000 3+ 3+ 3+ 3+ 3+

Blood(RBC/µL)
0 Neg. Neg. Neg. Neg. 4+
5 ± 1+ ± ± 4+
10 1+ 1+ 1+ 2+ 4+
250 4+ 4+ 4+ 4+ 4+

Leucocytes 0 Neg. Neg. Neg. Neg. Neg.

(WBC/µL) 10 ± ± Neg. Neg. Neg.
25 1+ 1+ Neg. Neg. Neg.
500 2+ 2+ Neg. Neg. Neg.

Nitrite(mg/dL) 0 Neg. Neg. Neg. Neg. Neg.

0.05 Pos. Neg. Neg. Neg. Neg.

Bilirubin(mg/dL) 0 Neg. Neg. Neg. Neg. Neg.

0.5 Neg. Neg. Neg. Neg. Neg.
1 ± ± ± Neg. Neg.
3 2+ 2+ 1+ ± Neg.

Table 4. Discrepant results due to the presence of vitamin C between the urine strip test and confirmatory reference
methods.

Specimen Test Urine strip Reference method Vitamin C

number results results concentration

1 Blood Negative 3-5 RBC/HPF 2+

2 Negative 3-5 RBC/HPF 2+
3 Negative 6-10 RBC/HPF 3+
4 Negative 3-5 RBC/HPF 3+
5 Negative 3-4 RBC/HPF 3+
6 Leukocytes Negative 3-5 WBC/HPF 1+
7 Negative 3-5 WBC/HPF 1+
8 Negative 3-5 WBC/HPF 2+
9 Negative 3-5 WBC/HPF 2+
10 Negative 3-5 WBC/HPF 2+
11 Negative 6-10 WBC/HPF 2+
12 Negative 3-5 WBC/HPF 3+
13 Negative 6-10 WBC/HPF 3+
14 Glucose Negative 122.5 mg/dL 3+

HPF: high power field.

 Influence of vitamin C on urine dip stick test results 105
Roche Diagnostics product insert), but additional C indicator is a preferred solution. For specimens
experiments are needed to clarify the exact cause(s). positive for vitamin C, the results should be report-
ed with a comment, such as “Vitamin C in urine
The Chemstrip uses iodate or potassium iodide can cause false-negative results for glucose and
chromogen as a scavenger to reduce interference blood cells. Repeating a test after restriction of vita-
from vitamin C [12,13]. The provider claims that it min C consumption is recommended,” or the spec-
is not influenced by vitamin C when the glucose imens should be subjected to additional follow-up
concentration exceeds 100 mg/dL, but it cannot analyses. Vitamin C-resistant strips do not elimi-
eliminate interference and provides no information nate possible interference by high concentrations of
about interference at lower concentrations of glu- vitamin C; thus, using a strip with a vitamin C in-
cose [3]. Berg demonstrated that with 30 mg/dL dicator is preferable.
vitamin C, 23 of 30 specimens with glucose levels
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