Blood in Urine Does Not Cause False Positive Nitrite on Urine Dipstick
No, blood in the urine (hematuria) does not cause a urine dipstick to test positive for nitrites. The nitrite test on a urine dipstick specifically detects the presence of nitrites formed by bacterial metabolism of urinary nitrates, particularly by gram-negative enteric bacteria 1.
Understanding Nitrite Testing
The nitrite test works through a specific biochemical mechanism:
- Nitrite is formed by bacterial metabolism of urinary nitrates, especially by gram-negative enteric bacteria 1
- This nitrite conversion requires extensive exposure of bacteria to urine 1
- The nitrite test has high specificity (87-98%) but lower sensitivity 1, 2
Factors That Can Affect Nitrite Results
Several factors can influence nitrite test results, but hematuria is not among them:
- Frequency of urination: Frequent voiding reduces the time for bacteria to convert nitrates to nitrites, potentially leading to false negatives 3
- Specimen processing time: Urine specimens should be processed within 1-4 hours to preserve accuracy 4
- Bacterial type: Certain bacteria like Enterococcus do not produce nitrite and will yield negative nitrite results despite causing UTI 5
Hematuria and Dipstick Testing
When blood is present in urine:
- It is detected by specific hemoglobin-sensitive areas on the dipstick, separate from the nitrite test area
- Hematuria is defined as three or more red blood cells per high-power field on microscopic evaluation 1
- Dipstick testing for blood has its own sensitivity and specificity profile, independent of nitrite testing 1
Interpreting Dipstick Results
For accurate UTI diagnosis, consider the following:
- The combination of leukocyte esterase and nitrite testing provides the highest diagnostic accuracy for UTI (sensitivity 93%, specificity 72%) 4
- A negative result for both nitrite and leukocyte esterase is useful to exclude UTI in most populations 6
- The positive predictive value of nitrite testing alone is highest in elderly patients and in family medicine settings (≥80%) 6
Clinical Implications
When evaluating dipstick results:
- Confirm hematuria detected by dipstick with microscopic evaluation of urinary sediment 1
- A positive nitrite test strongly suggests bacterial infection, regardless of whether blood is present 4
- The combination of negative nitrite and negative leukocyte esterase has a high negative predictive value (95%) for ruling out UTI 2
In summary, while both blood and nitrites can be detected on a urine dipstick, they represent entirely different biological processes and one does not cause false positive results for the other.