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Quick Reference
  • Normal Result: Negative
  • Specificity: >90% for bacterial UTI (highly specific)
  • Sensitivity: 35-85% (varies with conditions)
  • Primary Use: Detection of nitrate-reducing bacteria in urinary tract infections
  • Sample Type: Random urine specimen (first morning void preferred)
  • Key Point: Positive result is highly specific for bacterial UTI; negative does not rule out infection

Test Description

The urine nitrite test detects the presence of nitrites in urine, which are produced when certain gram-negative bacteria convert dietary nitrates to nitrites. This conversion occurs only in the presence of specific nitrate-reducing bacteria, making a positive nitrite test highly specific for bacterial urinary tract infection.

Normally, urine contains small amounts of nitrates from dietary sources but should not contain nitrites. When nitrate-reducing bacteria are present in sufficient numbers and have adequate time in the bladder, they convert nitrates to nitrites, which are detected by the urine dipstick test.

Mechanism of Action

  • Dietary Nitrates: Consumed in vegetables (leafy greens, beets) and converted to nitrates in the body, which are filtered by the kidneys into urine
  • Bacterial Conversion: Nitrate-reducing bacteria contain nitrate reductase enzymes that convert nitrate (NO₃⁻) to nitrite (NO₂⁻)
  • Detection: Nitrites react with reagent chemicals on the dipstick to produce a pink color change
  • Requirements: Adequate bacterial count (≥10⁵ CFU/mL), sufficient bladder dwell time (4-6 hours), and presence of dietary nitrates
Clinical Significance

Nitrate-Reducing Bacteria (Positive Nitrites)

The following gram-negative bacteria contain nitrate reductase enzymes and produce positive nitrite results:

  • Escherichia coli (E. coli): Most common cause of UTI (~80% of cases), reliably produces nitrites
  • Klebsiella species: Common nosocomial pathogen and community-acquired UTI
  • Proteus species: Associated with kidney stones and complicated UTIs
  • Enterobacter species: Opportunistic pathogen in healthcare-associated infections
  • Citrobacter species: Less common UTI pathogen
  • Pseudomonas aeruginosa: Catheter-associated UTIs and complicated infections

Non-Nitrate-Reducing Bacteria (Negative Nitrites)

The following bacteria commonly cause UTIs but do NOT produce nitrites, leading to false-negative results:

  • Enterococcus species: Common cause of complicated UTIs, catheter-associated infections, and post-surgical UTIs
  • Staphylococcus saprophyticus: Second most common cause of UTI in young sexually active women (after E. coli)
  • Acinetobacter species: Healthcare-associated infections and catheter-related UTIs
  • Other gram-positive bacteria: Group B Streptococcus, Staphylococcus aureus
Important Clinical Caveat: Approximately 15-20% of UTIs are caused by bacteria that do NOT produce nitrites (Enterococcus, Staph saprophyticus). A negative nitrite test does not rule out urinary tract infection. Always consider clinical symptoms and leukocyte esterase results.

Conditions Associated with Positive Nitrites

  • Uncomplicated Cystitis: Lower urinary tract infection with dysuria, frequency, urgency
  • Pyelonephritis: Upper urinary tract infection with fever, flank pain, systemic symptoms
  • Asymptomatic Bacteriuria: Positive nitrites without symptoms (treatment only in pregnancy or before urologic procedures)
  • Catheter-Associated UTI: UTI in patients with indwelling urinary catheters
  • Complicated UTI: Infection in presence of structural abnormalities, immunosuppression, or kidney stones
Interfering Factors

Medications and Substances That Interfere

  • Ascorbic Acid (Vitamin C): High doses (>500 mg/day) can cause false-negative results by reducing color development
  • Phenazopyridine (Pyridium, AZO): Orange-red urinary analgesic interferes with dipstick color interpretation
  • Antibiotics: Recent antibiotic use may reduce bacterial count below detection threshold
  • Beets and Rhubarb: Can cause red discoloration that may interfere with visual interpretation

Collection and Handling Factors

  • Specimen Storage: Room temperature storage allows bacterial overgrowth; refrigerate if testing delayed >2 hours
  • Contaminated Collection: Poor technique or contaminated collection containers can introduce bacteria
  • Delayed Testing: Bacterial multiplication in stored specimens can produce false-positive results
  • Exposure to Air: Prolonged air exposure can convert nitrites back to nitrates, causing false-negative results

Patient-Related Factors

  • Frequent Urination: Polyuria, overactive bladder, or frequent voiding prevents adequate dwell time
  • Diuretic Use: Increases urine volume and frequency, reducing dwell time and diluting nitrite concentration
  • Low Nitrate Diet: Vegetarian/vegan diets typically have higher nitrate content; low vegetable intake reduces substrate
  • Dehydration vs Overhydration: Extreme dilution reduces detection; severe dehydration may reduce bacterial conversion
Clinical Pearls
Clinical Pearl
"Positive Nitrites = Treat for UTI": A positive nitrite test is highly specific (>90%) for bacterial UTI. If the clinical picture is consistent, empiric antibiotic treatment is warranted even before culture results. Positive nitrites indicate significant bacteriuria with nitrate-reducing organisms.
Clinical Pearl
"Negative Nitrites ≠ No UTI": A negative nitrite test does NOT rule out urinary tract infection. Sensitivity is only 35-85% due to non-nitrate-reducing bacteria (Enterococcus, Staph saprophyticus), inadequate dwell time, low bacterial count, or vitamin C interference. Always consider clinical symptoms and leukocyte esterase results.
Clinical Pearl
First Morning Void is Best: Collect the first morning urine specimen whenever possible for maximum sensitivity. Overnight bladder dwell time (6-8 hours) allows optimal nitrate-to-nitrite conversion. Random specimens have significantly lower sensitivity, especially in patients with frequent urination.
Clinical Pearl
"Enterococcus and Staph Saprophyticus Don't Produce Nitrites": Remember that Enterococcus (common in complicated UTIs and elderly) and Staphylococcus saprophyticus (second most common cause of UTI in young women) do NOT reduce nitrates. These infections will have negative nitrites but positive leukocyte esterase.
LE + Nitrites = 95% Specific for UTI: The combination of positive leukocyte esterase AND positive nitrites has approximately 95% specificity for bacterial UTI. This is one of the most reliable dipstick findings and strongly supports empiric treatment.
Young Women with Dysuria: In young, sexually active women with acute dysuria, frequency, and urgency, up to 30% of culture-proven UTIs are caused by Staph saprophyticus, which does NOT produce nitrites. Rely on clinical symptoms and leukocyte esterase in this population.
Clinical Pearl
Vitamin C Interference: Patients taking high-dose vitamin C supplements (>500 mg/day) can have false-negative nitrite tests. Ask about vitamin supplementation if clinical suspicion for UTI is high but nitrites are negative.
Clinical Pearl
Rapid Urination = Low Sensitivity: Patients with urgency, frequency, polyuria, or overactive bladder often have negative nitrites even with true UTI because urine doesn't dwell long enough (need 4-6 hours minimum). This is especially common in children and elderly patients.
Asymptomatic Bacteriuria Decision: Positive nitrites without symptoms (asymptomatic bacteriuria) generally does NOT require treatment except in pregnant women and patients about to undergo urologic procedures. Overtreatment of asymptomatic bacteriuria contributes to antibiotic resistance.
Clinical Pearl
Catheterized Patients: Nitrite testing has very poor sensitivity in catheterized patients due to continuous bladder drainage (no dwell time) and polymicrobial infections. Rely more on clinical signs of infection (fever, leukocytosis, altered mental status in elderly) rather than urinalysis alone.
References
  1. Kratz, A., Ferraro, M., Sluss, P. M., & Lewandrowski, K. B. (2004). Laboratory reference values. New England Journal of Medicine, 351, 1548-1564.
  2. Lee, M. (Ed.). (2009). Basic skills in interpreting laboratory data. Ashp.
  3. Farinde, A. (2021). Lab values, normal adult: Laboratory reference ranges in healthy adults. Medscape. https://emedicine.medscape.com/article/2172316-overview?form=fpf
  4. Nickson, C. (n.d.). Critical Care Compendium. Life in the Fast Lane • LITFL. https://litfl.com/ccc-critical-care-compendium/
  5. Farkas, Josh MD. (2015). Table of Contents - EMCrit Project. EMCrit Project. https://emcrit.org/ibcc/toc/
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