Can a urinalysis (UA) with nitrites yield a false positive result followed by a negative Polymerase Chain Reaction (PCR)?

Can You Get a False Positive UA with Nitrites and Then a Negative PCR?

Yes, a positive nitrite test on urinalysis can occur without a true urinary tract infection, followed by a negative urine culture (or PCR), though this is uncommon given nitrite's high specificity of 98%.1

Understanding Nitrite Test Performance

The nitrite test has excellent specificity (98-100%) but poor sensitivity (53%), meaning false positives are rare but false negatives are common.12 When nitrite is positive, there are few false-positive results, making it one of the most reliable indicators when present.1

Mechanisms of False Positive Nitrite Results

Specimen handling issues are the most common cause of false positive nitrites:1

• Delayed processing: Urine must be processed within 1 hour at room temperature or 4 hours if refrigerated to maintain accuracy1
• Bacterial overgrowth in specimen: If urine sits too long before testing, contaminating bacteria can convert nitrates to nitrites ex vivo1
• Contaminated collection: Improper collection technique allowing perineal/vaginal flora contamination3

Dietary and medication factors can also contribute:4

• High dietary nitrate intake
• Certain oxidizing agents or medications
• Phenazopyridine (urinary analgesic)

Why Culture/PCR May Be Negative Despite Positive Nitrite

True False Positives (Specimen Issues)

Contamination during collection is the leading cause:3

• A study of disease-free women found 2.5% had positive nitrites even with ideal clean-catch technique3
• Culture contamination occurred in 63-77% of specimens regardless of collection method3
• Mixed bacterial flora with negative culture strongly suggests contamination, not true infection4

Asymptomatic Bacteriuria

Asymptomatic bacteriuria with subsequent clearance can explain this scenario:15

• Prevalence is 15-50% in long-term care residents and 0.7% in afebrile infants15
• Transient low-level bacteriuria may produce positive nitrite but clear before culture results
• The key distinguishing feature is absence of symptoms (no dysuria, frequency, urgency, fever, or hematuria)5

Technical Laboratory Factors

Culture technique limitations:4

• Fastidious organisms may not grow on standard culture media
• Prior antibiotic exposure (even brief) can sterilize urine while nitrite remains positive
• Inadequate culture incubation time or technique

Clinical Decision-Making Algorithm

When Nitrite is Positive but Culture is Negative:

Step 1: Assess for symptoms25

• If symptomatic (dysuria, frequency, urgency, fever, hematuria): Consider empiric treatment while awaiting culture, as combined positive leukocyte esterase and nitrite has 96% specificity5
• If asymptomatic: Do not treat—this represents asymptomatic bacteriuria or false positive25

Step 2: Evaluate specimen quality4

• Check for high epithelial cells (>few per HPF) indicating contamination4
• Review collection method—bagged specimens have 85% false-positive rate5
• Verify processing time—delayed processing causes false positives1

Step 3: Consider recollection if clinically indicated4

• Use catheterization for definitive specimen in women unable to provide clean-catch4
• Midstream clean-catch or condom catheter in cooperative men4
• Process immediately (within 1 hour at room temperature)1

Critical Pitfalls to Avoid

Do not treat based on urinalysis alone without symptoms—this leads to unnecessary antibiotic exposure and resistance development.25 The positive predictive value of pyuria for infection is exceedingly low in asymptomatic patients.4

Do not assume all positive nitrites require treatment—even with 98% specificity, false positives occur, especially with specimen handling issues.13

Do not ignore the clinical context—a positive nitrite in an asymptomatic patient with negative culture is most likely asymptomatic bacteriuria or contamination, neither of which requires treatment.25

Special Populations

In febrile infants <2 years: Always obtain culture regardless of urinalysis results, as 10-50% of culture-proven UTIs have false-negative urinalysis.25 However, the reverse (positive UA, negative culture) still suggests contamination if truly asymptomatic.5

In elderly/long-term care patients: Asymptomatic bacteriuria prevalence is 15-50%, making positive nitrite with negative culture common and not requiring treatment.25

In catheterized patients: Do not screen for or treat asymptomatic bacteriuria—reserve testing only for symptomatic patients with fever or hemodynamic instability.25