What is the significance of nitrites in urinalysis?

Nitrites in Urinalysis: Clinical Significance

Direct Answer

Nitrites in urinalysis indicate the presence of bacteria that convert dietary nitrates to nitrites (primarily Gram-negative organisms like E. coli), making a positive nitrite test highly specific (98%) for urinary tract infection, though a negative result does not rule out infection due to poor sensitivity (53%). 1

Biochemical Basis

• Nitrites are produced when Gram-negative enteric bacteria (particularly E. coli, Klebsiella, Proteus) convert dietary nitrates to nitrites in the bladder 1
• This conversion requires approximately 4 hours of bladder dwell time, which explains the test's limitations in certain populations 1
• Not all urinary pathogens reduce nitrate to nitrite—notably, Enterococcus species cannot perform this conversion, leading to false-negative results 1, 2

Diagnostic Performance

Positive Nitrite Test:

• Specificity: 98% (range 90-100%) - When nitrites are present, UTI is highly likely with very few false-positives 1
• Sensitivity: Only 53% (range 15-82%) - Many true UTIs will have negative nitrites 1
• In women with hematuria, combining positive nitrites with hematuria raises the probability of UTI to 93.3% at ≥10² CFU/ml 3

Negative Nitrite Test:

• Has little value in ruling out UTI due to poor sensitivity 1
• Particularly unreliable in infants and young children who void frequently, preventing adequate bladder dwell time for nitrate conversion 1
• Cannot exclude infection caused by Enterococcus or other non-nitrate-reducing organisms 2

Clinical Application Algorithm

When nitrites are POSITIVE:

• Highly specific for bacterial UTI—proceed with treatment if patient is symptomatic 1
• Combined with positive leukocyte esterase, sensitivity increases to 93% and specificity to 96% 1, 4
• Obtain urine culture before initiating antibiotics in symptomatic patients 5

When nitrites are NEGATIVE:

• Do NOT rule out UTI—must rely on other clinical and laboratory findings 1
• Evaluate leukocyte esterase, microscopy for bacteria and WBCs, and clinical symptoms 1
• Consider that frequent voiding (especially in infants) or non-nitrate-reducing organisms may cause false-negatives 1

Combined Testing Strategy

Optimal diagnostic approach combines multiple parameters:

• Leukocyte esterase OR nitrite positive: 93% sensitivity, 72% specificity 1
• Leukocyte esterase AND nitrite positive: 96% specificity for UTI 4
• When leukocyte esterase, nitrite, OR microscopy is positive: 99.8% sensitivity, 70% specificity 1
• Nitrites are more sensitive and specific than other dipstick components, particularly in elderly patients 6

Critical Pitfalls to Avoid

Do NOT use nitrite results to guide antibiotic selection:

• Studies show no significant difference in trimethoprim-sulfamethoxazole sensitivity between nitrite-positive (78% sensitive isolates) and nitrite-negative (82% sensitive isolates) UTIs 2
• Nitrite results do not predict bacterial resistance patterns and should not influence empiric antibiotic choice 2, 7

Do NOT rely solely on nitrites in high-risk populations:

• In hemodialysis patients, nitrites have very poor sensitivity (14-20%) despite high specificity (94%)—always obtain urine culture 8
• In infants 2-24 months with fever, negative nitrites cannot exclude UTI due to frequent voiding 1

Do NOT treat positive nitrites without clinical correlation:

• Positive nitrites with negative leukocyte esterase strongly suggests asymptomatic bacteriuria, which should NOT be treated in most populations 9
• The key distinction between true UTI and asymptomatic bacteriuria is the presence of pyuria (positive leukocyte esterase) 1

Specimen Requirements

• Urine must be fresh: within 1 hour at room temperature or 4 hours if refrigerated to ensure accuracy 1
• Contaminated specimens can produce false results—if epithelial cells are high, consider catheterization for definitive specimen 4