Are nitrites in urine normal during pregnancy?

Nitrites in Urine During Pregnancy: Normal or Abnormal?

Nitrites in urine are not normal during pregnancy and strongly indicate the presence of a urinary tract infection that requires treatment.

Understanding Nitrites in Urinalysis

Nitrites in urine are formed when certain bacteria (particularly gram-negative enteric bacteria) convert dietary nitrates to nitrites in the bladder. This conversion requires approximately 4 hours of bacterial exposure to urine 1. The nitrite test has:

• High specificity: 98% (range 90-100%)
• Lower sensitivity: 53% (range 15-82%) 1, 2

The lower sensitivity means that a negative nitrite test doesn't reliably rule out UTI, but a positive result is highly specific for bacterial infection 1.

Significance in Pregnancy

Pregnant women with asymptomatic bacteriuria have:

• 20-30 fold increased risk of developing pyelonephritis during pregnancy
• Higher risk of premature delivery and low birth weight infants 1

A recent study (2021) found that nitrites had the highest predictive value for UTI in pregnant women with:

• Specificity of 98.6%
• Positive predictive value of 86.6% 3

Diagnostic Approach in Pregnancy

When nitrites are detected in a pregnant woman's urine:

1. Confirm with urine culture: A positive nitrite test should be followed by urine culture before starting antibiotics, as it's the gold standard for diagnosis 2

2. Consider combined testing: Using both leukocyte esterase and nitrite tests improves diagnostic accuracy:

   • When both tests are positive: Specificity 82.7% (higher certainty of infection)
   • When at least one test is positive: Sensitivity 92.0% (fewer missed infections) 4

3. Be aware of limitations: A 2024 study found that in clinical practice, nitrite testing in pregnancy had 72.0% sensitivity and 73.4% specificity, which is lower than previously reported in controlled studies 4

Treatment Implications

Antimicrobial treatment of asymptomatic bacteriuria during pregnancy:

• Decreases risk of pyelonephritis from 20-35% to 1-4%
• Reduces frequency of low birth weight infants and preterm delivery 1

Potential Pitfalls

1. False negatives: Frequent urination in pregnancy may reduce nitrite formation due to insufficient bladder dwell time 1

2. Over-treatment: A 2024 study found only 16.8% of pregnant women treated with antibiotics based on dipstick results actually had positive cultures 4

3. Specimen collection: Ensure proper collection technique and specimen handling (fresh within 1 hour at room temperature or 4 hours if refrigerated) 1, 2

4. Not all pathogens produce nitrites: Some urinary pathogens don't reduce nitrate to nitrite 1

Bottom Line

Nitrites in urine during pregnancy are abnormal and warrant further investigation with urine culture. Given the significant risks of untreated UTI in pregnancy (pyelonephritis, preterm birth, low birth weight), positive nitrite tests should be taken seriously, but confirmation with culture is essential to avoid unnecessary antibiotic use.