Management of Trace Nitrites in Prenatal Patients
A prenatal patient with trace nitrites on urinalysis requires a urine culture before initiating antibiotics, and treatment should be started empirically if symptomatic while awaiting culture results. 1, 2
Diagnostic Interpretation of Trace Nitrites
Trace nitrites have high specificity (98%) but low sensitivity (53%) for urinary tract infection (UTI), meaning a positive result strongly suggests bacterial infection, but a negative result does not rule it out. 1, 2
- The nitrite test detects gram-negative bacteria (primarily E. coli) that convert dietary nitrates to nitrites, requiring approximately 4 hours of bladder dwell time 1
- In pregnancy, frequent voiding may reduce sensitivity further, making trace nitrites particularly significant when present 1
- When combined with positive leukocyte esterase, specificity increases to 96% with sensitivity of 93% 1, 3, 2
Mandatory Next Steps
Obtain a urine culture immediately before initiating any antimicrobial therapy. 1, 4
- All pregnant women with suspected UTI require urine culture confirmation, as screening for pyuria alone has only 50% sensitivity for identifying bacteriuria 1
- The culture should be obtained via clean-catch or catheterization method, not bag collection 1
- Treatment decisions should not rely solely on dipstick results without clinical correlation 2, 5
Treatment Decision Algorithm
If Symptomatic (dysuria, frequency, urgency, fever, suprapubic pain):
Initiate empirical antibiotic therapy immediately after obtaining urine culture. 1, 2, 4
First-line empirical options:
- Nitrofurantoin 100 mg twice daily for 7 days (preferred for asymptomatic bacteriuria and uncomplicated cystitis) 1, 6, 7
- Cephalexin 500 mg four times daily for 7 days (first-generation cephalosporin alternative) 6, 7
- Amoxicillin 500 mg three times daily for 3-7 days (if organism susceptibility known, though resistance rates are high) 7
Avoid trimethoprim-sulfamethoxazole in the first trimester due to teratogenic concerns, though it may be used in second trimester if needed 7
If Asymptomatic:
This represents asymptomatic bacteriuria, which requires treatment in pregnancy to prevent pyelonephritis. 1
- Asymptomatic bacteriuria occurs in 1.8-2.1% of pregnant women and progresses to pyelonephritis in 20-40% if untreated 1
- Treatment reduces pyelonephritis rates from 20-35% to 1-4% 1
- Use the same antibiotic regimens as for symptomatic infection, typically 7-day courses 1, 7
Critical Follow-Up
Obtain repeat urine culture 7 days after completing therapy to document microbiological cure. 1, 7
- Women with negative initial screening culture at 12-16 weeks still have 1-2% risk of developing pyelonephritis later in pregnancy 1
- If bacteriuria persists or recurs, consider suppressive prophylaxis for remainder of pregnancy 1
- Monthly screening cultures may be warranted after treatment completion 1
Special Considerations for Upper Tract Infection
If signs of pyelonephritis develop (fever >38°C, flank pain, nausea/vomiting), hospitalize immediately for parenteral therapy. 4
- Second-generation cephalosporins (cefuroxime) are first-line for hospitalized patients 4
- Aminoglycosides are second-line in second/third trimester only 4
- Switch to oral therapy after 48 hours of clinical improvement and adequate oral tolerance 4
- Total treatment duration should be 7-10 days 4
Common Pitfalls to Avoid
- Do not ignore trace nitrites in pregnancy - even trace amounts have high specificity and warrant culture and potential treatment 2, 8, 5
- Do not use nitrofurantoin for suspected pyelonephritis - it does not achieve therapeutic blood concentrations 3
- Do not delay culture collection - always obtain before starting antibiotics to guide subsequent therapy 1, 4
- Do not assume asymptomatic bacteriuria is benign in pregnancy - it requires treatment unlike in non-pregnant populations 1, 2
- Do not rely on urinalysis alone - up to 50% of culture-proven UTIs have negative urinalysis 1