Is it best practice to send urine for culture and sensitivity (C&S) in prenatal exams even if the patient is asymptomatic and urinalysis results are negative?

Urine Culture and Sensitivity Testing in Asymptomatic Pregnant Women with Negative Urinalysis

Urine culture and sensitivity (C&S) testing should be performed for all pregnant women at their first prenatal visit or at 12-16 weeks' gestation, regardless of symptoms or urinalysis results. 1

Rationale for Screening in Pregnancy

• Asymptomatic bacteriuria (ASB) occurs in 2-11% of pregnant women and is a clear predisposition to the development of acute pyelonephritis 2, 3
• Pyelonephritis is associated with significant maternal and fetal complications including septicemia, respiratory distress, low birth weight, and preterm birth 4
• Treatment of bacteriuria during pregnancy significantly reduces the incidence of pyelonephritis and associated complications 1, 4
• The U.S. Preventive Services Task Force (USPSTF) gives a "B" recommendation for screening all pregnant women for ASB using urine culture 1, 4

Limitations of Urinalysis for Detecting ASB

• Standard urinalysis tests (dipstick analysis and direct microscopy) have poor positive and negative predictive values for detecting bacteriuria in asymptomatic persons 1
• Screening for pyuria alone has limited sensitivity (approximately 50%) for identifying bacteriuria in pregnant women 5
• Studies evaluating reagent test strips show sensitivity as low as 33% when multiple parameters (blood, protein, nitrite, and leucocyte esterase) are used in combination 6
• No currently available screening tests have high enough sensitivity and negative predictive value to replace urine culture as the preferred screening test in pregnancy 1

Recommended Screening Protocol

• All pregnant women should provide a clean-catch urine specimen for culture at 12-16 weeks' gestation or at the first prenatal visit if later 1
• Significant bacteriuria is defined as ≥10^5 colony-forming units per mL of a single uropathogen in a midstream clean-catch specimen 7
• Women with a negative urine culture from a single screening specimen still have a 1-2% risk of developing pyelonephritis later in pregnancy 5
• The optimal frequency of subsequent urine testing during pregnancy remains uncertain 1

Treatment Considerations

• Pregnant women with ASB should receive antibiotic therapy directed at the cultured organism and follow-up monitoring 1
• Common pathogens include Escherichia coli, Klebsiella pneumoniae, Enterococcus faecalis, and Staphylococcus aureus 2, 8
• Group B Streptococcus in urine at any colony count requires treatment during pregnancy, as it indicates heavy genital tract colonization 7
• For confirmed pathogenic bacteriuria, 4-7 days of antimicrobial treatment is suggested 7

Common Pitfalls to Avoid

• Mistaking normal vaginal flora contamination (e.g., Lactobacillus) for true bacteriuria requiring treatment 7
• Relying solely on urinalysis or dipstick testing to rule out ASB in pregnancy 1, 6
• Failing to distinguish between non-pathogenic bacteria and true pathogens that require treatment 7
• Delaying specimen processing, which can lead to false-negative results 5

Contrast with Non-Pregnant Adults

• The USPSTF recommends against screening for ASB in non-pregnant adults (D recommendation) 1, 4
• For non-pregnant women, including those with diabetes, screening for or treatment of ASB is not indicated as studies show no benefit 5
• The potential harms of overuse of antibiotics (adverse effects, bacterial resistance) outweigh benefits in non-pregnant populations 1

In conclusion, despite negative urinalysis results in asymptomatic pregnant women, urine culture remains the gold standard for detecting ASB and should be performed at least once during early pregnancy to prevent serious maternal and fetal complications.