Purpose: Undiagnosed urinary tract infections (UTIs) in pregnancy are associated with adverse perinatal outcome. Urine microbiology cultures reported as 'mixed bacterial growth' (MBG) frequently present a diagnostic dilemma for healthcare providers. We investigated external factors contributing to elevated rates of (MBG) within a large tertiary maternity centre in London, UK, and assessed the efficacy of health service interventions to mitigate these.
Description: This prospective, observational study of asymptomatic pregnant women attending their first prenatal clinic appointment aimed to establish (i) the prevalence of MBG in routine prenatal urine microbiology cultures, (ii) the association between urine cultures and the duration to laboratory processing and (iii) ways in which MBG may be reduced in pregnancy. Specifically we assessed the impact of patient-clinician interaction and that of an education package on optimal urine sampling technique.
Assessment: Among 212 women observed over 6 weeks, the negative, positive and MBG urine culture rates were 66%, 10% and 2% respectively. Shorter duration from urine sample collection to laboratory arrival correlated with higher rates of negative cultures. Urine samples arriving in the laboratory within 3 hours of collection were most likely to be reported as culture negative (74%), and were least likely to be reported as MBG (21%) or culture positive (6%), compared to samples arriving > 6 hours (71%, 14% and 14% respectively; P < 0.001). A midwifery education package effectively reduced rates of MBG (37% pre-intervention vs 19% post-intervention, RR 0.70, 95% CI 0.55 to 0.89). Women who did not receive verbal instructions prior to providing their sample had 5-fold higher rates of MBG (P < 0.001).
Conclusion: As many as 24% of prenatal urine screening cultures are reported as MBG. Patient-midwife interaction before urine sample collection and rapid transfer of urine samples to the laboratory within 3 hours reduces the rate of MBG in prenatal urine cultures. Reinforcing this message through education may improve accuracy of test results.
What is already known on this subject? Undiagnosed urinary tract infections (UTIs) in pregnancy are associated with adverse perinatal outcome due to elevated rates of preterm birth, maternal sepsis and low birthweight. The prevalence of mixed bacterial growth in prenatal urine cultures is poorly reported, and is of uncertain clinical significance. Mixed bacterial growth may reflect either an undiagnosed urinary tract infection (UTI) or contamination by organisms colonising the peri-urethra.
What this study adds? As many as one-quarter of prenatal urine cultures in a healthy pregnant population are reported as mixed bacterial growth. This inevitably adds to physician workload and patient anxiety. Key policies to reduce rates of mixed bacterial growth include ensuring urine samples reach microbiology laboratories in a timely manner (< 3 h), provision of clear verbal instructions to women on optimal urine sampling technique, and regular simple education packages for clinicians to reinforce this message.
Keywords: Asymptomatic Bacteriuria; Mixed growth; Pregnancy; Urinary Tract Infection; Urine Culture.
© 2023. The Author(s).