Bacteriology and clinical outcomes of urine mixed bacterial growth in pregnancy

Introduction and hypothesis: The objective was to analyse the risk of significant bacteriuria in repeat urine cultures from pregnant women, following initial mixed bacterial results. Methods: This retrospective study examined maternal characteristics and clinical features of women who repeated urine cultures due to previous mixed cultures results. Results: Of 262 women included, 80 (30.5%) had negative cultures and 125 (47.7%) had mixed bacterial growth in their repeat cultures. Positive results (≥10⁴ CFU/ml of a urinary pathogen) were obtained for 57 women (21.8% [95% CI 17.1–27.0]). For 37 (14.1%), the repeat specimen grew 10⁴–10⁵ CFU/ml of microorganisms; whereas for 20 women (7.6% [95% CI 4.9–11.3]), it grew ≥10⁵ CFU/ml. Among women with positive (>10⁴ CFU/ml) compared with those with negative or mixed growth, rates of urinary symptoms were higher (38.6% vs 23.4%, p=0.028), abnormal dipstick results (49.1% vs 21.0%, p<0.001) and hydronephrosis, as demonstrated by renal ultrasound (12.3% vs 2.0, p=0.003). In a multivariate logistic regression analysis, hydronephrosis was associated with the occurrence of a positive repeat culture (aOR = 10.65, 95% CI 2.07–54.90). The sensitivity and specificity for predicting a repeat urine culture with ≥10⁵ CFU/ml were 12.9% and 94.3% respectively, for urinary symptoms; and 19.7% and 97.4% respectively, for abnormal dipstick results. Conclusions: Mixed bacterial growth might represent a true urinary tract infection in a considerable proportion of women who are symptomatic and have an abnormal dipstick urinalysis.