Urothelial cells may indicate underlying bacteriuria in pregnancy at term: a comparative study

BACKGROUND: Urinary tract infection is common in pregnancy. Urine is sampled from by mid-stream collection (MSU). If epithelial cells are detected, contamination by vulvo-vagial skin and skin bacteria is assumed. Outside pregnancy, catheter specimen urine (CSU) is considered less susceptible to contamination. We compared MSU and CSU methods in term pregnancy to test these assumptions. METHODS: Healthy pregnant women at term gestation (n = 32, median gestation 38 + 6 weeks, IQR 37 + 6–39 + 2) undergoing elective caesarean section provided a MSU and CSU for paired comparison that were each analysed for bacterial growth and bladder distress by fresh microscopy, sediment culture and immunofluorescent staining. Participants completed a detailed questionnaire on lower urinary tract symptoms. Epithelial cells found in urine were tested for urothelial origin by immunofluorescent staining of Uroplakin III (UP3), a urothelial cell surface glycoprotein. Urothelial cells with closely associated bacteria, or “clue cells”, were also counted. Wilcoxons signed rank test was used for paired analysis. RESULTS: Women reported multiple lower urinary tract symptoms (median 3, IQR 0–8). MSU had higher white blood cell counts (median 67 vs 46, z = 2.75, p = 0.005) and epithelial cell counts (median 41 vs 22, z = 2.57, p = 0.009) on fresh microscopy. The proportion of UP3+ cells was not different (0.920 vs 0.935, z = 0.08, p = 0.95), however MSU had a higher proportion of clue cells (0.978 vs 0.772, z = 3.17, p = 0.001). MSU had more bacterial growth on sediment culture compared to CSU specimens (median 8088 total cfu/ml vs 0, z = 4.86, p = 0.001). Despite this, routine laboratory cultures reported a negative screening culture for 40.6% of MSU specimens. CONCLUSION: Our findings have implications for the correct interpretation of MSU findings in term pregnancy. We observed that MSU samples had greater bacterial growth and variety when compared to CSU samples. The majority of epithelial cells in both MSU and CSU samples were urothelial in origin, implying no difference in contamination. MSU samples had a higher proportion of clue cells to UP3+ cells, indicating a greater sensitivity to bacterial invasion. Urinary epithelial cells should not be disregarded as contamination, instead alerting us to underlying bacterial activity. ELECTRONIC SUPPLEMENTARY MATERIAL: The online version of this article (10.1186/s12884-017-1606-z) contains supplementary material, which is available to authorized users.