asymptomatic bacteriuria of pregnancy

Last edited 07/2020 and last reviewed 11/2023

  • asymptomatic bacteriuria is significant if there are 10 ^5 organisms per ml of cultured urine. If this condition is not treated then women with asymptomatic bacteriuria are 4 times more likely to develop a symptomatic urinary tract infection than women without this condition

  • the definition of asymptomatic bacteriuria (ASB) was restated in 2019 by the Infectious Diseases Society of America (IDSA) as being>=10^5 CFU per ml or>=10^8 CFU per litre of a voided urine sample in people without indwelling catheters or signs or symptoms of UTI
    • IDSA recommended that in women, two consecutive samples should be collected within a two-week interval in order to confirm the presence of ASB, noting that between 10% and 60% of women (varying with population characteristics) have confirmed ASB on repeat testing following a first positive result (3)
    • the Scottish Intercollegiate Guidelines Network (SIGN) also recommends that ASB should be confirmed with a second urine culture (5) however, this is not current practice in England

  • thought that hormonal and physiological changes in pregnancy (e.g. compression of bladder, ureters and kidneys by the expanding uterus) can increase urinary stasis, making pregnant women susceptible to developing ASB

  • women with untreated ASB are thought to be at greater risk of developing pyelonephritis
    • pregnant women with pyelonephritis are at increased risk of maternal and foetal mortality and morbidity, including maternal fever, acute respiratory distress, acute renal failure, stillbirth and preterm birth.  Acute pyelonephritis is also associated with anaemia and pre-eclampsia

  • the usual organism responsible is E. coli (over 90%). Other possible organisms include Proteus, Klebsiella, Staphylococci and Pseudomonas
  • this condition should be treated because of implications to the mother (possible development of urinary tract infection) and to the pregnancy - during pregnancy asymptomatic bacteriuria is associated with premature delivery and low birthweight (1)
  • urological follow-up is usually only for women who develop acute recurrent symptomatic infections, or, in women in whom the bacteriuria persists despite treatment, or, in those women who develop recurrence post-partum

  • antibiotic treatment
    • prescribe antibiotics empirically
      • refer to local guidelines
      • if local guidelines are uavailable, suitable first-line antibiotics in pregnancy are (in order of preference) (2,3):
        • however see also notes below about use of trimethoprim and nitrofurantoin in pregnancy
          • nitrofurantoin 100 mg (modified-release) twice daily, for 7 days
            • OR
          • amoxicillin 500 mg three times a day for 7 days
            • OR
          • cefalexin 500 mg twice a day for 7 days

    • NICE state (3):
      • choose from nitrofurantoin 1,2, amoxicillin or cefalexin based on recent culture and susceptibility results
      • nitrofurantoin should be avoided at term because of risk of neonatal haemolysis

      1 avoid at term in pregnancy; may produce neonatal haemolysis (BNF, August 2018)

      2 may be used with caution if eGFR 30-44 ml/minute to treat uncomplicated lower UTI caused by suspected or proven multidrug resistant bacteria and only if potential benefit outweighs risk

NICE state that pregnant women should be offered routine screening for asymptomatic bacteriuria by midstream urine culture early in pregnancy. Identification and treatment of asymptomatic bacteriuria reduces the risk of preterm birth (4)

Notes:

Quinolones and tetracyclines should be avoided as empirical treatments. There are concerns about use of sulphonamides and trimethoprim in pregnancy:

  • trimethoprim - theoretical teratogenic risk (folate antagonist); manufacturers advise avoid; BNF states first trimester is the trimester of risk. Current NICE guidance states avoid
    • trimethoprim 200 mg twice daily, for 7 days (off-label use) (2)
      • if the woman is not folate deficient or taking a folate antagonist, and has not been treated with trimethoprim in the past year
        • give folic acid 5 mg daily if it is the first trimester of pregnancy
  • sulphonamides - neonatal haemolysis and methaemaglobinaemia; BNF states third trimester is trimester of risk
  • tetracyclines - avoid use during pregnancy; effects on skeletal development in animal studies if used during first trimester; dental discoloration and maternal hepatoxicity may occur if used during second or third trimesters
  • quinolones - should be avoided during pregnancy; arthropathy in animal studies

Nitrofurantoin should not be used at term because of the risk of neonatal haemolysis - during the last few weeks may precipitate haemolytic anaemia due to glucose-6-phosphate dehydrogenase deficiency in the newborn

  • BNF states third trimester is the trimester of risk associated with nitrofurantoin use

Consult local microbiology advice and latest edition of BNF for up-to-date guidance before definitive treatment.

Reference:

Related pages:

asymptomatic bacteriuria (ASB)

uncomplicated UTI (urinary tract infection) in pregnancy