Asymptomatic bacteriuria

Testing for asymptomatic bacteriuria in pregnancy allows treatment to be offered to reduce the risk of progression to pyelonephritis.

38.1 Background

Asymptomatic bacteriuria is the persistent bacterial colonisation of the urinary tract (usually by Escherichia coli) without symptoms. It is common in pregnancy.

38.1.1 Asymptomatic bacteriuria in Australia

• Incidence: Incidence of asymptomatic bacteriuria during pregnancy has been reported to be 2–10% in the United States Andrews & Gilstrap 1992, Sweet 1977 and 2–5% in the United Kingdom Little 1966, Campbell-Brown et al 1987, Foley et al 1987. In Australia, available estimates suggest that asymptomatic bacteriuria during pregnancy may be more common among Aboriginal and Torres Strait Islander women Hunt 2004, Bookallil et al 2005, Panaretto et al 2006.
• Risk factors: The prevalence of infection is most closely related to socioeconomic status and is similar in pregnant and non-pregnant women Turck et al 1962, Whalley 1967. Other factors associated with an increased risk of bacteriuria include a history of recurrent urinary tract infections, diabetes and anatomical abnormalities of the urinary tract (Golan et al 1989).

38.1.2 Risks associated with asymptomatic bacteriuria in pregnancy

While asymptomatic bacteriuria in non-pregnant women is usually benign, in pregnancy it increases the likelihood of kidney involvement (pyelonephritis), with an incidence of around 30% in affected women (Whalley 1967).

An association between untreated asymptomatic bacteriuria and low birth weight and preterm birth has also been suggested LeBlanc & McGanity 1964, Kincaid-Smith & Bullen 1965, Little 1966, Savage et al 1967. However, while a reduction in preterm birth and low birth weight is consistent with understanding of the role of infection in pregnancy complications Smaill 2007, Smaill & Vasquez 2007, other factors may be involved (eg other asymptomatic genitourinary infections) Campbell-Brown et al 1987, MacLean 2001 or links with socioeconomic status (Romero et al 1989). There may only be an association between asymptomatic bacteriuria and preterm birth if the infection progresses to pyelonephritis (Meis et al 1995).

38.2 Testing for asymptomatic bacteriuria

Universal testing for asymptomatic bacteriuria in pregnancy is recommended in the United Kingdom NICE 2008, the United States USPSTF 2004, Nicolle et al 2005, Canada (Nicolle 1994) and Scotland SIGN 2006, based on the effectiveness of available treatments and the reduced risk of pyelonephritis.

38.2.1 Benefits of testing

Testing for asymptomatic bacteriuria has been shown to reduce the number of women per 1,000 who experience pyelonephritis from 23.2 with no testing, to 16.2 with dipstick testing and 11.2 with urine culture (Rouse et al 1995). Both tests were found to be cost beneficial compared to no testing.

38.2.2 Effectiveness of interventions to treat asymptomatic bacteriuria

A Cochrane review found that antibiotic treatment compared with placebo or no treatment is effective in clearing asymptomatic bacteriuria (RR 0.25; 95%CI 0.14 to 0.48). The incidence of pyelonephritis was reduced by 75% (RR 0.23; 95%CI 0.13 to 0.41) (Smaill & Vasquez 2007).

38.2.3 Testing method 

Midstream urine culture is considered the standard for diagnosis of asymptomatic bacteriuria in pregnancy. 

Dipstick urinalysis of nitrites may be useful for excluding asymptomatic bacteriuria but is not accurate for diagnosis (Deville et al 2004). A meta-analysis (Deville et al 2004) and a small number of RCTs Teppa & Roberts 2005, Karabulut 2007, Eigbefoh et al 2008, Mignini et al 2009 have shown high specificity (89–100%) but low sensitivity (33–98%), with a mid range around 50%. Lower level studies have had similar results. 

38.2.4 Timing of the test

There is no consensus in the literature about the optimal timing and testing frequency for asymptomatic bacteriuria. However, in a prospective study (n=3,254), a single urine specimen obtained between 12 and 16 weeks gestation identified 80% of women who ultimately had asymptomatic bacteriuria (Stenqvist et al 1989).

38.2.5 Testing in rural and remote areas

Due to difficulties in transporting specimens to laboratories, dipstick tests are commonly used in remote areas to ‘rule out’ asymptomatic bacteriuria, with samples from women testing positive then sent for culture to confirm infection. While urine culture is the preferred method of testing, this process has been found to be cost effective (Rouse et al 1995). However, factors specific to conditions in rural and remote Australia (eg high humidity and ambient temperatures) may contribute to under diagnosis and overtreatment. Considerations in testing for asymptomatic bacteriuria in these areas include (Bookallil et al 2005):

• whether specimens can be provided to pathology services within the timeframe in which they can still be cultured (ideally within 24 hours)
• the availability of appropriate storage facilities for dipstick tests
• the consequences of treating all women with a positive dipstick result given the high rate of false positives and the risk of increased resistance to antibiotics associated with over-prescribing
• recall systems for women with a positive result on culture.

38.2.6 Repeat testing

Although most guidelines recommend a single urine culture at the first antenatal visit, two prospective studies have concluded that urine should be cultured in each trimester of pregnancy to improve the detection rate of asymptomatic bacteriuria McIsaac et al 2005, Tugrul et al 2005. There has been no prospective evaluation of repeated testing during pregnancy (Schnarr & Smaill 2008).

38.3 Practice summary: testing for asymptomatic bacteriuria

When

Early in antenatal care.

Who

• Midwife
• GP
• obstetrician
• Aboriginal and Torres Strait Islander health worker
• multicultural health worker.

What

• Discuss testing for asymptomatic bacteriuria
  Explain that identifying urinary tract infection enables women to be treated with antibiotics and avoids the risk of complications.
• Document and follow-up
  Note the results of testing in the woman’s record and have a follow-up system in place so that appropriate treatment is provided if a woman is found to have bacteriuria.

38.4 Resources

• Widmer M, Lopez I, Gülmezoglu A et al (2015) Duration of treatment for asymptomatic bacteriuria during pregnancy. Cochrane Database of Systematic Reviews 2015, Issue 11. Art. No.: CD000491.