Asymptomatic bacteriuria is defined as the presence of bacteria in the urine of a person without signs or symptoms of a urinary tract infection.¹ Among the general adult population, women (across all ages) have the highest prevalence of asymptomatic bacteriuria, although rates increase with age among both men and women.² The reported prevalence of asymptomatic bacteriuria ranges from 1% to 6% among premenopausal women to 22% among women older than age 90 years.^3-5 Asymptomatic bacteriuria is present in an estimated 2% to 10% of pregnant women.^3,6 The condition is considered rare in men.⁵

During pregnancy, physiologic changes affecting the urinary tract are believed to increase the risk of asymptomatic bacteriuria and symptomatic urinary tract infections, including pyelonephritis (a urinary tract infection in which one or both kidneys become infected).⁷ Pregnant women have a higher rate of hospitalization for pyelonephritis than nonpregnant women. Pyelonephritis is associated with perinatal complications including septicemia, respiratory distress, low birth weight, and spontaneous preterm birth.^8,9

The presence of asymptomatic bacteriuria has not been shown to increase the risk of negative health outcomes among nonpregnant persons.^3,10

Pregnant Persons

The USPSTF concludes with moderate certainty that screening for and treatment of asymptomatic bacteriuria in pregnant persons has moderate net benefit in reducing perinatal complications (Table). There is adequate evidence that pyelonephritis in pregnancy is associated with negative maternal outcomes and that treatment of screen-detected asymptomatic bacteriuria can reduce the incidence of pyelonephritis in pregnant persons. However, more recent evidence shows that the incidence of pyelonephritis among pregnant women with untreated asymptomatic bacteriuria has been low in recent decades. When direct evidence is limited, absent, or restricted to select populations or clinical scenarios, the USPSTF may place conceptual upper or lower bounds on the magnitude of benefit or harms. Therefore, the USPSTF bound the benefits of screening for asymptomatic bacteriuria in pregnant persons as no greater than moderate.

The USPSTF found adequate evidence of harms associated with treatment of asymptomatic bacteriuria, including side effects of antibiotic treatment. It also considered the potential effects of changes in the microbiome resulting from antibiotic use. Therefore, the USPSTF bound the harms of screening for asymptomatic bacteriuria in pregnant persons to be at least small.

The USPSTF found inadequate evidence on the harms of screening for asymptomatic bacteriuria in pregnant persons.

Nonpregnant Adults

The USPSTF concludes with moderate certainty that screening for and treatment of asymptomatic bacteriuria in nonpregnant adults has no net benefit (Table). There is adequate evidence that treatment of screen-detected asymptomatic bacteriuria in nonpregnant adults has no benefit. Based on the harms associated with antibiotic use, the USPSTF found adequate evidence to bound the harms of treatment of screen-detected asymptomatic bacteriuria in nonpregnant adults as at least small.

Patient Population Under Consideration

This recommendation applies to adults age 18 years and older and pregnant persons of any age without signs and symptoms of a urinary tract infection. It does not apply to persons who are hospitalized; persons who reside in an institution (e.g., a nursing home); persons who have chronic medical or urinary tract conditions, such as end-stage renal disease; persons who are transplant recipients; and persons with indwelling urinary catheters, urinary stents, or spinal cord injuries.

Definition of Asymptomatic Bacteriuria

Asymptomatic bacteriuria occurs when the urinary tract is colonized with significant amounts of pathogenic bacteria, primarily from the gastrointestinal tract, in the absence of symptoms or signs of a urinary tract infection. The most common pathogen is Escherichia coli, although other bacteria such as Klebsiella, Proteus mirabilis, and group B streptococcus can be involved.^5,11

Assessment of Risk

The risk of developing asymptomatic bacteriuria varies by age, sex, and pregnancy status. Because of the location and length of the female urethra, women are predisposed to infections of the urinary tract, including asymptomatic bacteriuria.^6,11 Physiologic changes in both pregnant and older women increase the risk of asymptomatic bacteriuria and a urinary tract infection.^7,12 In general, men are at low risk of developing asymptomatic bacteriuria, although rates increase with older age.¹² Persons with diabetes are also at increased risk of developing asymptomatic bacteriuria.⁵

Screening Tests

Screening for asymptomatic bacteriuria during pregnancy is done with a urine culture at 12 to 16 weeks of gestation or at the first prenatal visit. Urine culture is currently recommended for screening in pregnancy and is the established method for diagnosis.² A culture obtained using a midstream, clean-catch urine sample with greater than 100,000 CFU/mL of a single uropathogen is considered a positive test result.³ Greater than 10,000 CFU/mL of group B streptococcus is an indicator of vaginal colonization and is commonly used as the threshold for treatment of infection in pregnancy.¹³

Screening Intervals

In general, screening is performed once at the first prenatal visit per clinical guidelines. However, there is little evidence on the optimal timing and frequency of screening for asymptomatic bacteriuria in pregnancy.²

Treatment or Interventions

Pregnant persons with asymptomatic bacteriuria usually receive antibiotic therapy, based on urine culture results, and followup monitoring. The choice of antibacterial regimen for treatment of asymptomatic bacteriuria during pregnancy is based on its safety in pregnancy and patterns of antimicrobial resistance in the particular setting.

In this update, the USPSTF continues to recommend screening for asymptomatic bacteriuria in pregnant persons with urine culture and against screening in nonpregnant adults. The USPSTF changed the grade for pregnant persons from an “A” to a “B” based on a change in the understanding and applicability of the previous evidence and newer evidence that shows a significantly lower risk of pyelonephritis than found in previous reviews, leading to a reduction in the magnitude of benefit. In addition, there are newer concerns about antibiotic use, such as antimicrobial resistance and changes to the microbiome (not addressed in current studies), leading to an increase in the magnitude of potential harms.

Since 1996, the USPSTF has maintained an “A” recommendation for one-time screening for asymptomatic bacteriuria in pregnant persons with urine culture between 12 and 16 weeks of gestation. The original 1996 recommendation was reaffirmed in 2004 and again in 2008.^14-16 In 1996, the USPSTF found that there was insufficient evidence to recommend for or against screening in older adult women or women with diabetes and, in a separate recommendation, that screening was not recommended in other asymptomatic adults or older adults who reside in an institution.14 In 2004, these recommendations were combined into one recommendation against screening, which was subsequently reaffirmed in 2008.^15,16

Scope of Review

The USPSTF commissioned a systematic evidence review to evaluate the evidence on the potential benefits and harms of screening for and treatment of asymptomatic bacteriuria in community-dwelling adults, including pregnant persons.^2,9 This review was used to update the 2008 USPSTF recommendation statement.¹⁶

Evidence on Benefits of Screening and Treatment

Pregnant Persons

Two retrospective, observational cohort studies conducted in Spain and Turkey between 1987 and 1999^17,18 (n=5,289) examined outcomes in screened and unscreened pregnant women. Both studies included patients who were screened at the first prenatal visit with urine culture and treated upon detection of asymptomatic bacteriuria. In both studies, few cases of pyelonephritis developed in women in either cohort. Only one of the studies reported additional outcomes, including infant birth weight, prematurity, intrauterine death, and intrauterine growth restriction, although the study was not adequately powered to detect differences in these outcomes.^2,17

Twelve trials of pregnant women (n=2,377) screened for asymptomatic bacteriuria and randomized to either a treatment or control condition (placebo or no treatment) were included in the review.^19-30 Most studies were conducted in hospital-based obstetrics-gynecology clinics. Seven studies reported screening at the first prenatal visit, two studies reported the specific gestational age at which screening was performed, and three studies did not report the timing of screening.² All but two studies were published in the 1960s or 1970s, with the most recent studies published in 1987 and 2015.^24,30 In the older studies, there was sparse reporting on many patient characteristics such as age and race/ethnicity. In addition, treatment regimens for screen-detected asymptomatic bacteriuria varied according to the medication used, timing, duration, and dosage. Antibiotics were used in all studies except one, although several antibiotics tested in the trials are no longer recommended for treatment of urinary tract infections in pregnancy.² Rates of pyelonephritis in the control groups were considerably higher in the 10 older studies than in the two more recent ones (7% to 36% vs. 2.2% and 2.5%, respectively).

Control groups had higher rates of pyelonephritis than treatment groups in all but one of the studies. Pooled analysis demonstrated a 76% reduction in pyelonephritis among the intervention groups (pooled relative risk [RR], 0.24 [95% confidence interval (CI), 0.14 to 0.40]; 12 studies; n=2,068).² A sensitivity analysis that removed studies deemed to have high risk of bias demonstrated a similar risk reduction (pooled RR, 0.19 [95% CI, 0.11 to 0.34]; 7 studies; n=1,184).²

Seven treatment studies reported on the incidence of low birth weight. The pooled estimate found statistically significant reductions in the incidence of infants with low birth weight (pooled RR, 0.64 [95% CI, 0.46 to 0.90]; 7 studies; n=1,522).² Preterm birth and perinatal mortality were reported in three and six studies, respectively. For both outcomes, results were mixed and pooled estimates did not demonstrate statistical significance.

Nonpregnant Adults

No studies were identified that directly evaluated the benefits of screening for asymptomatic bacteriuria in the general adult population. Five trials (n=777)^31-35 addressed the benefits of treatment of screen-detected asymptomatic bacteriuria in general adult populations. All five studies included participants who had two consecutive positive screening urine cultures using a midstream, clean-catch urine sample and using a cutoff of greater than 100,000 CFU/mL. Across all studies, 84% to 100% of participants were women. One study included women ages 25 to 65 years without diabetes, one study included only women with diabetes (mean age, 55 years), and three studies included only older patients living in independent living facilities. In general, characteristics of study participants were sparsely reported across studies, with none reporting on race/ethnicity. Treatment varied across trials, ranging from a single dose to 3 months of antibiotics. No study found a difference in the rates of symptomatic infections or mortality between treated and untreated groups.²

Evidence on Harms of Screening and Treatment

Pregnant Persons

One cohort study (n=186) comparing screened and unscreened pregnant women reported on potential harms (congenital abnormalities) associated with the screening program, with no meaningful differences reported.¹⁷

Seven studies reported on harms associated with treatment of screen-detected asymptomatic bacteriuria.^{19,21-23,27,28,30} Five studies (n=961) reported on congenital malformations. All but one study reported fewer cases in the intervention group, although the number of cases was small and pooled estimates were not statistically significant.² Other infant or fetal harms, such as jaundice (two studies), respiratory distress (one study), and neonatal sepsis (one study) were sparsely reported and had low event rates, which limited comparisons.² Adverse reactions to medications were reported in two studies; vaginitis and diarrhea were associated with ampicillin and nausea and rashes were reported with use of nalidixic acid and nitrofurantoin.² Complications of pregnancy and delivery (such as third-trimester hemorrhage, premature rupture of the membranes, nonspontaneous onset of labor, or cesarean delivery before onset of labor) were inconsistently and sparsely reported, limiting any conclusions.²

Nonpregnant Adults

Two studies of treatment in nonpregnant women^33,35 and two studies in older adults^31,34 reported on rates of adverse events associated with treatment of asymptomatic bacteriuria. Overall, harms were not reported consistently, which limited the conclusions that could be drawn from the current evidence base.

No studies were identified that addressed the harms of screening for asymptomatic bacteriuria in nonpregnant adults.

How Does Evidence Fit With Biological Understanding?

The relationship between asymptomatic bacteriuria and adverse pregnancy outcomes is related to a combination of factors. Women are at increased risk of urinary tract infections, including asymptomatic bacteriuria, because of the anatomic placement of the urethra.² Conditions such as increased blood glucose levels and urinary stasis (in which the bladder is unable to completely empty) can lead to symptomatic urinary tract infections and pyelonephritis. Pregnancy further increases the risk because of changes in urine pH, bladder compression, and urethral dilation. Pyelonephritis in pregnancy has been associated with worse pregnancy outcomes.^9,36,37 Screening for and treatment of asymptomatic bacteriuria in pregnant persons could prevent cases of pyelonephritis and associated negative pregnancy outcomes.

Antibiotics are the mainstay treatment for urinary tract infections, but there are consequences to their use. The use of antibiotics is known to lead to antimicrobial resistance. In addition, there is emerging evidence that bacterial colonization of the gastrointestinal and genitourinary tracts plays a protective role. Antibiotic use can disrupt these effects.

The USPSTF identified several gaps in the evidence where more research is needed:

• There were few studies that examined asymptomatic bacteriuria and risk of serious outcomes (i.e., pyelonephritis or urosepsis) in modern pregnant populations. Epidemiologic evidence suggests that the prevalence of asymptomatic bacteriuria has been low in recent decades, and many antibiotics used in older studies are no longer recommended for use in pregnancy. More observational studies examining this would help improve the applicability of the evidence base.
• Trials, observational studies, and natural experiments in settings where asymptomatic bacteriuria screening and treatment are not the standard of care or where guidelines are changing would be useful in assessing benefits and harms.
• Newer understandings of the human microbiome suggest that bacterial colonization may play a protective role in both mothers and babies. For pregnant and nonpregnant populations, research is needed to better understand the microbiology of a healthy urinary tract and the natural history of asymptomatic bacteriuria.
• The role of current patterns of antibiotic use in the epidemiology of asymptomatic bacteriuria is unclear. Antibiotic use increases the risk of antimicrobial resistance and can change the microbiome. More research is needed to better understand potential harms of treatment and the effects of antibiotic use on newborn, child, and longer-term health.

The American College of Obstetricians and Gynecologists and the American Academy of Pediatrics have no specific recommendation on screening for asymptomatic bacteriuria in pregnant women. If urine culture is performed, however, both organizations recommend that clinicians treat screen-detected asymptomatic bacteriuria and test that is cured. In 2008, the American Academy of Family Physicians recommended screening in pregnant women at 12 to 16 weeks of gestation or at the first prenatal visit, whichever comes first.³⁸

The Infectious Diseases Society of America recommends screening for asymptomatic bacteriuria in pregnant women and treatment for those who screen positive.³⁹ The Canadian Task Force on Preventive Health Care recommends screening in pregnant women with urine culture once during the first trimester, although this was issued as a “weak” recommendation and the quality of evidence was considered low.⁴⁰

The American College of Obstetricians and Gynecologists, the American Academy of Pediatrics, the American Academy of Family Physicians, and the United Kingdom’s National Institute for Health and Care Excellence all recommend against screening for and treatment of asymptomatic bacteriuria in nonpregnant adults.^38,41-43

1. Foxman B. Urinary tract infection syndromes: occurrence, recurrence, bacteriology, risk factors, and disease burden. Infect Dis Clin North Am. 2014;28(1):1-13.
2. Henderson JT, Webber EM, Bean SI. Screening for Asymptomatic Bacteriuria in Adults: An Updated Systematic Review for the U.S. Preventive Services Task Force.Evidence Synthesis No. 183. AHRQ Publication No. 19-05252-EF-1. Rockville, MD: Agency for Healthcare Research and Quality; 2019.
3. Nicolle LE, Bradley S, Colgan R, et al; Infectious Diseases Society of America; American Society of Nephrology; American Geriatric Society. Infectious Diseases Society of America guidelines for the diagnosis and treatment of asymptomatic bacteriuria in adults. Clin Infect Dis. 2005;40(5):643-654.
4. Hooton TM, Scholes D, Stapleton AE, et al. A prospective study of asymptomatic bacteriuria in sexually active young women. N Engl J Med. 2000;343(14):992-997.
5. Ferroni M, Taylor AK. Asymptomatic bacteriuria in noncatheterized adults. Urol Clin North Am. 2015;42(4):537-545.
6. Schnarr J, Smaill F. Asymptomatic bacteriuria and symptomatic urinary tract infections in pregnancy. Eur J Clin Invest. 2008;38(Suppl 2):50-57.
7. Macejko AM, Schaeffer AJ. Asymptomatic bacteriuria and symptomatic urinary tract infections during pregnancy. Urol Clin North Am. 2007;34(1):35-42.
8. Hill JB, Sheffield JS, McIntire DD, Wendel GD Jr. Acute pyelonephritis in pregnancy. Obstet Gynecol. 2005;105(1):18-23.
9. Wing DA, Fassett MJ, Getahun D. Acute pyelonephritis in pregnancy: an 18-year retrospective analysis. Am J Obstet Gynecol. 2014;210(3):219.e1-6.
10. Dull RB, Friedman SK, Risoldi ZM, Rice EC, Starlin RC, Destache CJ. Antimicrobial treatment of asymptomatic bacteriuria in noncatheterized adults: a systematic review. Pharmacotherapy. 2014;34(9):941-960.
11. Glaser AP, Schaeffer AJ. Urinary tract infection and bacteriuria in pregnancy. Urol Clin North Am. 2015;42(4):547-560.
12. Juthani-Mehta M. Asymptomatic bacteriuria and urinary tract infection in older adults. Clin Geriatr Med. 2007;23(3):585-594, vii.
13. Verani J, McGee L, Schrag S; Division of Bacterial Diseases, National Center for Immunization and Respiratory Diseases, Centers for Disease Control and Prevention (CDC). Prevention of perinatal group B streptococcal disease--revised guidelines from CDC, 2010. MMWR Recomm Rep. 2010;59(RR-10):1-36.
14. U.S. Preventive Services Task Force. Guide to Clinical Preventive Services: Report of the U.S. Preventive Services Task Force. Baltimore, MD: Williams & Wilkins; 1996.
15. U.S. Preventive Services Task Force. Guide to Clinical Preventive Services: Report of the U.S. Preventive Services Task Force. Rockville, MD: Agency for Healthcare Research and Quality; 2007.
16. U.S. Preventive Services Task Force. Screening for asymptomatic bacteriuria in adults: U.S. Preventive Services Task Force reaffirmation recommendation statement. Ann Intern Med. 2008;149(1):43-47.
17. Uncu Y, Uncu G, Esmer A, Bilgel N. Should asymptomatic bacteriuria be screened in pregnancy? Clin Exp Obstet Gynecol. 2002;29(4):281-285.
18. Gratacós E, Torres PJ, Vila J, Alonso PL, Cararach V. Screening and treatment of asymptomatic bacteriuria in pregnancy prevent pyelonephritis. J Infect Dis. 1994;169(6):1390-1392.
19. Wren BG. Subclinical renal infection and prematurity. Med J Aust. 1969;2(12):596-600.
20. Williams GL, Campbell H, Davies KJ. Urinary concentrating ability in women with asymptomatic bacteriuria in pregnancy. Br Med J. 1969;3(5664):212-215.
21. Furness ET, McDonald PJ, Beasley NV. Urinary antiseptics in asymptomatic bacteriuria of pregnancy. N Z Med J. 1975;81(539):417-419.
22. Elder HA, Santamarina BA, Smith S, Kass EH. The natural history of asymptomatic bacteriuria during pregnancy: the effect of tetracycline on the clinical course and the outcome of pregnancy. Am J Obstet Gynecol. 1971;111(3):441-462.
23. Little PJ. The incidence of urinary infection in 5000 pregnant women. Lancet. 1966;2(7470):925-928.
24. Foley ME, Farquharson R, Stronge JM. Is screening for bacteriuria in pregnancy worthwhile? Br Med J (Clin Res Ed). 1987;295(6592):270.
25. Pathak UN, Tang K, Williams LL, Stuart KL. Bacteriuria of pregnancy: results of treatment. J Infect Dis. 1969;120(1):91-103.
26. Kincaid-Smith P, Bullen M. Bacteriuria in pregnancy. Lancet. 1965;1(7382):395-399.
27. Savage W, Hajj S, Kass E. Demographic and prognostic characteristics of bacteriuria in pregnancy. Medicine (Baltimore). 1967;46(5):385-407.
28. Gold E, Traub F, Daichman I, Terris M. Asymptomatic bacteriuria during pregnancy. Obstet Gynecol. 1966;27(2):206-209.
29. Brumfitt W. The effects of bacteriuria in pregnancy on maternal and fetal health. Kidney Int Suppl. 1975;4:S113-119.
30. Kazemier BM, Koningstein FN, Schneeberger C, et al. Maternal and neonatal consequences of treated and untreated asymptomatic bacteriuria in pregnancy: a prospective cohort study with an embedded randomised controlled trial. Lancet Infect Dis. 2015;15(11):1324-1333.
31. Giamarellou H, Dontas AS, Zorbas P, Staszewska-Pistoni M, Xirouchaki E, Petrikkos G. Asymptomatic bacteriuria in freely voiding elderly subjects. Long-term continuous vs pulse treatment with ofloxacin. Clin Drug Investig. 1998;15(3):187-195.
32. Abrutyn E, Mossey J, Berlin JA, et al. Does asymptomatic bacteriuria predict mortality and does antimicrobial treatment reduce mortality in elderly ambulatory women? Ann Intern Med. 1994;120(10):827-833.
33. Asscher AW, Sussman M, Waters WE, et al. The clinical significance of asymptomatic bacteriuria in the nonpregnant woman. J Infect Dis. 1969;120(1):17-26.
34. Boscia JA, Kobasa WD, Knight RA, Abrutyn E, Levison ME, Kaye D. Therapy vs no therapy for bacteriuria in elderly ambulatory nonhospitalized women. JAMA. 1987;257(8):1067-1071.
35. Harding GK, Zhanel GG, Nicolle LE, Cheang M; Manitoba Diabetes Urinary Tract Infection Study Group. Antimicrobial treatment in diabetic women with asymptomatic bacteriuria. N Engl J Med. 2002;347(20):1576-1583.
36. Smaill FM, Vazquez JC. Antibiotics for asymptomatic bacteriuria in pregnancy. Cochrane Database Syst Rev. 2015;(8):CD000490.
37. Farkash E, Weintraub AY, Sergienko R, Wiznitzer A, Zlotnik A, Sheiner E. Acute antepartum pyelonephritis in pregnancy: a critical analysis of risk factors and outcomes. Eur J Obstet Gynecol Reprod Biol.2012;162(1):24-27.
38. American Academy of Family Physicians. Clinical Preventive Service Recommendation: Asymptomatic Bacteriuria. https://www.aafp.org/patient-care/clinical-recommendations/all/bacteriuria.html. Accessed April 3, 2019.
39. Nicolle LE, Gupta K, Bradley SF, et al. Clinical practice guideline for the management of asymptomatic bacteriuria: 2019 update by the Infectious Diseases Society of America. Clin Infect Dis. 2019 Mar 21.
40. Canadian Task Force on Preventive Health Care. Asymptomatic Bacteriuria in Pregnancy (2018). https://canadiantaskforce.ca/guidelines/published-guidelines/asymptomatic-bacteriuria/. Accessed April 3, 2019.
41. National Institute for Health and Clinical Excellence. Urinary tract infection (lower): antimicrobial prescribing. 2018. https://www.nice.org.uk/guidance/ng109. Accessed April 3, 2019.
42. American College of Obstetricians and Gynecologists. ACOG Practice Bulletin No. 91: treatment of urinary tract infections in nonpregnant women. Obstet Gynecol. 2008;111(3):785-794.
43. American Academy of Pediatrics and American College of Obstetricians and Gynecologists. Guidelines for Perinatal Care. 7th ed. Itasca, IL: American Academy of Pediatrics; 2012.