Asymptomatic bacteriuria is common in young women, but little is known about its pathogenesis, natural history, risk factors, and temporal association with symptomatic urinary tract infection.

We prospectively evaluated 796 sexually active, nonpregnant women from 18 through 40 years of age over a period of six months for the occurrence of asymptomatic bacteriuria (defined as at least 10 ⁵ colony-forming units of urinary tract pathogens per milliliter). The women were patients at either a university student health center or a health maintenance organization (HMO). Periodic urine cultures were taken, daily diaries were kept, and regularly scheduled interviews were performed. Escherichia coli strains were tested for hemolysin, the papG genotype, and the ribosomal RNA type.

The prevalence of asymptomatic bacteriuria (the proportion of urine cultures with bacteriuria in asymptomatic women) was 5 percent (95 percent confidence interval, 4 percent to 6 percent) among women in the university group and 6 percent (95 percent confidence interval, 5 percent to 8 percent) among women in the HMO group. Persistent asymptomatic bacteriuria with the same E. coli strain was rare. Symptomatic urinary tract infection developed within one week after 8 percent of occasions on which a culture showed asymptomatic bacteriuria, as compared with 1 percent of occasions when asymptomatic bacteriuria was not found (P<0.001). Asymptomatic bacteriuria was associated with the same risk factors as for symptomatic urinary tract infection, particularly the use of a diaphragm plus spermicide and sexual intercourse.

Asymptomatic bacteriuria in young women is common but rarely persists. It is a strong predictor of subsequent symptomatic urinary tract infection.