Investigators assembled a cohort of 224 women attending college and followed the

Question

Investigators assembled a cohort of 224 women attending college and followed them up for 6 months. They had to be free of urinary tract infection (UTI) at baseline. The occurrence of UTI for the first and second time were ascertained using molecular epidemiologic methods such as genotyping of E. coli strains. These young women contributed to 1,140 months of exposure. The investigators looked among other exposures into a previously reported risk factor: the use of certain contraceptive methods. College female students on oral contraceptives contributed to 436 person-months and there were 22 cases of secondary UTI. Among 20 person-months contributed by young women using diaphragm or cap there were 5 secondary UTI. Compared to women using oral contraceptive, what is your estimate of the effect or association between use of diaphragm or cap and risk of secondary UTI?

Explanation / Answer

The higher rate of second UTI when the first UTI is cause by E. coli than when it is cause by non-E. coli. Women with first UTI due to E. coli are more likely than are those with non-E. coli first UTI to have engaged in vaginal intercourse prior to their first UTI. Although risk of second UTI is strongly influenced by sexual behavior, E. coli first UTI are more likely than are non-E. coli first UTI to result in a second UTI within 6 months.

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