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. What percent of the risk  of secondary UTI  among those who used diaphragm or cap is due to them using diaphragm or cap ?

Explanation / Answer

The incidence rate of secondary UTI among female students using oral contraceptives is,

= 22/436

= 0.05

The incidence rate of secondary UTI among female students using diaphragm is,

= 5/20

= 0.25

Therefore, the risk of secondary UTI among those who used diaphragm or cap is due to them using diaphragm or cap is,

= 0.25 - 0.05

= 0.2

= 20%

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