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Relative Risk Reduction vs Absolute Risk Reduction Part 3

Posted May 16 2012 3:00am
Remember last month's review of absolute vs relative risk ?  We looked at how the use of drospirenone doubled to tripled the relative risk of blood clots compared to women not taking any contraception, yet the overall or absolute risk of a deep venous thromboembolism (DVT) remained quite small compared to the number expected during pregnancy and for the 1st three months post-partum..

Therefore I read w/interest a Danish cohort study published in the British Medical Journal last week that concluded that use of transdermal patches, vaginal rings & even subcutaneous implants increased relative risk of DVT while those who received levonorgestrel intrauterine contraception had equivalent risk compared to those women who did not use any hormonal therapy.

But let's look deeper at the incidence of events per 10,000 exposure years.  Last month's study quoted a baseline rate of 1-5 thrombotic events per 10,000 women-years, w/use of oral contraception increasing the absolute rate of thrombotic events to 3-9 per 10,000 women-years.  As I noted, this still compared favorably w/baseline incidence of 5-20 thrombotic events during pregnancy over 10,000 women-years and 40-65 thrombotic events in the 12 weeks immediately post-partum.

This month's study claimed an incidence of 3.84 events per 10,000 exposure years, which compared favorably to last month's 1-5 events.  Depending upon the type oral contraceptive used, rate of DVT increased to 6.63-8.68 events per 10,000 exposure years, again similar to last month's event rates.  
However, the patch, vaginal ring and levonorgestrel implant increased event rates to 11.33, 10.93 & 5.09 events per 10,000 exposure years, which is still comparable to risk during pregnancy and much lower than during the 3 months immediately postpartum..  
So while the relative risk may look amazingly elevated, this still compares favorably to the absolute risk of clots engendered during pregnancy & during the immediate postparum period.  It's too bad that it's easier to get new/more attention w/overblown relative risk than the more truthful absolute risk.  As always, when it comes to contraception, choose wisely!  And don't forget that you still need to wear a barrier method to decrease the risk for sexually transmitted infections .
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