When relative and absolute risks disagree (Part 2 of 4)
Click here to see part 1
When did your doctor last prescribe you a quinolone antibiotic? My last was in 2017, which was before the FDA warning about aortic aneurysm, but after warnings about severe nerve damage. Now I’m curious: Next time I get a urinary tract infection, what will my doctor prescribe…?
For the whole picture, make sure you consider both relative and absolute risks
Our journal club class recently reviewed a cohort study of the risk of abdominal aortic aneurysm (AAA) associated with quinolone antibiotics.1 As we discussed the article, we observed a nice illustration of the phenomenon that absolute and relative risks often disagree. In this study, the overall finding was that quinolones increase the risk of aortic aneurysm by 66%. In subgroup analyses, the study authors reported the results separately for males and females, shown in the Figure 1. Interestingly, the relative risk showed that the risk was more than doubled in women (a 114% increase), a finding that was statistically significant. In men, the relative risk was only increased 48% in patients who received quinolones.
However, when absolute risk was considered, we saw that the risk difference associated with quinolones was smaller for women than for men. For women, quinolones only contributed to an increase of 0.4 events per 1000 patient-years. For men, the increase was 50% larger at 0.6. Despite the smaller absolute risk difference for women, the finding was statistically significant for them, but it wasn’t for men. Curious.
Click here to find out why.
- Pasternak B, Inghammer M, Svanström H. Fluoroquinolone use and risk of aortic aneurysm and dissection: nationwide cohort study. British Medical Journal. 2018;360:k678.