Keeping up to date – the shifting sands of evidence.
NOTE: These blog posts are written from the perspective of a GP colleague who still seems to accumulate a list of unanswered questions at the end of each day in practice. Each post describes how I went about trying to answer the specific questions. These posts are not intended to be used for specific clinical guidance but hopefully provide links to some useful resources which the registrar can explore for themselves.
Just a few months ago I searched the literature about imaging and screening for Abdominal Aortic Aneurysms (AAA) because several patients had asked me about their own situations with family history. I discovered that the UK, US and Sweden had screening programs in which 65 year old males were the most common targets. It is this group for which the evidence was the strongest, particularly among smokers which I found quite convincing. I thought I had all the issues sorted to my own satisfaction but there is always the caveat regarding the need to keep up to date with the evidence.
And then, an article came to my attention which was published in the Lancet in June. “Benefits and harms of screening men for abdominal aortic aneurysm in Sweden”: a registry-based cohort study M Johansson et al. I must admit I saw the original link on Twitter. This was a report of a large sample in Sweden which was followed up over a three-year period. More than 25,000 were invited to participate in the screening and included a matched control group of more than 100,000 who were not screened. It’s so useful when countries actually collect data and have programs that are widely implemented.
They documented a concurrent large decrease in the incidence of AAA and related mortality which means that the results and conclusions of previous trials of screening had likely become outdated. The underlying reduction in cases may therefore influence the benefits and harms of screening (which include over diagnosis and needless surgery). The bottom line was that an estimated two deaths were avoided for every 10,000 men screened after six years (not statistically significant). It is worth reading the article for the details.
The crunch appears to be the significant decline in smoking rates which the authors suggest might be the key to the declining mortality from AAA. As mentioned in my previous blog, this was noted in other studies. Another interesting observation was that many cases of AAA are identified incidentally outside of the screening programs, probably through the increase in imaging for other reasons. There were some caveats such as the relatively short follow-up period of six years. They noted that smoking rates in Sweden reduced from 44% to 15% in the forty years to 2010. For countries with similar decreases it is likely that similar conclusions may apply. It is also noted that about 80% of deaths from abdominal aortic aneurysm involve smokers.
So what does all this mean for us in General Practice? Interestingly, it was not relevant to my patients who spurred me to do the original literature search. They were all female, non-smokers with a family history and the study does not enlighten me further. It does however highlight the importance of smoking and my last question had been what to do about the small number of older male smokers among my patients. It may certainly have an influence on Australian guidelines if some are developed at some point in the future.
It does however nicely illustrate how it can be difficult to apply what appears to be sound evidence and even harder to keep up with the changing evidence base. Evidence is often gathered in other populations and in this case it is the changing characteristics (eg smoking) of the population that are crucial. It also underlines the fact that public health issues may be more important than well implemented screening programs.
My final point, as always, is to keep an eye on the literature and make your own judgments. It’s always interesting and it’s useful to be able to give patients an evidence-based explanation. I find I am continually making the caveat to them that the evidence base may have changed by the next time they see me and, in this instance, that was the case.