Friday, June 17, 2011

Peripheral arterial disease (PAD) and smoking, now there is a real relative risk

While I thought there was little doubt remaining about the relationship between cigarette smoking and PAD, a recent study published in the Annals of Internal Medicine (see here for abstract) provided more convincing data, this time in women. Yes, cigarettes are bad for women's peripheral arteries as well.

This study from the Women's Health Study generated some robust, relative risk numbers.I am not talking about the puny 1.2-1.4 relative risks (RRs) we often see in the typical data dredging articles and certainty not the ridiculous RR of 1.01 (not a typo) that was the alleged increased risk of death from vitamin E use.See here for that silliness.

Here are the age adjusted incidence numbers for symptomatic PAD

0.12 never smoked
0.34 former smoker
0.45 smoked less than 15 cigarettes per day
1.63 smoked greater than 15 cigarettes per day

1.63/0.12 =13.6

You are not likely to see RRs greater 10 from the typical data dredge and the WHS data also demonstrated a dose-response effect.

So, how large should a RR be before one worries about it or seriously believes we may have a causal relationship?

Sackett ,of McMaster EBM fame, asked one of the giants of epidemiology that question. Sir Richard Doll said that if the RR were 20 or greater that would be almost sufficient to indicate causality.Sackett was not quite that cautious and indicated that a RR of greater than 3 was "convincing".

Some courts use a RR greater than 2 to reach the threshold of "more likely than not".This is the current level of proof in most tort cases.

Michale Thun, who at the time was vice-president of epidemiology and Surveillance at the American Cancer Society, said:

With epidemiology you can tell a little thing from a big thing.What's very hard to do it to tell a little thing from nothing at all.

With cigarettes and PAD, we have big thing and we will not likely see battling statisticians debating the data. However, we did see that when Nissen's NEJM article claimed a RR of 1.43 for of Avandia and heart disease and we will likely get to see another again with the current breaking news of a RR around 1.4 with Actos and bladder cancer.

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