Prevention of Cardiovascular Events and Death with Pravastatin in Patients
with
Coronary Heart Disease and a Broad Range of Initial Cholesterol Levels
N Engl J Med 1998;339:1349-57.
I had a go at converting table 2 into NNTs last evening using Douglas
Badenoch's CATmaker.
NNT (death any cause over 6.1 years) is 33 (23 to 58) and
NNT (Death CHD) = 52 (33 to 118) and so on.
But we have to remember that these patients were high-risk patients
(post-infarction or unstable angina, mostly men, smokers that had quit, 82%
used aspirin, 46% used beta-blockers, 9% diabetics). I also calculated the
RRR,
which the industry clings to: 22% for the first outcome.
My next step was to assume that the drug companies would extrapolate the
findings to the total population.
So I thought that maybe the baseline risks for the rest of us was 10% of the
study patients'.
CER (Control event rate for death any cause) per 6.1 yr in the studypop
14.1%
EER (Experiment event rate for death any cause) in the statin group was
11.0%
RRR (rel riskred) = (CER-EER)/CER= 22%
ARR= absolute riskred.=CER-EER = 14.1% - 11.0% = 3.1%
NNT=1/ARR= 32 i.l.a. 6.1 år, 95% CI (23,60)
Proposed linical bottom-line: NNT is low enough and the 95%CI is narrow enough
to recommend statins as part of secondary prophylaxis to patients with
established CHD.
***********************
Then look at the low risk population. Assume that the population risk is 1/10
of the study patients'.
CER= 1.41%
EER= 1.10%
RRR=(CER-EER)/CER=22% Still 22%, and the drug reps play this game with us
ARR=0.31%, not-significant.
NNT= 333 over 6.1 yr, 95% CI (132,infinity). So- here the NNT is calculated
on a
non-significant difference. Today's BMJ teaches us how we should handle this:
Take a look at
Confidence intervals for the number needed to treat
Douglas G Altman
BMJ 1998;317 1309-1312
http://www.bmj.com/cgi/content/full/317/7168/1309
Proposed clinical bottom-line: The NNT is NOT low enough, and the CI limit
goes
to infinity, meaning that there might be an effect as high as given by an NNT
of 133 to no effect at all for the general population who do not have CHD.
Is this how it should be termed?
Atle Klovning, MD GP
Research Fellow
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