Hi guys,
That is fascinating. Of course, the thing that immediately struck me
was that the outcome was a composite of a biochemical marker (serum
creatinine), a chronic disease state (ESRD), and death. All of these
have different weights when it comes to patient values (I would hope).
Therefore, before even doing the statistical test, the authors have
created a 'void of reality' situation. Therefore we really shouldn't
care about which test is being used.
The use of the Cox test may be a legitimate of controlling for the
differential times in teh study, but you have to ask why is that
difference there in the first place. Was one group more likely to be in
the study longer and could this bias the outcome?
Cheers,
Dan
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Dan Mayer, MD
Professor of Emergency Medicine
Albany Medical College
47 New Scotland Ave.
Albany, NY, 12208
Ph; 518-262-6180
FAX; 518-262-5029
E-mail; [log in to unmask]
****************************************************************************
>>> Mike/Linda Stuart <[log in to unmask]> 07/15 3:11 PM >>>
We are evaluating the literature for managing chronic kidney disease
and
would very much appreciate opinions regarding the following issue in
analyzing the results of an RCT comparing enalapril with placebo in
diabetics.
Ref: Brenner BM, Cooper ME, de Zeeuw D, et al., for the RENAAL Study
Investigators. Effects of losartan on renal and cardiovascular
outcomes
in patients with type 2 diabetes and nephropathy. N Engl J Med. 2001
Sep
20;345:861-9.
The patients were type 2 diabetes with nephropathy. Baseline
characteristics, including disease severity, were similar in the two
groups.
Outcomes were -
--the composite of a doubling of the baseline serum creatinine level,
end-stage renal disease (ESRD), or death,
--and the composite of cardiovascular morbidity or mortality.
The authors used the Cox regression model and reported significant
RRRs
for all of the endpoints except death.
The authors explain that there is often a difference between the risk
reduction as determined on the basis of the Cox regression model and
the
risk reduction as determined on the basis of the crude rates of
events.
They state that the difference results in part from the fact that the
Cox regression model accounts for the time at risk - i.e., the longer
average follow-up in the losartan group than in the placebo group.
In this study the mean follow-up was 3.4 years and actual follow-up
ranged from 2.3-4.6 years. The authors presented the numbers of
events
per 100 patient-years of follow-up.
The data is shown below:
We recalculated the P value for the primary outcome, basing our
calculations on crude event rates. We did not get statistically
significant results (p=0.235).
We are not statisticians but believe that the crude event rates should
also be statistically significant for the study to be valid.
We would be most grateful for opinions regarding this question and our
belief that we should be able to use crude event rates for these
outcomes.
-- Thanks, Michael Stuart MD and Sheri Strite
Delfini Group, LLC
[log in to unmask]
www.delfini.org <http://www.delfini.org/>
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