Systematic reviews are not automatically “level 1 evidence”. First the systematic review has to be well conducted and appropriately combine content. Second, the evidence reviewed has to have sufficient quality for the conclusions to support “level 1 evidence”. Equating systematic reviews (or randomized trials) with level 1 evidence without checking the details is problematic.
On the other hand “many previous epidemiologic studies, animal studies, …” does not establish high-quality evidence and could be prone to biases related to confounding factors.
In this case the Cochrane review conducted a systematic review of randomized trials (seeking unconfounded data) and really found insufficient evidence to make a “level 1 evidence” claim of benefit or to make a “level 1 evidence” claim of no benefit. Despite many studies and “time-honored practice” (which just means many have accepted this to be true without evidence for a long time) we have never had evidence clearly establishing benefit or no benefit for salt restriction. This Cochrane review basically looks at the totality of evidence today (in case it changed since the last time this was systematically evaluated) and still finds we do not have a highly reliable answer.
But interpreting the Cochrane review should still involve critical analysis. For example when evaluating this Cochrane review for DynaMed we considered that the meta-analyses combining all the trials were not the ideal representation of the data because combining normotensive patients, hypertensive patients, and patients with heart failure does not represent a typical patient or general population. So instead we reported the detailed results separately (with numbers of trials, numbers of patients, and confidence intervals for differences or trends) for:
· normotensive patients (non-significant trend toward lower mortality and lower systolic blood pressure at end of trial, but no significant differences in mortality or cardiovascular morbidity at longest follow-up)
· hypertensive patients (no significant differences in all-cause mortality or cardiovascular morbidity, but trend toward reduced cardiovascular mortality)
· patients with heart failure (significant decrease in systolic blood pressure but significant INCREASE in mortality in 1 trial)
Ultimately we gave the review a conclusion of:
· unknown whether reduced salt intake reduces cardiovascular morbidity or mortality (level 2 [mid-level] evidence)
- based on Cochrane review of trials with insufficient power to exclude clinically important differences
Brian S. Alper, MD, MSPH
Editor-in-Chief, DynaMed (www.ebscohost.com/dynamed)
From: Evidence based health (EBH) [mailto:[log in to unmask]] On Behalf Of Dr. Abdelhamid Attia
Sent: Sunday, September 18, 2011 8:06 PM
To: [log in to unmask]
Subject: Interpreting seemingly odd evidence from systematic reviews
Dear EBMers,
I have been asked by a colleague about a systematic review that has been heavily criticized “Reduced dietary salt for the prevention of cardiovascular disease”.
Being away from my specialty, I did not follow the reactions to its publication and what followed.
Anyhow, here is a quote of the questions posed by this colleague.
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a new cochrane review has concluded there is no clear evidence of benefit—in terms of preventing CV events and deaths—from advising people to reduce dietary salt intake[1,2]. This type I evidence came against many previous epidemiological studies, animal studies, randomized trials and sure against a time-honored practice. This caused many physicians to shout "FOUL" over it [3].
This situation raises many questions like;
a) How many systematic reviews show conflicting results?
b) How will the practicing physician interpret conflicting evidences?
c) Do we need expert opinions of respected authorities to evaluate level I evidence (Systematic reviews)?
d) If so, will this threaten the whole concept of evidence based medicine? Or at least turn the balance again towards medicine as a profession?
1. Taylor RS, Ashton KE, Moxham T, et al. Reduced dietary salt for the prevention of cardiovascular disease. Cochrane Database of Systematic Reviews 2011; 7:CD009217.
2. Taylor RS, Ashton KE, Moxham T, et al. Reduced dietary salt for the prevention of cardiovascular disease: a meta-analysis of randomized controlled trials (Cochrane Review). Am J Hypertension 2011; 8:843-853.
3. He FJ, MacGregor GA. Salt reduction lowers cardiovascular risk: meta-analysis of outcome trials. Lancet 2011; 378:380-382.
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I will appreciate any insight about this case and specifically the answer to question b) when an ordinary clinician finds a strong reaction against a systematic review as the one published in the lancet.
Best of wishes,
Abdelhamid Attia
Prof. Dr. Abdelhamid Attia
Prof. Of Obstetrics & Gynecology; Cairo University
Assistant Secretary General Of The Egyptian Board (Fellowship)
President Of The Arab Federation of Evidence-Based Medicine
Secretary General of the Scientific Council of OBGYN, The Arab Board
Editor-in-Chief of Kasr Al-Aini Journal of Obstetrics & Gynecology
Editor Of Kasr Al-Aini Medical Journal
Associate Editor of The Middle East Fertility Society journal
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