Dear Abdelhamid,
Can I expand a little on Bob Phillips point? The right study and
hierarchy depends on the questions you want to answer.
For diagnostic ACCURACY a cross sectional study with full verification
with a blinded adequate gold standard is fine.
That works even if we can't or don't want to treat, e.g, an untreatable
tumour or Osgood-Schlatter's disease.
But we may also want to know whether having the test available as part
of a complex management strategy improves outcomes. But that is a
complex brew of:
1. the test accuracy
2. the alternative tests
3. the available treatments
4. how clinicians interpret the test and behave as a result
So the RCT tests all this brew together (and probably more), not the
test accuracy. That is of greater interest, but usually less generalisable.
However, I agree that some tests need this, e.,g. cardiotocography in
labour. But others clearly don't - I am happy that the whispered voice
test* is accurate for diagnosing deafness without need an RCT to know
that it changes my or the patient's behaviour, or that the jolt
accentuation test can rule out meningitis (though better cross sectional
studies are needed to verify this).
Cheers
Paul Glasziou
* Pirozzo, et al BMJ 2003.
> Dear listers,
>
> I have been discussing with a colleague the best evidence
> about diagnostic tests. In our discussion, I found that some tutors
> take the oxford hierarchy of evidence at
> http://www.cebm.net/levels_of_evidence.asp
> at its face value while I see that RCTs can give also better evidence
> than cross-sectional studies if they are feasible to perform. One of
> the pitfalls of the hierarchy is that we may miss a better piece of
> evidence as it doesn't mention RCTs or systematic reviews of RCTs.
>
> Teaching EBM, I always concentrate on the concept of bias in different
> study designs and their implications on the process of evidence
> generation rather than restricting my students to a "rigid" scheme and
> I always ask them to start with searching on SR of RCTs, RCTs, etc..
> down the hierarchy of evidence.
>
> For a live example:
> In the Cochrane Database of SRs there is a systematic review for RCTs
> comparing the accuracy and safety of chorionic villous biopsy and
> amniocentesis in the diagnosis of genetic abnormalities.
> http://www.mrw.interscience.wiley.com/cochrane/clsysrev/articles/CD003252/frame.html
>
> This piece of evidence about diagnosis is of course better than 10s of
> cross-sectional studies. I think that we should add RCTs and their
> systematic reviews at the very top of the evidence pyramid for
> diagnosis as we do for therapy and harm?
>
> Any thoughts?
>
> Best of wishes,
> Abdelhamid Attia
--
Paul Glasziou
Director, Centre for Evidence-Based Medicine,
Department of Primary Health Care,
University of Oxford www.cebm.net
ph +44-1865-227055 fax +44-1865-227036
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