Ben - while no one would argue that the treatment should be withheld if the
risk of CVD is below AT using your terminology, the converse is not so
clear cut. Evidence that a treatment results in more good than harm is a
necessary but not sufficient reason for treatment. Alternative uses of
scarce health resources need to be considered and this is very dependant on
the wealth of the community in which you live. In New Zealand we suggest a
10-15% 5 year risk of CVD is a reasonable level at which to discuss
pharmacological treatment of raised blood pressure with patients. As it
happens in NZ if we only treated people with a 15% 5 yr CVD risk, this
would lead to little net change in numbers treated (although many would
stop and many others would start) and we would inccrease the number of
events prevented over 5 yrs by about 30%. We have a different threshold
for lipid lowering treatment because despite a similar effect on risk, the
drugs cost alot more in NZ. In the ideal world we would estimate $/QALY
for all interventions and use these to help prioritise who should be
treated with what.
Rod
>As I argued it at this group before and as we described it elsewhere (for
>details see Cancer Control 1998;5:394-405; also at
>http://www.moffitt.usf.edu/pubs/ccj/v5n5/article2.html , Med Decision
>Making 1998;18:464
>; Comp Biomed Res; or, see
>http://www.hsc.usf.edu/~bdjulbeg/Programs/BR/br-java.htm ), normatively
>highest (maximum) NNT at which treatment is worth administering in
>prophylactic setting is equal to:
> NNT<NNH or NNT<1/ R
>where R is risk of the treatment .
>For example, for a toxicity of 2%, the treatment NNT has to be at most 50 to
>be worth administering the treatment, for a toxicity of 4%, the NNT has to
>be at most 25, etc.
>
>If this condition is not satisfied, no NNT is "good NNT".
>
>In the setting of selection of Rx1 vs Rx2, in order to even consider the
>treatment Rx1 as opposed to the alternative treatment Rx2, the following
>inequality must be satisfied:
>
>NNT1 <= 1/(R1 - R2 + 1/NNT2)
>
>
>This is not only normatively correct, but also makes sense intuitively: we
>should not consider measures of treatment benefits without consideration of
>other side of therapeutic coin (that is, treatment harm).Again, as I wrote
>before, it is important to realize that same units of benefits and harms are
>used when these calculations are performed.
>
>I am not aware of any normative model that successfully integrated patients
>values with NNT, NNH or with any other EBM summary measures of therapeutic
>effects (although, I should say that we are currently working on one such
>model)
>
>Intertwined with this issue is the question of "action threshold". I will
>try to illustrate it from perspective of ongoing discussion of CVD risk and
>New Zealand tables for CVD. The tables use data from Framingham Heart study.
>These tables suggest threshold for therapeutic action if risk of CVD is
>10-15% at 5yrs (NNT=25 for 5 years) (see:
>http://cebm.jr2.ox.ac.uk/docs/prognosis.html ) However, this treatment
>threshold appears to be quite arbitrary. As I argued before, normatively
>this action threshold (AT) is equal to
>
>AT= NNT/NNH or AT=risk of treatment*NNT=treatment
>risk/(RRR*morbidity/mortality without Rx) (for details see references
>provided above).
>
>We should treat if estimated risk of clinical event (in this case, CVD) is
>above AT; conversely, we should withhold Rx if risk of CVD is below AT. For
>example, package insert for pravastatin indicated that at median treatment
>of 4.8 years, 0.10% of patients treated with pravastatin developed elevation
>of AST as opposed to 0.03% of placebo treated patients. This would amount
>roughly to NNH=1428. Using the action threshold equation this means that we
>can consider treatment with statins if risk of CVD exceeds >1.7% at 5 years
>(AT=25/1428). Even when I vary risk in sensitivity analysis, I still obtain
>threshold for action, which is below recommended 2% or 3% per year. It is
>not clear to me how current recommendations (for treatment actions) have
>been derived. Can somebody care to comment? (Cardiovascular medicine is not
>my field, and I am perhaps missing something here).
>
> (Incidentally, I developed a simple BASIC program that integrates this
>threshold model with Framingham risk equations. If anybody is interested in
>the program, I will be happy to send it to him/her.)
>
>
>
>hope this clarifies this issue
>
>ben d
>
>Benjamin Djulbegovic, MD
>Associate Professor of Medicine
>H. Lee Moffitt Cancer Center & Research Institute
>at the University of South Florida
>Division of Blood and Bone Marrow Transplant
>12902 Magnolia Drive
>Tampa, FL 33612
>
>e-mail:[log in to unmask]
>http://www.hsc.usf.edu/~bdjulbeg/
>phone:(813)979-7202
>fax:(813)979-3071
>
>> -----Original Message-----
>> From: Takeo Saio [SMTP:[log in to unmask]]
>> Sent: Friday, March 05, 1999 5:32 PM
>> To: Amit Ghosh
>> Cc: [log in to unmask]
>> Subject: RE: What is a good number for NNT?
>>
>> Hello! That's very good question.
>>
>> I think the NNTs of 2-4 are for therapeutic usage of drugs,and the NNTs
>> over 20 are for preventitive usage of drugs.
>> i.e. for acute condition,NNTs of 2-4:for chronic condition,NNTs of over
>> 20.
>>
>>
>> > A question I have , is what is a good number for NNT? Bandolier 12
>> >indicated that a NNT of 2-4 is suggestive of good NNT ( i.e., ARR of
>> >0.25-0.50). However most the the RCT reported in journal s NEJM, Annals
>> >etc have statistical significance however their NNT are usually over 20,
>> >ie, finasteride for BPH( NNT= 30). I reckon one of the disadvantages of
>> >NNT maybe that despite the NNT being over 20 , these patients may have
>> >significant improvement in quality of life and other subjective issues.
>> >Or is the fact that NNT of 2-4 an over enthusiastic expectation when it
>> >comes to a clinical response.I guess in the case of chronic diseases
>> >with outcomes occurring over decades a NNT of 50 would be acceptable,
>> >however in other more acute conditions one would consider a smaller
>> >number for NNT as desirable. Could the members throw some additional
>> >light in the defining what is a good number for NNT.
>> **********************************************
>> Saio,Takeo
>> ///////////////CHIAKI hospital,JAPAN/////////
>> e-mail;[log in to unmask]
>> **********************************************
>>
>>
>>
Dr Rodney Jackson MBChB PhD FAFPHM
Associate Professor of Epidemiology
Head of Department
Dpt of Community Health, School of Medicine
University of Auckland
(Grafton Mews, 52-54 Grafton Rd)
Private Bag 92019, Auckland, New Zealand
Phone: +64 (0)9-3737599 ext 6343
Fax: +64 (0)9-3737503
e-mail: [log in to unmask]
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