BoringEM CaRMS Classics

In the ramp up to CaRMS, BoringEM is committed to bringing you some classic content that we think will be useful.  Periodically over the next few weeks, we will be revising and posting some of the “Classics” from BoringEM around the CaRMS application period.

And so I announce to you that for the next few weeks BoringEM will be posting 3 times a week!  Sometime around midweek we will post one of our pieces about the Canadian Residency Match… the CaRMS Classics!

Also, with our new emphasis on the Expert Peer Review / Attending Review process, I will be ensuring that each piece is read, vetted and supplemented by a staff person for their perspective.

So, stay tuned for tomorrow when we release our first BoringEM CaRMS Classics piece.

– Teresa (@TChanMD)

Author information

Teresa Chan
Managing Editor at BoringEM
Emergency Physician. Medical Educator. #FOAMed Supporter, Producer and Researcher. Assistant Professor, Division of Emergency Medicine, Department of Medicine, McMaster University. + Teresa Chan

The post BoringEM CaRMS Classics appeared first on BoringEM and was written by Teresa Chan.

Case Report: Synthetic Cannabinoid K2 and Myocardial Infarction

K2-Weed-thumb-300x20012.5 out of 5 stars

K2—Not the Spice of Life; Synthetic Cannabinoids and ST Elevation Myocardial Infarction: A Case Report. McKeever RG et al. J Med Toxicol 2014 Aug 26 [Epub ahead of print]

Abstract

Use of synthetic cannabinoids has been associated with a broad range of adverse effects, including tachycardia, hypertension, seizures, psychosis, renal injury, and cyclic vomiting.

This article describes 16-year-old male who presented to hospital with 1 day of substernal chest pressure associated with nausea, vomiting, and dyspnea that started 2 hours after he smoked the synthetic cannabinoid K2. Workup revealed elevated ST segments in the inferolateral leads and elevated troponin that peaked at 8.29 ng/ml (normal 0-0.3 ng/ml). Echocardiogram and cardiac catheterization were unremarkable. Urine drug screen was positive only for opiates and benzodiazepines, both of which the patient received in the hospital before the specimen was obtained. Specimens for additional toxicology tests were sent to an outside lab but never arrived.

The authors claim that this is “the first report of ST-elevation myocardial infarction (STEMI) in the setting of synthetic cannabinoid use without concomitant marijuana use.” To my mind this claim is a bit disingenuous. A 2011 paper by Mir et al — cited in this article — described several teenagers who presented with chest pain and STEMI after smoking K2 but had no history of proximate marijuana use.

IC-HU Project in New York, thanks to The Arnold P. Gold Foundation

Hola a tod@s, my dear friends.

I want to share a great happines and a great surprise with everybody.

Today, seven months after start the Blog,  The Arnold P. Gold Foundation  published today a post I wrote to promote our Project in United States. Of course, this time in English!!




Because it´s time to human being returns to the center of health care. 

Thanks so much to be here, my rogue!
Gabi

On Sick Kids, Sore Throats, Swabs and Such

Dear Fellow Doctors,

I would like to take a moment of your time to discuss a point of medical practice which seems to remain controversial despite it being really common and a well-researched problem.  I would like to tell you about the modern management of the humble throat infection.  Yes, it is a really common, dare I say dull topic.  However, it would appear that our profession has the wrong end of the proverbial stick when it comes to this quotidian diagnosis.

Firstly, let me introduce myself and tell you why my perspective ought to be relevant and enlightening to you.  I am a small town doctor working in primary care and Emergency Medicine.  I work in a beautiful part of remote northern Australia.  I have an interest in paediatrics and specifically the acute care of Aboriginal kids.  I see a lot of children with common throat infections – don’t we all!  The reason I feel somewhat qualified to discuss this topic is because I work in a population where the complications of acute streptococcal infections still produce massive morbidity.

Our population have a burden of disease rarely seen in the developed world.  Skin infections, STIs, malnutrition, fetal alcohol syndrome… the list goes on.   Last year I saw 3 new cases of Sydenham’s chorea.  I see children with acute rheumatic fever and carditis on a weekly basis.  We are currently in the throes of an epidemic of post-streptococcal glomerulonephritis.  You might say that pus is our bread and butter.  The incidence of those dreaded complications of the common strept throat is astronomical in my neck of the woods.  So that is why I want to chat to you about a few things – namely: throat swabs and antibiotics.

So, how do we decide who has a true “strept throat” and not just a virus?

Let me start on the practice of throat swabbing children with acute tonsillo-pharyngitis.

So, when do I swab a child’s throat?  Easy – never.  I can honestly state that I have not swabbed a child’s throat in order to make the diagnosis of “strept throat” in the last decade.

I have certainly swabbed a number of throats in that time – but only in order to satisfy the diagnostic criteria of the diseases which we worry about i.e. acute rheumatic fever or post-strept GN.  To be clear, I really only swab them after the fact – that is after the diagnosis is clinically apparent and we really just need a microbiological sample to prove the case, satisfy the diagnostic criteria and provide information to the folk in the Public Health office and a few Microbiologists whom might be interested.  This is not a tool for making routine management decisions.

I hear that in much of North America that the rapid throat swab test is used to decide which children require antibiotics.  Well, that opens another whole can of worms – do they really need antibiotics?  But before we get onto that vexed question – lets look briefly at the diagnostic strategy that is “throat swabbing” and why it just doesn’t make much sense.

Before we discuss the “diagnosis” of strept throat – please recall that many children have asymptomatic colonisation with group A streptococci – they are well.  The background rate is between 9 and 14% (Link).  This seems to surprise a lot of doctors when you point this out to them.  We do prefer the binary answers i.e. positive swab equals disease, negative equals no-disease.  But life just does not work that way.

So can we use clinical findings to work out who is likely to have a true strept throat, rather than a viral infection?  This is well studied.  There are a number of ‘clinical decision rules’ that we can utilise.  For example, the modified Centor Criteria or the Fever PAIN score  are easy to use – just a few questions and a cursory exam give you all the data you need to get a score.  Here is how it pans out from the studies:

Kids get +1 point just for being kids[ aged 3 - 14 yrs] .  Then if you have any of the other criteria – tender anterior nodes, exudate or swollen tonsils, fever > 100.4 (38 C) or the absence of a cough you are up to 2 points and you have an 11 – 17% chance of having a positive throat culture.  Or basically – the same as our pretest guess based on what we know about carriage rates.  So having a Centor score of 0, 1 or 2 is pretty good evidence that you are wasting your, and your patient’s time by doing a swab.  If they had a score of 0 or 1 – then you probably wouldn’t be asking the question.  The 3 year old with a runny nose and low-grade fever has a virus 99% of the time.  Don’t apply this rule to those kids!

Incidentally if you have 3 Centor points then your rate of swab positive is about 1/3 and if you have 4 or 5 points – you are up to 50% – even money as they say.  So a kid with a full house of clinical features suggesting strept pharyngitis has a 50:50 chance of having a + swab.  Save your cash – and toss a coin instead.

So do swabs help predict response to antibiotics? Tougher question.  Are they any better than simple clinical exam and history?

There is a very recent large clinical trial out of the UK  – the PRISM trial in the UK.  This looked at the predictive ability of a clinical score (FeverPAIN) and a rapid antigen detection test (RADT) for symptom reduction with targeted antibiotic prescribing.  They found that using the FeverPAIN score was useful to target antibiotic prescription for symptom reduction.  The use of RADT tests also did the same thing.  BUT adding a RADT did NOT add anything to simple clinical scoring when used in tandem.  Therefore it would seem like a waste of resources to use a test where simple clinical exam will do the same job.

So we can use this to identify kids whom are unlikely to benefit from having a swab – i.e. the Centor 0 – 2 group will more likely have commensal strept than actual disease – so do not swab them.  So what about the others…

This is where it gets fun.  Having a high Centor score makes it more likely that you will have a positive swab test.  It DOES NOT predict that you will have a more serious clinical course or be at higher risk of the rare but nasty sequelae [Reference].  However I have had in numerous discussions with folk who genuinely get extra-worried if their patient has a Centor score of 5.  I have even seen a kid get a whack of IM ceftriaxone – purely because he had a high Centor score – not because he looked particularly septic!

Summary of Swabbing:  It is:

– unhelpful,

– can lead to a lot of false positives,

– does not predict “strept throat” any better than a simple clinical scoring tool and

– costs a lot more.

Even in the kid with all the features of Strept throat – there is not much evidence to suggest swabs can help predict who should get antibiotics.  The question of whether or not antibiotics actually help is to be covered next….

So lets assume you have taken a look at a kid and clocked up a full suite of symptoms suggestive of strept (eg. large, kissing exudative tonsils, tender nodes and a fever without a preceding viral prodrome, coryza or cough.)   The Centor scores suggest 50% rate of swab positivity.  It certainly is not 100%!.  Now we come to the big issue – to treat with antibiotics or not….   I am going to break this down into 3 areas for discussion.

(1) patient-oriented benefits i.e. pain and suffering,

(2) preventing suppurative complications : Quinsy, otitis media, sinusitis, cellulitis / impetigo

(3) preventing non-suppurative complications [i.e rheumatic fever, PSGN].

Let’s start with the most likely actual potential benefit of antibiotics.

What do antibiotics do for kids with sore throats in terms of symptoms?

This question should be easy enough to answer – search the Cochrane database.  The reviews there are huge – and they include papers from the last 50 years. So a heterogeneous group of patients, actually there are surprisingly inadequate trials in kids specifically.  There are not many recent trials – so we may be seeing a bias – with antibiotic resistance emerging and improved hygiene etc in the last 50 years.

The simple summary of the data is:

– antibiotics reduce pain after 3 days.  This effect was seen in subjects whom were swab positive.  About half of non-treated patients were improved by day 3, more in the AB groups. The NNT was about 6 for “pain reduction at 3 days”.

– by the 7 day mark the difference was much less with an NNT of 21 for pain resolution by day 7.  That is most of the untreated kids were better, the gap between the treated and untreated cohorts was smaller at one week.

OK, excellent… so what were the downsides?  Unfortunately a lot of these trials do not report harms accurately.  So if you want to know exact rates – it is tricky.  The common adverse effects attributed to penicillin and similar antibiotics commonly used for sore throat are:  diarrhoea, nappy rash, cadidiasis, skin reactions and the big one – anaphylaxis.  Studies looking at the common, minor reactions would put the NNH [number needed to harm] between 6 and 9.  Anaphylaxis is a bit more bothersome – though rarer – maybe 1 in 1,000 to 1 in 10,000 depending on the paper you believe.

To be honest – if you are having a risk vs. benefit chat with the parents about ABs – then anaphylactic reaction does seem a bit over the top.  I just don’t think that it really weighs on a parent’s mind enough to change the way they feel about “fixing” their sick child ASAP.

So here is how I would discuss the risks  and benefits of antibiotics in a kid with a high Centor score / clinical impression of strept:

Jimmy has a nasty throat infection.  Based on how he looks, his age and other symptoms I would estimate that there is a 50:50 chance that he has a “true strept throat” which antibiotics have been shown to help.  When we say ‘help’ – we mean that he is more likely to have less pain etc in 3 days.  For every six kids we treat with antibiotics, one will get this benefit.  The other 5 will not.  However, there are a number of other ways we can reduce Jimmy’s pain without using antibiotics – and they work a lot faster.  On the downside, roughly the same number of kids that we treat with antibiotics will get a side effect – like diarrhoea or a rash from the antibiotics.  So I reckon it is a close balance between risks and harms.  Would you like to hear more about antibiotics or I can give you a plan for managing Jimmy’s pain and fever?”

I would not even delve into the murky waters of the benefits of antibiotics for more serious sequelae – these are so rare and the evidence is so sparse that it would not be a meaningful discussion.  If the family want to discuss problems like quinsy or rheumatic fever then I would imagine they are the type of folk whom are going to insist on at least a “wait and see prescription” – which seems fair enough.  I would however make it clear that the ABs will do very little for their child’s symptoms in the short term – and insist they use a good analgesia plan.  My favourite line in this scenario:  “So are you specifically wanting antibiotics or for your son’s symptoms to get better?”  It really helps parents think about what they want for their kid.  We need to separate the concept of ‘reducing suffering’ from ‘antibiotic prescription’.

There are a few papers out there that suggest the simple NSAIDs give more rapid and significant relief of throat pain. This review of the data from the Brit Journ of GP in 2000 suggests that ibuprofen and paracetamol are both efficacious, quick and well tolerated.  This seems like a reasonable option – paracetamol and ibuprofen are omnipresent in parent’s medicine cupboards – so I am sure most will have already dosed their munchkins – just need advice on how to do this safely and effectively.

I am a fan of a “dose of Dex” for the patient with a nasty sore, swollen throat – as a Paeds Anaesthetic doc – we give a lot of Dexamethasone for prophylaxis of swelling, nausea etc  – it seems to be good.  So is there much data on using steroids for sore throat?  Well – yes… and no.    There have been a series of trials looking at steroids – but they all gave antibiotics concurrently – so tough to say if they are testing steroids, or a combination effect of ABs and steroids.  The Cochrane folk did a review in 2012 – and concluded that the addition of steroids (oral, IM..) roughly tripled your chanced of being symptom-free in 24 hours. So I do use these, but would love to see the trial of Dex vs placebo  vs dex + ABs vs placebo.

Can we prevent local purulent complications of pharyngitis / tonsillitis?

Lets look at the dreaded local suppurative complications of tonsillitis – quinsy, otits media, sinusitis, cellulitis / impetigo.  Do we have any good data on this ?  Yep – the Brits recently published a huge prospective clinical cohort study with over 14,000 patients! Check it out here: Predictors of suppurative complications from acute sore throat in primary care Brit Med Journ  Nov 2013.  And here are some raw numbers for you…. have a guess – how common are these?

Of the 13,288 patients they have data on for complications:  47 developed quinsy (0.35%),   38 got sinusitis (0.29 %),   69 had acute otitis media (0.52 %)   and only 20 (0.15 %)had cellulitis / impetigo at follow up.  So if you take all comers – about 1.3 % will get a suppurative complication – regardless of the antibiotic prescription.  The numbers are tiny!  I am sure most doctors would guess these rates would be closer to 10 or 20 % if asked.

So any benefit is going to be tiny in absolute terms. The NNT for preventing OM or sinusitis was 193 in a Lancet Infectious Dis 2014 paper from the same data set.  Can you sell a reduction in otitis media from 0.6 % down to 0.5 %??  It is just not worth discussing.

Actually if you look at the raw data the rate of quinsy actually went up in the antibiotic groups – from about 0.2 % to roughly 0.4 % [a 100% increase! - see recent blog post on the evils of relative risks.]  I am sure this was biased by the GPs tending to prescribe ABs to folk with really bad looking tonsils that were ‘near quinsy’.  In this older paper from the Brit Journ of GP 2007 – only about a third of patients diagnosed with quinsy actually presented with a sore throat prior to being diagnosed with a quinsy.  So even if antibiotics were effective [maybe they are not?] – you will not see two-thirds of the patients anyway – they will just rock up with a hot, red, angry quinsy de novo – and you should drain it – not so much use antibiotics then! [Oh, use an ultrasound to drain it - very cool.]

So – for suppurative complications the summary discussion ought to be – the risk of another local infection is very low – about 1 in a hundred.  Antibiotics may reduce this a bit, but it is not really clinically significant.  The preferred strategy here should be to safety net and ask patients to come back for review if they develop new symptoms.  Even then – are we going to treat otitis media or sinusitis any more so with antibiotics?

What about rheumatic fever and glomerulonephritis?  Can we prevent these?

OK – now onto the really rare complications – the “non-suppurative” ones.  Namely acute rheumatic fever and post-strept glomerulonephritis.  As I mentioned earlier – we here in tropical Australia are sadly world experts on these diseases.  They remain relatively common despite ours being a very rich country with a decent, socialised health care system.  It just is not too hard to get some antibiotics in most places, and for free!   This is the part where I might get a little controversial as there is a lot of political and medical debate about how we should deal with these diseases.  But to cut a long story short – I am not convinced that antibiotics are going to prevent these problems.

First lets look at a real, first world population – western Scotland .  The incidence of rheumatic fever in the developed world is low, very low, super-super low! In 1985 (30 years ago) this paper in Scotland [Howie, Journ RCGP, 1985] estimated it would take 12 GPs working a lifetime to identify one single new case of acute rheumatic fever.  That is cool: in a medium-sized town – only one single case of ARF would have been diagnosed since that paper was published!

Now in my part of the world acute rheumatic fever is common.  We drill the Jones criteria into our RMOs and medical students.  They need to be able to pick it clinically.  It is common enough.

We throw a lot of antibiotics around – the unofficial policy is to treat any Aboriginal kid with a febrile, sore throat, ears ache or skin infection aggressively.  There is a really proactive immunisation system – with great rates of coverage – over 95% in most communities.  We have been doing this for a few decades now… and yet we are still seeing these complications of streptococcal disease.  Why is this?  Why are these kids still getting sick despite the marvels of modern medicine?

Well – lets take a quick history lesson in streptococcal disease.  These diseases were common in Australia and other first world countries a hundred years ago.  In the early 20th century the rates of rheumatic fever (or scarlet fever) began to decline in urban, developed countries.  Here is a typical graph (this one from the UScarlet fever mortalityK).  Note that the steep part of the decline occurred between 1860 and 1900.

So -this makes it unlikely that antibiotics are the cause of this dramatic decline.  In fact – antibiotics had very little impact of the rates of disease or morbidity when they were introduced towards the middle of the 20th century.

McKinlay & McKinlay out of Harvard wrote this paper: The questionable contribution of medical measures to the decline of mortality in the United States in the twentieth century. – it included streptococcal disease.  I have taken this table from the text and underlined the streptococcal stats for you: as you can see the impact of penicillin between 1946 and 1973 was almost exactly ZERO.

Scarlet fever decline post penicillinOK.  So why did all those nasty diseases go away? Well, the prevailing wisdom would suggest that a change in the “social determinants of health” occurred.  Rheumatic disease is the result of a chronic exposure to strept antigen in a susceptible host.  Streptococci thrive in overcrowded, malnourished people with poor access to basic hygiene (running water, washing facilities, refuse disposal).  This is what we see in the north of Australia – these are diseases of poverty – it is unusual to see rheumatic disease in well nourished, suburban Aboriginal kids.

What happened in that 50 years from 1860 – 1910?  People got running water, toilets and the average size of the household declined.  Folk were able to wash regularly, nutrition generally improved.  So I would say that the decline of these diseases was more the work of plumbers, builders and town planners rather than doctors and microbiologists.

So should we really be worrying about all of those kids with sore throats?

There is a large body of evidence to suggest that rheumatic heart disease are associated with pyoderma (skin infections) rather than pharyngitis. [McDonald, Clinical Infect Diseases 2006]  and as such – treating children with throat infections ay be a wildly misguided strategy in the first instance.  Currie et al also found a strong relationship with impetigo and subsequent post-strept GN disease.  Pharyngitis just does not seem to be particularly common in these kids.  In fact in my time in the tropics – I cannot recall a single case of “non-suppurative” disease in which the doctors were able to identify a clear, antecedent sore throat.  We just do not see this!  Although it is common to do swabs and return a “positive” – but as we know – that is expected in a good number of these kids – based on th known carriage rates.

The majority of the antecedent infections that cause non-suppurative complications fly under our radar – they are not seen by doctors, and hence do not get treated.

For example during the outbreak of ARF in  Salt Lake County, Utah in the mid-80s it was noted that at least  two thirds of the cases reported no clear antecedent sore throat. [NEJM, Veasy et al 1987]  And there was no change in the community rate of acute pharyngitis to suggest an epidemic.  They didn’t mention the rate of antecedent pyoderma!

So – in summary  – for preventing acute rheumatic fever and PSGN…

–  You’ll need to wait a long time to see a case in any first world practice – it is just really rare.

–  Antibiotics seems to reduce the rate of these complications – but we are talking about a near-infinite NNT!

–  There is a good amount of data that the infections that lead to ARF etc are (1) subclinical / not coming to our attention, (2) more likely skin infections than throat infections and (3) the result of chronic host-antigen interactions as a result of chronic exposure.

–  This is a disease of poverty.  If we want to prevent it we need primordial measures – better housing, running water and access to good food.  Doctors and antibiotics (i.e primary prevention) are unlikely to help.

[NB: secondary prevention with long-term antibiotics in children whom have confirmed ARF does benefit that high risk cohort. The Australian Heart Foundation makes it clear: "Secondary prophylaxis with regular benzathine penicillin G (BPG) is the only RHD control strategy shown to be effective and cost-effective at both community and population levels."]

How do I practice?

I live and work amongst some of the sickest kids in the developed world.  I would really love to be able to prevent as much of the morbidity that I see every day.  And yet…  I do not believe that aggressive investigation or antibiotic use will achieve this goal.

Many of my colleagues opt to give antibiotics to Aboriginal kids with a sore throat – and I think this is reasonable.  If you can identify a “high risk” cohort – then the equation tilts in favour of treatment.  I would argue that even within this group – we can further define the “at risk” kids by looking for signs of chronic illness, malnutrition and recurrent skin disease [scabies, impetigo etc].  Asking about the household – how many live there, diet, and the access to hygiene facilities etc.  Then treat this with acute and subsequent follow-up care.

In children with the usual background, “developed world” risk there appears to be no real basis to prescribing antibiotics with the aim of preventing suppurative or non-suppurative complications.

There is clinical equipoise when it comes to using antibiotics for “symptom reduction”.  The NNT was ~6 on day 3.  However, there are more effective and less harmful medications which will achieve this goal faster.  I only prescribe antibiotics after a realistic discussion about the likely risk and benefit has been had, and failed to convince the parents.   I routinely tell them to stop the antibiotics as soon as their child is improved.  Why run the risk of more adverse effects?

I give a written analgesia program – with doses and suggested times.  I see a lot of under-dosing of pain meds in this scenario.

I offer a stat dose of Dexamethasone (0.15 – 0.3 mg/kg) to kids with nasty swelling, bad pain or swallowing difficulty.

 

Final thoughts…

So that is why I do not swab, treat symptomatically and rarely prescribe antibiotics for an acute sore throat.

If we really want to improve the lot of our patients – and significantly reduce the burden of streptococcal disease – then we need a systematic approach to “primordial prevention”.  We need better houses, schools, nutrition and access to the basic services that we all take for granted in the First world.  We need plumbers, not doctors!

As always – I would be very happy to hear your thoughts.  There is a huge diversity of practice all over the world when it comes to treating kids with sore throats.  I hope that my perspective from the coal-face of post-strept sickness will give you a new perspective in your own practice.

Casey

MUBEE#13. Leggere i risultati di un studio: riduzione assoluta e relativa del rischio

 

Dott. Paolo Balzaretti, redazione Blog SIMEU

@P_Balzaretti

 

Supponiamo che dobbiate scegliere quale tra due farmaci di nuova produzione introdurre nella vostra pratica clinica. Sareste più propensi ad adottare:

a) il farmaco A, che riduce dal probabilità di morte dal 6,6% al 2,8%, o

b) il farmaco B, che riduce il rischio di morte del 57%?

Come vedremo, molti di noi, sia medici che pazienti, sono più interessati al farmaco B sebbene in realtà il farmaco A e il farmaco B siano lo stesso (un antibiotico) e quelli riportati siano i dati riguardanti l’efficacia nei pazienti con riacutizzazione di BPCO nei pazienti ospedalizzati (1).

Questo piccolo esperimento ci permette di farci una prima idea di quanto possa essere importante il modo con cui i dati vengono presentati sulle decisioni che prendiamo, motivo per cui ho pensato di dedicare all’argomento un post. Esistono 3 principali misure di efficacia degli interventi terapeutici: la riduzione assoluta del rischio (RAR), riduzione relativa del rischio (RRR) e numero di pazienti da trattare (in inglese number needed to treat, NNT). In questo post ci occuperemo delle prime due, riservando l’NNT ad un prossimo post.

Come calcolare RAR e RRR

Come si calcolano RAR e RRR? Iniziamo con la definizione di un linguaggio comune, riportato nella tabella seguente.

 

Gruppo di controllo

Gruppo di trattamento

Dimensioni del gruppo

nC

nT

N° di eventi

xC

xT

Tasso di eventi

(xC/nC) * 100

(xT/nT) * 100

 

• Riduzione Assoluta del Rischio = tasso di eventi nel gruppo di controllo – tasso di eventi nel gruppo di trattamento

• Riduzione Relativa del Rischio = ((tasso di eventi nel gruppo di controllo – tasso di eventi nel gruppo di trattamento( / tasso di eventi nel gruppo di controllo) * 100. (2)

Proviamo a riprendere l’esempio iniziale:

Gruppo di controllo

Gruppo di trattamento

Dimensioni del gruppo

273

351

N° di eventi

18

10

Tasso di eventi (%)

6,6

2,8

 

• Riduzione Assoluta del Rischio = 6,6% – 2,8% = 3,8%

• Riduzione Relativa del Rischio = ((6,6 – 2,8) / 6,6) = 57,6%

 

Entrambe i valori descrivono l’efficacia del medesimo intervento ma hanno sicuramente un impatto diverso: mentre saremmo senza dubbio propensi ad adottare un trattamento che ci viene presentato come in grado di dimezzare il rischio di morte nei pazienti con cui viene applicato, l’adozione di una terapia di cui ci viene segnalata la capacità di ridurre in termini assoluti il rischio di morire di meno del 4% probabilmente ci vedrebbe meno pronti, spingendoci a prendere in considerazione i potenziali effetti avversi, i costi e le difficoltà organizzative.

Il limite principale della RAR è che non prende in considerazione la classe di rischio di partenza (o baseline risk, ovvero il tasso di eventi nel gruppo di controllo). Per capire meglio, proseguiamo con il nostro esempio sull’impiego di antibiotici nella riacutizzazione di BPCO (1).

Se suddividiamo i pazienti in due gruppi, quelli ricoverati nei reparti di Medicina Interna e quelli ricoverati in Unità di Terapia Intensiva, avremo due risultati ben diversi:

• Medicina Interna: mortalità nei pazienti non trattati: 3,5%, RAR: 2%

• Unità di Terapia Intensiva: mortalità dei pazienti non trattati: 21,7%. RAR: 12,4%

In altri termini, nel primo caso il trattamento evita 2 morti ogni 100 pazienti trattati, nel secondo 12, dimostrando una maggiore efficacia nei pazienti più gravi. Chiaramente, si assume che la RAR per un certo outcome sia costante per tutte la classi di rischio di partenza, una proprietà valida nella maggior parte dei casi (3).

Quali conseguenze?

Non c’è da stupirsi dunque che le modalità di presentazione delle stime di efficacia nella forma di riduzione relativa del rischio garantisca giudizi più favorevoli verso un trattamento rispetto alla riduzione assoluta (4) in quanto spesso è numericamente consistente anche per rischi di base bassi. Per avere un’informazione completa, sarebbe dunque sufficiente che nei report degli studi fossero presenti sia le stime assolute di rischio e non solo quelle relative. Ciò avviene meno frequentemente di quanto si possa immaginare: un’analisi del 2006 segnalava che le stime assolute erano assenti dagli abstract del 68% degli articoli analizzati (222 lavori provenienti dalle principali riviste mediche generali); inoltre, solo il 48% di questi riportava queste misure nel resto dell’articolo (5).

Considerazioni finali

Lo scopo di questo post non è certo quello di suggerire di imparare a memoria tutti i rischi di base e i RRR per ogni patologia e ogni trattamento bensì quello di tenere sempre presente, prendendo una decisione, che, a parità di RRR, un trattamento può avere un impatto differente a seconda della classe di rischio di partenza e ciò può modificare il bilancio tra rischi e benefici (1). Inoltre, quando leggiamo un lavoro scientifico, è sempre bene ricercare la presenza di entrambe le stime di efficacia del trattamento. Nel caso sia riportata unicamente la RRR, potrebbe essere utile cercare il tasso di eventi nel gruppo di controllo per calcolare RAR autonomamente.

 

Bibliografia

1. Vollenweider DJ, Jarrett H, Steurer-Stey CA, Garcia-AimerichJ, Puhan MA. Antibiotics for exacerbations of chronic obstructive pulmonary disease. Cochrane Database of Systematic Reviews 2012; 12: CD010257. Link

2. Barratt A, Wyer PC, Hatala R, McGinn T, Dans AL, Keitz S, Moyer V, Guyatt G, for the Evidence-based Medicine teaching Tips Working Group. Tips for learners of evidence-based medicine: 1. Relative risk reduction, absolute risk reduction and number needed to treat. Can Med Ass J 2004; 171: 353-358. Link to free full text

3. Furakawa TA, Guyatt GH, Griffith LE. Can we individualize the “number needed to treat”? An empirical study of summary effect measures in meta-analysis. Int J Epidemiol 2002; 31: 72-76. Link

4. Cowey J. A meta-analysis of the effects of presenting treatment benefits in different formats. Med Decis Making 2007; 27: 638 – 654. Link

5. Schwartz LM, Woloshin S, Dvorin EL, Welch HG. Ratio measures in leading medical journal: structured review of accessibility of underlying absolute risks. BMJ 2006; 333: 1248. Link to free full text