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Semantic sMatter

We aim to bring some semantics back into critical care, so, we are going to buff up your clinical lexicon, put clarity into your vocabulary and enhance the elegance of your diction - So you can talk gooderer!

Semantic topics

Semantic sMatter #6: Study mix'n match

Semantic sMatter #5: Procalcitonin

Semantic sMatter #4: Drowning terminology

Semantic sMatter #3: The number needed to treat

Semantic sMatter #2:  Wellen's syndrome

Semantic sMatter #1: The Clagett procedure and the Eloessar flap


ICU consultant: "Yeah, we never thought the story sounded much like cholecystitis. The pattern was much more suggestive of acute alcoholic hepatitis, so we've been sticking with supportive treatment."

Well-read ICU senior registrar (He' is into FOAMed and all that kooky young person stuff!): "What's his Maddrey's discrimination factor?"


So, you've come up against an intervention for which you're a bit shaky on the evidence supporting it. What do you do? Why, jump on to PubMed or Medline of course and do a search.

You're in luck, there's an RCT. Ooh, and there's another one. And lookie here, some thoughtful soul has performed a metanalysis. You can just read this and go with the recommendations. Sorted!

But then an annoying little voice in your head says "It might not be that simple. What about heterogeneity?"


ICU Registrar: "That new fever is a pain. I suppose we'll have to send a septic screen and start some broad spectrum antibiotics. I bet it's just his pancreas though. We'll probably have to arrange a CT to look for necrotic areas for the radiologist to do a needle aspirate."

ICU Resident: "What about sending one of those PCT thingies? That'll tell us what to do, won't it?"


Can't you just hear the "Critically appraise the role of PCT" question yelling out to you? Well, read on for the low-down on that PCT thingy.


ED registrar: "Hi there. You're on for ICU outside reviews, are you?... Great. Look, we're expecting in a near drowning to the resus bay in the next few minutes. Sounds like he needed a bit of work at the pool-side and they'll be here fairly soon. Can you start coming down here to give us a hand? Thanks." *Click*

There are a plethora of terms that describe statistical outcomes out there in medical literature land. One of the more practical and clinically helpful is the Number Needed to Treat, or NNT.

Do you know what it means?


What is meant by "Number Needed to Treat" and what are the considerations when interpreting it?


The Number Needed to Treat (NNT) is an expression of the average number of patients that are needed to receive an intervention in order to prevent one additional adverse outcome, or, the number of patients that are needed to receive an intervention to have one extra patient benefit compared to a control group of similar patients. (The corollary, number needed to harm is the number of patients that are needed to receive an intervention in order to prevent one additional beneficial outcome.)

The ideal NNT is 1 => Everyone benefits with the intervention; no one benefits from being in the control group.

The NNT is the inverse of the absolute risk reduction and so is calculated by NNT = 1/Absolute risk reduction. So watch out for trials that only quote the relative risk reduction and do not at least provide the numbers to work out the absolute RR.

The NNT is both time and population dependant:

  1. If the NNT for an intervention is 10 over 5 years, then it will be 50 over 1 year and 2.5 over 20 years
  2. The NNT is what it is for the population it was calculated for. So if the outcome being measured is mortality and your population's mortality is different from that of the trial's control population, then you cannot be sure what the NNT for your population will be.


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