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Life in the Fast Lane • LITFL • Medical Blog

Emergency medicine and critical care medical education blog

Critical Care Compendium | Risk and Numbers Needed to Treat

Risk and Numbers Needed to Treat

by Chris Nickson, Last updated August 28, 2015

Revised and reviewed 26 August 2015

OVERVIEW

  • Risk is a nebulous term in evidence-based medicine, as it may refer to either relative or absolute risks
  • Overall, absolute risks are more meaningful and can be converted in ‘numbers need to treat’ (NNT), which are useful in the clinical setting

RISK

Absolute Risk

  • the actual event rate in the placebo or treatment group

Relative Risk

  • the ratio of the incidence of disease among exposed to the incidence among non-exposed
  • a measure of the strength of an association between groups
  • prospective studies (RCTs and cohort studies)
  • also called the incidence risk

RR = risk of disease in the exposed (a/a+b) / risk of disease in the non-exposed (c/c+d)

  • RR of 3 -> there is three times the risk
  • RR of 0.5 -> the risk is halved
  • RR of 1 -> there is no association
  • if the RR is reported with a CI that includes 1 -> then the RR is not significant.

Relative Risk Reduction

  • expressed as a percentage reduction in events in treated vs untreated groups
  • = 1 – (incidence in exposed / incidence in unexposed)

Attributable Risk

  • a measure of association that provides information about the absolute effect of the exposure or excess risk of disease of those exposed compared to unexposed

Absolute Risk Reduction (ARR)

  • incidence in exposed – incidence in unexposed
  • a measure of treatment effect
  • reverse of attributable risk

NUMBER NEEDED TO TREAT

  • NNT is the number of patients who need to be treated in order to avoid one adverse event, which is the reciprocal of the absolute risk reduction
  • NNT gives the RR some relevance in terms of the magnitude of clinical effect
  •  ie. if the incidence of an adverse event = 0.06% & relative risk reduction = 0.33 the absolute risk reduction -> 0.02% and thus numbers needed to treat = 1/0.0002 = 5000
  • however, if the incidence of an adverse event was 6% the absolute risk reduction -> 2% and thus the NNT = 1/0.02 = 50

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About Chris Nickson

FCICM FACEM BSc(Hons) BHB MBChB MClinEpid(ClinTox) DipPaeds DTM&H GCertClinSim

Chris is an Intensivist at the Alfred ICU in Melbourne and is an Adjunct Clinical Associate Professor at Monash University. He is also the Innovation Lead for the Australian Centre for Health Innovation and the Chair of the Australian and New Zealand Intensive Care Society (ANZICS) Education Committee. He has a passion for helping clinicians learn and for improving the clinical performance of individuals and collectives. After finishing his medical degree at the University of Auckland, he continued post-graduate training in New Zealand as well as Australia's Northern Territory, Perth and Melbourne. He has since completed further training in emergency medicine, clinical toxicology, clinical epidemiology and health professional education. He coordinates the Alfred ICU's education and simulation programmes and runs the unit’s education website, INTENSIVE. He created the 'Critically Ill Airway' course and teaches on numerous courses around the world. He is one of the founders of the FOAM movement (Free Open-Access Medical education) and is co-creator of Lifeinthefastlane.com, the RAGE podcast, the Resuscitology course, and the SMACC conference. His one great achievement is being the father of two amazing children. On Twitter, he is @precordialthump.

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