Evidence-based clinical practice uses published medical literature to improve patient care. Number Needed to Treat (NNT) is a measure of clinical effectiveness that is driving out-of-hospital practice and procedures.
The Centre for Evidence-Based Medicine provides this summary: “The Number Needed to Treat (NNT) is the number of patients you need to treat to prevent one additional bad outcome (death, stroke, etc.). For example, if a drug has an NNT of 5, it means you have to treat 5 people with the drug to prevent one additional bad outcome.”
Calculating NNT
Richard Cook and David Sackett described calculating NNT in a 1995 British Medical Journal (BMJ) article:
“The effect of treatment is usually measured by comparing the probabilities of events in the two groups of patients. Point estimates of these measures are obtained by substituting the observed rate of events for the probabilities.
“For example, the absolute risk reduction is the difference in the probabilities of an event in the control and treatment groups and is estimated as the corresponding difference in the event rates. If the event rate in the treatment group is less than that in the control group this suggests a potential benefit from the active treatment.
“Similarly, if the event rate is greater in the treatment group than the control group (negative absolute risk reduction) the active treatment may be harmful. Before recommendations can be made regarding the treatment more formal analyses of the treatment effect are needed to quantify the strength of evidence: this is done by tests of significance or confidence intervals.”
Absolute Risk Reduction is the difference in event rates between the control and treatment groups. NNT is the reciprocal of Absolute Risk Reduction. In calculating NNT, you need to have analyzed the baseline risk reduction (without therapy) and risk reduction with therapy.
Andreas Laupacis, David Sackett and Robin Roberts identified weaknesses in NNT in a 1988 New England Journal of Medicine article. The first weakness is that calculating NNT combines baseline risk and risk reduction into a single number.
“The fact that a physician must treat 11 patients in order to prevent one adverse event tells us nothing about the fate of the other 10 patients. … we are unable to identify the patients in whom therapy will have side effects,” they said.
In addition, the benefit of treatment may vary considerably due to different patient populations, trial designs and chance. These two weaknesses are important considerations when evaluating out-of-hospital clinical treatment options.
Other limitations include expressing NNT in disease-specific terms, patient compliance and the consequences of continuing therapy beyond the period of trial. These are not as important when considering out-of-hospital clinical care.
New EMS benchmarks
The U.S. Metropolitan Municipalities EMS Medical Directors Consortium referred to NNT when they released their position paper “Evidence-Based Performance Measures for Emergency Medical Services Systems: A Model for Expanded EMS Benchmarking” in the April/June 2008 issue of Prehospital Emergency Care.
Their recommendations covered six clinical areas:
1. ST-Elevation Myocardial Infarction (STEMI):
- Administration of aspirin (ASA), unless contraindicated
- Acquisition of 12-lead electrocardiograph (ECG) with appropriate, training-based interpretation by a paramedic and/or transmission to emergency physician
- Direct transport to percutaneous coronary intervention (PCI) facility with a written plan to activate cardiac catheterization team prior to EMS arrival
- Elapsed time from acquisition of diagnostic ECG (STEMI identified) to balloon inflation < 90 minutes
2. Pulmonary edema:
- Administration of nitroglycerin (NTG) unless contraindicated and prehospital Noninvasive Positive Pressure Ventilation (NPPV) to avoid endotracheal intubation
3. Asthma:
- Administration of a beta-agonist by earliest-arriving trained personnel
4. Seizure:
- Blood glucose measurement and administration of benzodiazepine by IV, IM, rectal or intranasal routes
5. Trauma:
- Limit on-scene, non-entrapment time to < 10 minutes
- Direct transport to trauma center for those meeting criteria, particularly those over 65 (with time consistent caveats for air medical transport situations)
6. Cardiac arrest:
- Response interval < 5 minutes for basic CPR and automated external defibrillators (AEDs). No response interval was specified for ALS arrival.
- In justifying its cardiac arrest recommendation, the group noted that much of the clinical research used to establish acceptable ALS response time intervals was conducted prior to the widespread dissemination of AEDs and at a time in which the compression component of CPR was not emphasized as it is now.
- As a result, the consensus group proposed that EMS systems not focus response time measurement on ALS ambulances, but rather pay greater attention to first response/BLS response time to measure what it called the “most important predictive elements for optimal outcome: time elapsed until initiation of basic chest compressions and time elapsed until defibrillation attempts.”
APPLICATION: WAKE COUNTY EMS ANNUAL REPORT
Wake County, North Carolina, used NNT as part of their annual report for Fiscal Year 2010. Of the four clinical treatments measured, pulmonary edema provided the most dramatic: “Every 6 patients treated results in the prevention of one endotracheal intubation (usually associated with admission to the hospital ICU rather than a non‐critical care unit).”
In FY 2010 Wake EMS treated 295 pulmonary edema patients, “59 ICU admissions/ventilator placements avoided.”
Transport to Level 1 trauma center was equally impressive: “Every 11 patients appropriately transported to a Level I trauma center results in the prevention of one death.” Wake transported 498, “45 patients for whom death was avoided.”
Implications for EMS administrators and managers
Number Needed to Treat is a clinical measurement that has influenced prehospital care, including the use of CPAP and going from ABC to CAB in cardiopulmonary resuscitation practice.
It can also be used to demonstrate the difference EMS is making in the community.
Duke University Medical Center Library has an online tutorial, “Introduction to Evidence Based Practice,” here: http://www.hsl.unc.edu/services/tutorials/ebm/
References
Cook RJ, Sackett DL. “The number needed to treat: a clinically useful measure of treatment effect.” British Medical Journal. 1995;310:452–454.
Gordon Guyatt, Drummond Rennie, Maureen O. Meade, and Deborah J. Cook (2008). User’s Guide to the Medical Literature: A Manual for Evidence-Based Clinical Practice, 2nd edition. American Medical Association/McGraw-Hill, ISBN 978-0-07-159036-5.
Laupacis A, Sackett DL, Roberts RS. “An assessment of clinically useful measures of the consequences of treatment.” New England Journal of Medicine 1988;318:1728-33.
J. Brent Myers, Corey M. Slovis, Marc Eckstein, Jeffrey M. Goodloe, S. Marshal Isaacs, James R. Loflin, C. Crawford Mechem, Neal J. Richmond, Paul E. Pepe. “Evidence-Based Performance Measures for Emergency Medical Services Systems: A Model for Expanded EMS Benchmarking.” Prehospital Emergency Care Jan 2008, Vol. 12, No. 2: 141–151.
Wake County EMS (2011) FY 2010 End of Year Report http://www.wakegov.com/NR/rdonlyres/D7C98B96-24AB-4A49-B212-EB396E32001E/0/EMSDepartmentInformation.pdf
Centre for Evidence Based Medicine: http://www.cebm.net/index.aspx?o=1044
