Date: December 17th, 2019  

Reference: Reeves and Reynolds. The NNT-WET and NNT-DRI: (Mostly) Satirical New Metrics to Emphasize the Inherent Inefficiency of Clinical Practice. AEM Dec 2019.

Guest Skeptics: Dr. Mathew Reeves is a Professor and Interim Chair of the Department of Epidemiology and Biostatistics at the College of Human Medicine at MSU.

Dr. Joshua Reynolds is an Associate Professor of Emergency Medicine at the College of Human Medicine at MSU. Outside of his academic duties, he works clinically in the adult ED at Spectrum Health in Grand Rapids, Michigan, the tertiary care center for Western Michigan.

This is an SGEM Xtra and is a result of the December AEM publication suggesting new metrics to emphasize the inherent inefficiency of clinical practice. This (mostly) satirical article seems to be in the same theme of the annual BMJ holiday edition.

The BMJ has published some great studies in their holiday edition. We have covered two of them on the SGEM:

* SGEM#6: Orthopedic Surgeons: Strong AND Smart!

* SGEM#23: A Bump Up Ahead (Diagnosis of Appendicitis)

One of my other favourite BMJ holiday edition articles has been the classic parachute trial (Smith and Pell BMJ Dec 2003). Parachutes have been used for years to prevent orthopaedic, head and soft tissue injuries after a gravitational challenge (jumping out of planes). There was observational data that showed parachute use led to injury and there were case reports of people surviving falling/jumping out of a plane without a parachute or it opening properly. They could find no randomized controlled trials (RCT) to include in their systematic review and meta-analysis (SRMA).

The authors suggested taking evidence-based medicine (EBM) advocates up in a plane and have them randomized in a double-blinded fashion to a parachute or a sham (backpack). It would be a cross over trial. Those participants who survived the first jump would be randomized into the opposite group. Only then would there be definitive evidence for the efficacy of parachutes.

Since that SRMA published in 2003, there has been a randomized control trial conducted and published on the topic of parachutes. It was published last year in the 2018 BMJ holiday edition (Yeh et al BMJ Dec 2018). It will be covered as an SGEM Xtra in 2020.

NNT: Number Needed to Treat

The NNT stands for the Number Needed to Treat. It estimates the average number of patients who need to be treated to positively impact one person with therapeutic benefit. It was originally described in 1988 by Andreas Laupacis, an internist and clinical epidemiologist who was at McMaster University in Ontario at the time. (Laupacis A et al NEJM 1988).

How is the NNT Described Mathematically?

The “number needed to do anything” is the inverse of the absolute change in risk. So in this case, the number needed to treat is the inverse of the absolute risk reduction (ARR): NNT=1/ARR

An Example of Calculating the NNT