One and Done: Steroids for Adult Asthma William T. Davis, MD, Tyler W. Barrett, MD, MSCI  Annals of Emergency Medicine  Volume 69, Issue 4, Pages 503-512 (April 2017) DOI: 10.1016/j.annemergmed.2016.11.009 Copyright © 2016 American College of Emergency Physicians Terms and Conditions

Figure The horizontal axis shows the difference between dexamethasone and prednisone in the percentage of patients experiencing asthma relapse within 10-14 days of receiving systemic corticosteroids in the ED for acute asthma. A positive difference indicates a greater relapse rate in the dexamethasone treatment group. A, A representation of the results of Rehrer et al1 with a point estimate of 2.3% more patients experiencing relapse when treated with dexamethasone. The 95% CI extends up to 8.6%, beyond the 8% difference that was specified by the authors as the noninferiority margin. Because of this (red), the strict frequentist interpretation of Rehrer et al1 is that the study failed to reject the null hypothesis of inferiority. B, Illustrates Bayesian interpretation of the data using the data from Kravitz et al8 as the prior distribution. This prior (blue line) is combined with the Rehrer et al1 data (black line) to create a posterior distribution (red dashed line) which illustrates our belief after combining prior knowledge with the current trial. Compared to the experimental data, the posterior distribution has narrowed and shifted toward no difference, providing support for the notion that dexamethasone is noninferior to prednisone. C, Similar to B, except that the prior is based both on Kravitz et al8 and Keeney et al37 (see text) which, together, represent 963 patients. Again, the posterior distribution is pulled toward no difference as compared to the data, but less so because Keeney et al37 favors prednisone slightly whereas Kravitz et al8 favors dexamethasone. Because of the large N, the prior exerts more influence on the posterior than the prior in B, so the posterior (red dashed line) looks more like the prior (blue line) than the experimental data (black line). D, Demonstrates how one can test the effect of different priors when examining data in a Bayesian manner. We use the data from C but adjust the prior to reflect prior knowledge based on 120 patients instead of 963 (effectively reducing the N informing prior knowledge by eight-fold). This reflects our uncertainty because Kravitz et al8 used a two-day dexamethasone dosing strategy and Keeney et al37 was a meta-analysis of 3 pediatric studies. The posterior distribution (red dashed line) in D is much wider than C, reflecting the increased uncertainty of our prior (blue line). The posterior distribution is still pulled in the direction of no difference, and 95% of the posterior distribution lies below the specified 8% noninferiority margin. E, Illustrates the difference between the interpretation of a CI and a Bayesian posterior interval. A 95% Bayesian posterior distribution, sometimes called a “95% credible interval,” is the region that has a 95% chance of containing the true value. Those who try to interpret a frequentist 95% CI as such are assuming that the prior is a flat line. This prior (blue line) does not change the shape of the data distribution, producing a posterior distribution (red dashed line) that is identical to the data (black line). Such a flat prior is extremely unreasonable. A flat prior means that every value is equally plausible, that there is equal likelihood of prednisone being 100 times worse than dexamethasone, 100 times better than dexamethasone, or anything in between. Annals of Emergency Medicine 2017 69, 503-512DOI: (10.1016/j.annemergmed.2016.11.009) Copyright © 2016 American College of Emergency Physicians Terms and Conditions