The CORTICUS Trial: Power, Priors, Effect Size, and Regression to the Mean

The long-awaited results of another trial in critical care were published in a recent NEJM: (http://content.nejm.org/cgi/content/abstract/358/2/111). Similar to the VASST trial, the CORTICUS trial was "negative" and low dose hydrocortisone was not demonstrated to be of benefit in septic shock. However, unlike VASST, in this case the results are in conflict with an earlier trial (Annane et al, JAMA, 2002) that generated much fanfare and which, like the Van den Berghe trial of the Leuven Insulin Protocol, led to widespread [and premature?] adoption of a new therapy. The CORTICUS trial, like VASST, raises some interesting questions about the design and interpretation of trials in which short-term mortality is the primary endpoint.

Jean Louis Vincent presented data at this year's SCCM conference with which he estimated that only about 10% of trials in critical care are "positive" in the traditional sense. (I was not present, so this is basically hearsay to me - if anyone has a reference, please e-mail me or post it as a comment.) Nonetheless, this estimate rings true. Few are the trials that show a statistically significant benefit in the primary outcome, fewer still are trials that confirm the results of those trials. This begs the question: are critical care trials chronically, consistently, and woefully underpowered? And if so, why? I will offer some speculative answers to these and other questions below.

The CORTICUS trial, like VASST, was powered to detect a 10% absolute reduction in mortality. Is this reasonable? At all? What is the precedent for a 10% ARR in mortality in a critical care trial? There are few, if any. No large, well-conducted trials in critical care that I am aware of have ever demonstrated (least of all consistently) a 10% or greater reduction in mortality of any therapy, at least not as a PRIMARY PROSPECTIVE OUTCOME. Low tidal volume ventilation? 9% ARR. Drotrecogin-alfa? 7% ARR in all-comers. So I therefore argue that all trials powered to detect an ARR in mortality of greater than 7-9% are ridiculously optimistic, and that the trials that spring from this unfortunate optimism are woefully underpowered. It is no wonder that, as JLV purportedly demonstrated, so few trials in critical care are "positive". The prior probability is is exceedingly low that ANY therapy will deliver a 10% mortality reduction. The designers of these trials are, by force of pragmatic constraints, rolling the proverbial trial dice and hoping for a lucky throw.

Then there is the issue of regression to the mean. Suppose that the alternative hypothesis (Ha) is indeed correct in the generic sense that hydrocortisone does beneficially influence mortality in septic shock. Suppose further that we interpret Annane's 2002 data as consistent with Ha. In that study, a subgroup of patients (non-responders) demonstrated a 10% ARR in mortality. We should be excused for getting excited about this result, because after all, we all want the best for our patients and eagerly await the next breaktrough, and the higher the ARR, the greater the clinical relevance, whatever the level of statistical significance. But shouldn't we regard that estimate with skepticism since no therapy in critical care has ever shown such a large reduction in mortality as a primary outcome? Since no such result has ever been consistently repeated? Even if we believe in Ha, shouldn't we also believe that the 10% Annane estimate will regress to the mean on repeated trials?

It may be true that therapies with robust data behind them become standard practice, equipoise dissapates, and the trials of the best therapies are not repeated - so they don't have a chance to be confirmed. But the knife cuts both ways - if you're repeating a trial, it stands to reason that the data in support of the therapy are not that robust and you should become more circumspect in your estimates of effect size - taking prior probability and regression to the mean into account.

Perhaps we need to rethink how we're powering these trials. And funding agencies need to rethink the budgets they will allow for them. It makes little sense to spend so much time, money, and effort on underpowered trials, and to establish the track record that we have established where the majority of our trials are "failures" in the traditional sence and which all include a sentence in the discussion section about how the current results should influence the design of subsequent trials. Wouldn't it make more sense to conduct one trial that is so robust that nobody would dare repeat it in the future? One that would provide a definitive answer to the quesiton that is posed? Is there something to be learned from the long arc of the steroid pendulum that has been swinging with frustrating periodicity for many a decade now?

This is not to denigrate in any way the quality of the trials that I have referred to. The Canadian group in particular as well as other groups (ARDSnet) are to be commended for producing work of the highest quality which is of great value to patients, medicine, and science. But in keeping with the advancement of knowledge, I propose that we take home another message from these trials - we may be chronically underpowering them.