Wednesday, August 5, 2009

Defining sample size for an a priori unidentifiable population: Tricks of the Tricksters

During a recent review of critical care literature for a paper on trial design, a few trials (and groups) were noted to have pulled a fast one and apparently slipped it by the witting or unwitting reviewers and editors. This has arisen in the case of two therapies which have in common a targeted population in which efficacy is expected which population cannot be identified at the outset. What's more, both of the therapies are thought to be mandatory to begin early for maximal efficacy, at a time when the specific target population cannot be identified. These two therapies are intensive insulin therapy (IIT) and corticosteroids for septic shock (CSS). In the case of IIT, the authors (Greet Van den Berghe et al) believe that IIT will be most effective in a population that remains in the ICU for at least some specified time, say 3 or 5 days. That is, "the therapy needs time to work." The problem is that there is no way to tell how long a person will remain in the ICU in advance. The same problem crops up for CSS because the authors (Annane, et al) wish to target non-responders to ACTH, but they cannot identify them at the outset; and they also believe that "early" administration is essential for efficacy. The solution used by both of these groups for this problem raises some interesting and troubling questions about the design of these trials and other trials like them in the future.

An "intention-to-treat" population must be identified at the trial outset. You need to have some a priori identifiable population that you target and you must analyze that population. If you don't do that, you can have selective dropout or crossovers that undermine your randomization and with it one of the basic guarantors of freedom from bias in your trial. Suppose that you had a therapy that you thought would reduce mortality, but only in patients that live at least 3 days, based on the reasoning that if you died prior to day three, you were too sick to be saved by anything. And suppose that you thought also that for your therapy to work, it had to be administered early. Suppose further that you enroll 1000 patients but 30% of them (300) die prior to day three. Would it be fair to exclude these 300 and analyze the data only for the 700 patients who lived past three days (some of whom die later than three days)? Even if you think it is allowable to do so, does the power of your trial derive from 700 patients or 1000? What if your therapy leads to excess deaths in the first three days? Even if you are correct that your therapy improves late(r) mortality, what if there are other side effects that are constant with respect to time? Do we analyze the entire population when we analyze these side effects or do we analyze the entire population, the "intention-to-treat" population?

In essence what you are saying when you design such a trial is that you think that the early deaths will "dilute out" the effect of your therapy, much as people who drop out of a trial or do not take their assigned antihypertensive pills dilute out an effect in a trial. But in these trials, you would account for drop-out rates and non-compliance by raising your sample size. Which is exactly what you should do if you think that early deaths, ACTH responders, or early-departures from the ICU will dilute out your effect. You raise your sample size.

But what I have discovered in the case of the IIT trials is that the authors wish to have their cake and eat it too. In these trials, they power the trial as if the effect they seek in the sub-population will exist in the intention-to-treat population (e.g., ; inadequate information is provided in the 2001 study.) In the case of CSS ( ), I cannot even justify the power calculations that are provided in the manuscript, but another concerning problem occurs. First, note that in Table 4 ADJUSTED odds ratios are reported so these are not raw data. Overall there appears to be a trend toward benefit in the overall group in terms of an ADJUSTED odds ratio with an associated P-value of 0.09. But look at the responders versus non-responders. While, (AFTER ADJUSTMENT) there is a statistically significant benefit in non-responders (10% reduction in mortality), there is a trend towards HARM in the responders (10% increase in mortality)! [I will not even delve into the issue of presenting risk as odds when the event rate is high as it is here, and how it inflates the apparent relative benefit.] This is just the issue we are concerned about when we analyze what are basically subgroups, even if they are prospectively defined subgroups. A subgroup is NOT an intention-to-treat population, and if we focus on the subgroup, we risk ignoring harmful effects in the other patients in the trial, we inflate the apparent number needed to treat, and we run the risk of ending up with an underpowered trial because we have ignored the fact that patients who are enrolled who don't a posteriori fit our target population are essentially drop-outs and should have been accounted for in sample size calculations.

This is very similar to what happened in an early trial of a biological agent for sepsis ( ). The agent, HA-1A human monoclonal antibody against endotoxin, was effective in the subgroup of patients with gram negative infections, which of course could not be prospectively identified. It was not effective in the overall population. It was never approved and never entered into clinical use, because, like the investigators, clinicians will have no way of knowing a priori which patients have gram negative infections and which ones will not, so their experience with the clinical use of the agent is more properly represented by the investigation's result in the overall population.

[I am reminded here of the 2004 Rumbak study in Critical Care Medicine in which a prediction was made as to who would require 14 or more days of mechanical ventilation as a requirement for entry into a study which randomized patients to tracheostomy or conventional care on day 2. In this study, an investigator made the prediction of length of mechanical ventilation, based on unspecified criteria, which was a major shortcoming of the study in spite of the fact that the investigator was correct in about 80% of cases. See: ]

I propose several solutions to this problem. Firstly, studies should be powered for the expected effect in the overall population, and this effect should account for dilution caused by enrollment of patients who a posteriori are not the target population (e.g., ACTH responders or early-departures from the ICU.) Secondly, only overall results from the intention-to-treat population should be presented and heeded by clinicians. And thirdly, efforts to better identify the target population a priori should be undertaken. Surely Van den Berghe by now have sufficient data to predict who will remain in the ICU for more than 3-5 days. And surely those studying CSS could require a response or non-response to a rapid ACTH test as a requirement for enrollment or exclusion.

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