Retention Metrics, Simplified

In my experience, most clinical trials do not suffer from significant retention issues. This is a testament to the collaborative good will of most patients who consent to participate, and to the patient-first attitude of most research coordinators.

However, in many trials – especially those that last more than a year – the question of whether there is a retention issue will come up at some point while the trial’s still going. This is often associated with a jump in early terminations, which can occur as the first cohort of enrollees has been in the trial for a while.

It’s a good question to ask midstream: are we on course to have as many patients fully complete the trial as we’d originally anticipated?

However, the way we go about answering the question is often flawed and confusing. Here’s an example: a sponsor came to us with what they thought was a higher rate of early terminations than expected. The main problem? They weren't actually sure.

Here’s their data. Can you tell?

If you can, please let me know how! While this chart is remarkably ... full of numbers, it provides no actual insight into when patients are dropping out, and no way that I can tell to project eventual total retention.

In addition, measuring the “retention rate” as a simple ratio of active to terminated patients will not provide an accurate benchmark until the trial is almost over. Here's why: patients tend to drop out later in a trial, so as long as you’re enrolling new patients, your retention rate will be artificially high. When enrollment ends, your retention rate will appear to drop rapidly – but this is only because of the artificial lift you had earlier.

In fact, that was exactly the problem the sponsor had: when enrollment ended, the retention rate started dropping. It’s good to be concerned, but it’s also important to know how to answer the question.

Fortunately, there is a very simple way to get a clear answer in most cases – one that’s probably already in use by your  biostats team around the corner: the Kaplan-Meier “survival” curve.

Here is the same study data, but patient retention is simply depicted as a K-M graph. The key difference is that instead of calendar dates, we used the relative measure of time in the trial for each patient. That way we can easily spot where the trends are.

In this case, we were able to establish quickly that patient drop-outs were increasing at a relatively small constant rate, with a higher percentage of drops coinciding with the one-year study visit. Most importantly, we were able to very accurately predict the eventual number of patients who would complete the trial. And it only took one graph!