How Can I Measure Clinical Trial Recruitment by Enrolled Participants?

How do I tell whether a clinical trial recruitment campaign is bringing in qualified participants?

A campaign brings in qualified participants when it moves people who meet the study criteria through screening and consent into enrollment. Start with one funnel and give every stage a consistent definition: ad click or referral, completed screener, eligible candidate, consented participant, and enrolled participant.

The difference between these stages matters. An eligible candidate has passed the criteria measured at screening; a consented participant has agreed to join; an enrolled participant has completed the study’s enrollment requirements. Report each count separately. Otherwise, a large pool of clicks can look like success while the study team is still waiting for usable participants.

A 2021 Facebook campaign for a telehealth knee osteoarthritis trial illustrates the gap. The Journal of Medical Internet Research reports 33,319 ad viewers and 9,879 clicks, but only 423 completed an online screener. Seventy people were deemed eligible, and 32 signed informed consent. The authors described a steep drop-off from clicks to enrollment.

Those counts point to separate questions. Did the ad attract the intended audience? Did the screener get completed? Did eligible candidates consent? A campaign can perform well at one stage and poorly at the next. The useful diagnosis is the stage where the count falls, not a single headline rate.

Eligibility criteria, protocol demands, and site capacity shape that pattern. A campaign can deliver interested people who fail a key criterion, while a site with limited capacity can slow the movement from consent to enrollment. Record the reason for screen failure where available, and distinguish a marketing problem from a protocol or operational bottleneck.

What metrics should I track beyond clicks and form submissions?

Track conversion, quality, cost, and time at each funnel stage. Clicks and form submissions measure activity; they do not establish eligibility, consent, or enrollment.

Define the denominator before comparing rates. For example, completed-screen rate can mean completed screeners divided by clicks; eligibility rate can mean eligible candidates divided by completed screeners. Keep those definitions fixed across reports, sites, and channels. A rate without its denominator is hard to interpret.

Published results show why cost needs an outcome attached. In a study of 18–25-year-olds, Translational Behavioral Medicine reported screening costs of US$9 per person on Instagram, US$8 on Facebook, and US$178 on Twitter. Cost per enrolled participant was US$16 for Facebook, US$23 for Instagram, and US$678 for Twitter. Unpaid staff posts or referrals cost US$1 per person screened. Those are results from that study, not universal benchmarks; the study also found differences in enrollment and participant representativeness across approaches.

Use the metrics as a linked set. A low cost per screener has limited value if few screeners qualify. A strong eligibility rate does not settle performance if eligible people do not consent. The final cost per enrolled participant keeps the whole funnel visible, while stage-level rates show what to fix.

How much does it cost to recruit one clinical trial participant through digital ads?

There is no single digital-ad cost per enrolled participant in the available studies; reported costs vary by study and targeting approach. Use published figures as examples of variation, then calculate the comparable cost for your own campaign using actual enrolled participants as the denominator.

In the knee osteoarthritis campaign, the Journal of Medical Internet Research reported US$51.94 per participant who signed informed consent. Consent is the reported outcome, so it should not be relabelled as cost per final enrollee without confirmation that the measures match.

A cancer-survivor trial shows a different cost picture. In Clinical Trials, Facebook ads produced 4,401 clicks from 3 million impressions, followed by 24 contacts with study staff, six prescreenings, and four enrollments. The reported Facebook cost was US$794 per enrollee for general targeting and US$1,426 when targeting strategies focused on African Americans or men. Other methods in that study also had different costs per enrollee: US$917 for community fairs, US$799 for direct mail, and US$436 for periodicals.

These figures cannot be ranked as if they came from one standardized test. The trials, audiences, targeting, and recruitment methods differed. Their shared lesson is narrower and more useful: report spend against the same endpoint. Cost per click describes traffic. Cost per enrollee describes recruitment yield.

Calculate channel cost by dividing the channel’s attributable recruitment spend by its enrolled participants over a stated period. Report cost per qualified candidate separately, and state whether the endpoint is eligibility, consent, or enrollment. Include recruitment time: a channel that produces participants slowly may not meet a site’s schedule even when its final cost appears favorable.

Compare sites and channels using the same funnel rules, but do not assume their conditions are identical. In decentralized CHIEF-HF, a Journal of Cardiac Failure report found site opt-in rates ranging from 0.05% to 16.4%. At the highest-enrolling site, opt-in was 7.8% with electronic medical-record portal messages versus 4.4% with email alone. Variation was substantial even within one trial.

Recruitment pace also belongs in the comparison. A review in BMJ Open found that 63% of 388 publicly funded randomized trials reached their final recruitment target, and 32% of those trials needed an extension. The median recruitment rate was 0.95 participants per centre per month. These findings are a reminder to pair campaign conversion and cost with site-level capacity and time to enrollment.

The practical scorecard is simple: show counts at every stage, stage conversion rates, spend per qualified candidate, spend per enrollee, and time to enrollment. A channel earns its place by bringing the right participants through the whole process at a cost and pace the study can support.