Choosing the right outreach channel for clinical trial recruitment is not a theoretical question. It directly affects how quickly you hit enrollment targets, what it costs to get there, and how representative your participant pool ends up being. Yet many recruitment strategies default to whichever channel the team used last time rather than matching the channel to the population, the study design and the compliance requirements.
This article breaks down the three main outbound outreach channels used in patient recruitment, SMS, telephone and direct mail, across the metrics that matter most: response rates, speed, cost per qualified participant, audience reach and compliance.
SMS recruitment
How it performs
SMS is the highest-response outbound channel available for patient recruitment. Research published in Applied Clinical Trials found that text message recruitment can generate response rates of more than 30%, compared to the 5% to 10% typically seen with traditional letter-based outreach.
The mechanics behind that gap are straightforward. Additional research from Applied Clinical Trials suggests that 98% of text messages are read, and 90% are read within the first three minutes of receipt. Email, by comparison, is read by roughly 22% of recipients.
In the United States, a case study involving vaccine trials at Johnson County Clin-Trials, switched from email to two-way SMS and produced five times the response rate with fewer contacts sent. Enrolment results exceeded the trial sponsor’s expectations.
Response is also fast. Where mail campaigns take weeks to generate replies and phone campaigns require staffing and call-centre scheduling, SMS responses arrive within minutes of sending. One CRO using SMS for Phase I recruitment saw a 32% increase in response rates and found it necessary to stagger text messages to manage the volume of incoming interest.
Who it reaches well
SMS performs strongly across most age groups under 65 and is particularly effective for studies needing fast recruitment at scale. It is less reliable for older populations, where mobile phone use and comfort with text-based communications varies more significantly.
Compliance considerations in Australia
In Australia, SMS outreach for clinical trial recruitment must be conducted using opted-in data and must comply with the Spam Act 2003 and ACMA requirements. All messages must identify the sender, include a functional opt-out mechanism and be sent only to contacts who have provided consent for marketing or research communications. Recruitment materials used alongside SMS outreach require HREC approval before deployment.
Cost profile
SMS is generally the lowest cost per contact of the three channels, with minimal per-send costs at scale. The high read rate means cost per response is significantly lower than email or untargeted broadcast channels, though the cost per qualified, screened participant will depend on the eligibility criteria and screening conversion rate specific to your study.
Telephone outreach
How it performs
Telephone outreach produces lower initial response rates than SMS but delivers something the other channels cannot: a direct conversation. That conversation is often what converts an interested contact into a screened, consented participant, particularly for studies involving complex eligibility criteria or procedures that require explanation.
Research on outreach for a pragmatic advance care planning trial found that including phone outreach alongside mailed surveys lifted the overall response rate to 20%, with 35% of all completed responses coming from participants who only engaged after a phone call following an unreturned mail piece.
Phone outreach also reached a more diverse participant pool. Participants recruited via phone compared to mail alone were more socially vulnerable, more likely to identify as a racial or ethnic minority, and more likely to be non-English speaking. For sponsors with diversity and inclusion requirements in their protocol, this is a meaningful finding.
Research published by Cochrane-affiliated researchers found that phone calls following unreturned postal invitations successfully converted hesitant candidates into active participants, and that direct human contact is one of the most reliable methods for improving recruitment in trials where initial outreach has underperformed.
Who it reaches well
Telephone outreach is particularly effective for older demographics, populations with lower digital engagement, studies with complex inclusion criteria that benefit from verbal explanation, and as a follow-up channel to convert contacts who did not respond to a mail or SMS first touch.
Compliance considerations in Australia
All telemarketing lists used in Australia must be washed against the Do Not Call Register before use, in accordance with the Do Not Call Register Act 2006. Exemptions apply to certain research and survey calls, but the scope of those exemptions should be confirmed with your legal team and HREC before any outbound calling program begins.
Cost profile
Telephone is the most resource-intensive of the three channels. It requires trained callers, call centre capacity or site staff time, and a higher cost per contact than either SMS or mail. However, when measured against qualified enrolments rather than raw contacts, phone outreach can be cost-effective in studies where the conversion from initial contact to screened participant is higher than other channels.
Direct mail
How it performs
Direct mail produces lower raw response rates than SMS but consistently outperforms untargeted digital channels. A head-to-head comparison published in Applied Clinical Trials found a 3.6% response rate for third-party direct mail versus 0.5% for third-party email, with a cost per interested individual of $30 for mail versus $100 for email.
A postal outreach study using electronic medical records to identify participants and direct mail for outreach found the approach cost-effective and noted it increased participation from under-represented groups, making it a practical tool for sponsors with diversity requirements.
Direct mail is slower than SMS and phone in terms of time to response. Campaigns require print and production lead time, and response cycles are measured in weeks rather than hours. However, for studies targeting older populations, households in regional and rural areas, or participants unlikely to be reached via digital channels, mail remains one of the most reliable first-contact methods.
Who it reaches well
Direct mail reaches populations that SMS and email do not. Older Australians, regional and rural households, and demographics with lower smartphone penetration all engage more reliably with physical mail. For studies targeting these groups specifically, mail is often the primary channel rather than a fallback.
Compliance considerations in Australia
Direct mail must comply with the Privacy Act 1988 and the Australian Privacy Principles governing the use of personal information for marketing purposes. All postal data should be sourced from opted-in, verified consumer records. Recruitment mail pieces require HREC approval before distribution.
Cost profile
Direct mail has a higher cost per contact than SMS, driven by print, production and postage. However, when the list is accurately targeted, the cost per qualified response is competitive. One study found direct mail at $30 per interested respondent compared to $100 for email, making targeted mail considerably more cost-effective than untargeted digital outreach for the right population.
Channel comparison summary
| SMS | Phone | Direct mail | |
|---|---|---|---|
| Read/open rate | ~98% | N/A (live contact) | Variable |
| Response rate | 30%+ | Moderate, higher with follow-up | 1.5%–3.6% |
| Speed to response | Minutes | Immediate | Weeks |
| Best audience | Under-65s, high digital engagement | Complex criteria, older adults, minority groups | Older adults, regional, low digital engagement |
| Cost per contact | Low | High | Medium |
| Compliance requirement | Spam Act 2003, ACMA opt-in | Do Not Call Register Act 2006 | Privacy Act 1988 |
Which channel should you use?
There is no single answer. The right channel depends on your target population, your study timeline, your budget and what your HREC has approved. For most studies, a multi-channel approach outperforms any single channel used in isolation. SMS generates fast, high-volume initial responses. Direct mail reaches audiences SMS misses, particularly older and regional populations. Phone outreach converts hesitant contacts and reaches under-represented groups who do not respond to written outreach.
Using direct mail as a first contact, followed by phone for non-responders, is a well-evidenced sequencing strategy. SMS works well as a fast-activation channel for studies needing rapid enrolment or as a supplement to existing patient databases. The quality of your outreach list underpins the performance of all three channels. An accurately targeted, regularly updated, opted-in consumer list gives every channel a better starting point and reduces the cost per qualified enrollment regardless of which channel you lead with.
Lead Lists supplies opted-in, compliant Australian consumer data for direct mail, telemarketing and SMS outreach, drawn from a database of 21 million+ verified records. All telemarketing lists are washed against the Do Not Call Register before delivery. We can build targeted lists filtered by age, gender, location, household type and socio-economic profile to support participant outreach campaigns where direct-to-consumer recruitment is part of your approved approach.
Our data cleansing service can also be used to clean and update existing participant databases before your next recruitment campaign.
Get in touch to discuss data options for your study.
Disclaimer: This article is intended as general guidance and does not constitute legal, regulatory or ethical advice. Recruitment activities must be conducted in accordance with your study’s HREC approval and applicable Australian regulations.
