Improving the availability of stem cell donors - A letter and email intervention

Last registered on February 07, 2022


Trial Information

General Information

Improving the availability of stem cell donors - A letter and email intervention
Initial registration date
February 06, 2022

Initial registration date is when the trial was registered.

It corresponds to when the registration was submitted to the Registry to be reviewed for publication.

First published
February 07, 2022, 1:43 PM EST

First published corresponds to when the trial was first made public on the Registry after being reviewed.


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Primary Investigator

University of Tuebingen

Other Primary Investigator(s)

PI Affiliation
University of Tuebingen
PI Affiliation
Johns Hopkins University, Carey Business School
PI Affiliation
University of Technology Sydney
PI Affiliation
University of Cologne

Additional Trial Information

In development
Start date
End date
Secondary IDs
Prior work
This trial does not extend or rely on any prior RCTs.
For many patients suffering from blood cancer, such as leukemia, who lack a related donor, a hematopoietic stem cell transplantation from a matching unrelated donor offers the best treatment and chance of survival. However, a substantial proportion of registered donors is unavailable to donate stem cells when invited to confirmatory typing (CT). At this decisive stage in the donation process, a matching patient has been found for the potential donor, and the donor is required to undertake medical checks to confirm genetic and medical suitability. Worldwide, unavailability ranges from about 25% to 50%. Reducing the unavailability rate can save lives, and is a key outcome of stem cell donation studies.
External Link(s)

Registration Citation

Haylock, Michael et al. 2022. "Improving the availability of stem cell donors - A letter and email intervention." AEA RCT Registry. February 07.
Experimental Details


In a natural field experiment, we randomly vary invitations to an existing activity of a major stem cell donor center, aimed at increasing the willingness of registered donors to be available at the confirmatory typing stage. The activity asks donors to fill out a health questionnaire, and to report any periods longer than three weeks that preclude donation. The experimental interventions comprise of an invitation, in form of a letter and an email, both with consistent framing, as well as the activity. We run two treatments and a baseline group. In the treatments, letters and emails are both framed so donors either belong to (i) a team of “quickly available donors,” or (ii) a team of “highly engaged” donors. The baseline group receives a letter (and email) with neither framing. The activity itself is the same for all invitees, and is voluntary. Not participating has no consequences for the donor. No one can participate in the activity who is not invited.
Intervention Start Date
Intervention End Date

Primary Outcomes

Primary Outcomes (end points)
Our primary outcome variable is a potential donor’s availability at the confirmatory typing (CT) stage.
Primary Outcomes (explanation)
CT availability measures the donor’s availability (i.e., willingness to move forward) at the confirmatory typing stage to follow through with the stem cell donation process, after a matching patient needing a stem cell transplant has been identified, based on genetic characteristics (such as HLA loci, blood group, and immunity to cytomegolavirus). More specifically, CT availability includes going to a clinic for an extra medical screening, and filling out a health questionnaire.

Secondary Outcomes

Secondary Outcomes (end points)
Our secondary outcome variables include the take-up rate of the activities and unavailability reporting behavior.
Secondary Outcomes (explanation)
The take-up rate measures whether an invited registry member has actively agreed to take part in the activity. This measure will be evaluated continually.

Absence reporting behavior measures how often participants in each experimental group reported being absent.

Experimental Design

Experimental Design
In a natural field experiment, we randomly vary invitations to an existing activity of a major stem cell donor center, aimed at increasing the willingness of registered donors to be available at the confirmatory typing stage.
Experimental Design Details
Not available
Randomization Method
For this activity, the donor center determined the frequencies of donor genotypes (for genes that are to date considered relevant for transplantation). For each genotype a certain number of donors are designated with a maximum allowed age. Thereby for more frequent genotypes the maximum age is set lower than for rare genotypes. The number of donors per genotype remains the same, but over the years people drop out of the desired age and so free spots are generated.

On a rolling basis an algorithm selects donors to fill up the free spots in the activity. For the intervention, the generation of a random number was implemented in the algorithm. Based on this number, the donor randomly receives a specific treatment for the intervention.
Randomization Unit
Individual (potential stem cell donor)
Was the treatment clustered?

Experiment Characteristics

Sample size: planned number of clusters
Sample size: planned number of observations
6,000 observations for primary outcome. Sample for analysis: We will send 133,333 invitations each year (comprising all experimental treatments), in 2022, and 133,333 in 2023, and 133,333 in 2024. In addition to our field experiment, the donor center will collect data on a control group of about 133,333 persons who are randomly not invited to the letter intervention, but share same characteristics as experimental subjects. Other data collected by the donor center: Additionally, the donor center will collect data on 133,333 invitations using the letter they used prior to the intervention, only for internal monitoring.
Sample size (or number of clusters) by treatment arms
2,000 from each experimental manipulation (plus ca. 2,000 from a control group (without invitation) constructed by the donor center).
Minimum detectable effect size for main outcomes (accounting for sample design and clustering)
For the experimental treatments, we plan to send around 400,000 invitations out over 4 years, which will in expectation lead to 6,000 CT requests in total. Estimated with past donor center data, there is a 1.5% probability of being requested within 2 years of receiving the letter. If we evaluate the results with a two-sided proportion test, and correct for multiple hypothesis testing over the two treatments and a control letter (giving alpha=0.025), we have a power of 0.8, and a minimum detectable effect size of 3.75pp (intention-to-treat), given a baseline availability rate of 80%. We also expect to add 2,000 CT requests to the data from the group that receives no letter in order to test for overall letter effects. The corresponding MDE in this case (comparing treatments to the control group that receives no letter) amounts to 4pp.

Institutional Review Boards (IRBs)

IRB Name
Ethics Committee of the Faculty of Economics and Social Sciences, University of Tuebingen
IRB Approval Date
IRB Approval Number
IRB Name
Homewood Institutional Review Board, Johns Hopkins University
IRB Approval Date
IRB Approval Number