Background: WhatsApp® is one of the most used apps in Brazil. One of its main features is the potential to overcome geographical, cultural, and socioeconomic disparities. This app is increasingly being used as a tool for mobile health (m-health) interventions; however, with regard to blood donation, scientific studies are still lacking. The recruitment, return, and loyalty of blood donors are public health challenges, especially in developing countries. Objective: To verify the effectiveness of WhatsApp as a tool to increase the return rates of blood donors (first-time and loyal donors). Methods: The study was carried out with 548 individuals who voluntarily went to a private blood bank in the city of Maringá, south of Brazil, to donate blood. The participants were divided randomly into an intervention group (IG) and a control group (CG). Four messages were created following a strategy of persuasive communication and sent using an automatic system (Bulk System) to the list of participants in the IG after they had donated blood. Results: In contrast to our expectations, after the intervention, no significant difference in the return rates was observed between the IG and the CG. Loyal donors, regardless of whether they received the messages or not, returned in higher numbers compared with the first-time and sporadic donors. Conclusion: The intervention using WhatsApp was not effective at increasing the blood donors' return rates, suggesting that the motivations of blood donors are different from those of people who are undergoing self-care interventions. Furthermore, the difference in the blood donors' return rates suggests that specific interventions should be created according to the different stages of the donors' careers.
Keywords: blood donation; health intervention; health promotion; m-health; persuasive communication.