Double Dippers in Clinical Trials: A Risk to Research Integrity and Patient Safety

Double dippers are individuals who enroll in multiple clinical trials simultaneously or repeatedly − often in violation of study protocols. In today’s evolving landscape of clinical research, ensuring data integrity and patient safety is paramount. The phenomenon of double dipping poses a serious threat to both and may affect the credibility of research outcomes.

At Fortrea, we are committed to advancing clinical research with a patient-first approach. Our recent webinar, “Double Dippers: The Secret Lives of Serial Study Subjects,” brought together industry experts to explore this complex issue and share strategies to mitigate its impact.

This blog explores insights shared during the Xtalks webinar, featuring experts from Fortrea and Verified Clinical Trials.

Why It Matters

Clinical trials are designed with rigorous protocols to ensure participant safety and generate reliable data. When individuals enroll in multiple studies:

• Adverse event rates may be inaccurately reported.
• Efficacy data can be skewed, especially in placebo-controlled or non-inferiority trials.
• Safety risks increase due to potential drug interactions or overlapping treatments.

The Scope of the Problem

Andrew Cwiertniewicz opened the discussion by defining the term "professional patient" participants who enroll in the same study more than once or violate washout periods. He shared a striking example of one patient having enrolled 8 times in one single study. He emphasized the difficulty in identifying such participants and the significant impact they can have on study timelines and costs, “We ended up spending thousands of hours over a 6 - 8 month period of time to identify that we had hundreds of duplicate subjects in a program.”

Why Do Participants Enroll in Multiple Trials?

This issue became more visible during the COVID-19 pandemic as there was a huge desperation to be vaccinated and that drove a number of people to enroll in the same trial at multiple different locations. Dr. Patrick McLeroth explained why vaccine trials are particularly vulnerable to double enrollment. These trials often involve healthy, compensated adults, widely advertised and are conducted across multiple sites within the same city. They typically feature broad inclusion criteria and fast-paced enrollment timelines, especially for seasonal vaccines.

Prevention is more effective than detection

Dr. Patrick McLeroth said during the webinar, it’s very difficult to detect unless you’re looking at source data; hence, prevention is far more effective than detection. With that in mind, several proactive strategies were discussed to reduce the risk of duplicate enrollment:

• Reducing reimbursement to lower financial incentives
• Avoiding compensation-based language in advertising
• Offering minimal payment for screening visits
• Obtaining consent to access medical records
• Incorporating objective testing methods (e.g., urine drug screens)
• Leveraging subject registries to track participation across studies

Failure to prevent double enrollment can distort trial outcomes. For instance, adverse event rates may appear falsely elevated if injection site reactions are mistakenly attributed to multiple vaccines across different trials, when in fact they stem from a single injection in one of those trials. Similarly, efficacy data can be skewed, potentially favoring non-inferiority when active medication is given across the trial arms and undermining superiority trials when placebo recipients receive active product at another site—compromising the scientific validity of the study.

The Human Side of the Story

Melissa Harris brought a compassionate lens to the discussion, urging the industry to consider the patient’s perspective: “Can you imagine being desperate enough to enroll your child for multiple studies as a cancer patient?” The patients might be unaware that multi-participation is not allowed, concerned that they are on low dose or a placebo, or facing economic hardships. Addressing these root causes is essential to designing more inclusive and ethical trials.

Melissa outlined Fortrea’s passion for incorporating the voice of the patient and site insights across the drug development paradigm. This involves designing protocols that are livable for the patient and their family to ensure fewer patients drop out or search for a new study, communicating with patient advocates, reviewing epidemiology data or real-world data to make sure that the inclusion/exclusion criteria are representative of the patient population. Lastly, replace complex protocols with compassionate and intelligent protocol design and provide health-literate education materials to ensure patient compliance.

Fortrea has a large board of sites and medical professionals, and our core objective is to bring these great leaders together across sites and industry to make an impact on how we run a clinical trial.

Collaboration

Fortrea and its collaborators, including Justin Goodarz from Verified Clinical Trials (VCT), advocate for a proactive approach to safeguard against the same subject enrolling in multiple trials. Study teams and CROs spend time and funds to try and detect these participants after screening or enrollment. Poor quality participants entered the study, resulting in faulty data with potential for increased placebo and adverse events. The VCT tool can flag duplicate enrollments by evaluating participants’ research history and study history. The responsibility to prevent double dipping requires a unified, sponsor-led commitment to prioritize patient safety, protect data integrity, design trials that are accessible, equitable and ethical.

Fortrea’s Role

At Fortrea, we believe that every patient deserves a safe, transparent, and respectful research experience. By listening to patients, empowering sites, and leveraging technology, we can build a more trustworthy and effective clinical research ecosystem. Learn more about our experience in advancing vaccine drug development.