­­Evaluating blood sampling strategies within the SIREN study: the experience from a large cohort of healthcare workers in the UK

Background Delivering research studies that require a large number of samples to monitor specific populations is complex, often resulting in high costs and intricate logistics. We aim to describe the processes for blood sample collection and management and evaluate alternative sampling methods within a large cohort of healthcare workers in the UK (the SIREN study). Methods We conducted a process evaluation. First, we described blood sample collection and management across different study periods from June 2020 to March 2024 and how these evolved over time. Secondly, we compared alternative methods of blood sampling: venous phlebotomy (hospital-based) vs. capillary sampling (at-home). Results The main challenges with blood sampling within SIREN stemmed from the scale and use of decentralised phlebotomy across 135 hospital sites during the COVID-19 pandemic. We adapted our sampling processes as the study progressed, overcoming most of these challenges. When comparing hospital-based and at-home sampling, overall, return rates of samples taken at home were higher than site- based samples (80% vs 71%, respectively). At-home samples took less time to be returned to UKHSA Laboratory for testing compared to hospital-based samples (median 2 days; interquartile (IQ) 2-3) vs 6 days; IQ 3-8). However, at-home samples were more likely to be considered void (4%) when tested compared to hospital-based samples (0%). Cost for hospital-based sampling was almost 3-times higher than at-home sampling (£34.05 vs £11.50, respectively), however larger sample volumes were obtained via hospital-based sampling when compared to at-home sampling (8 ml vs 600 µl of whole blood). Conclusions Sample collection and management in large scale research studies are complex. Our results support at-home blood sampling as an effective and cheaper strategy when compared to hospital-based phlebotomy and therefore should be considered as alternative sampling method for future research. Trial registration number: ISRCTN11041050 - registration date 12/01/2021.