Abstract: Objective To assess the extent to which changes in age-group and collection-site composition accounted for the annual difference, we compared repeat donation outcomes among first-time whole blood donors between 2023 and 2024. Methods A retrospective annual cohort design was used to include first-time whole blood donors in 2023 and 2024. Repeat donation was defined as any subsequent donation recorded in the blood collection information system within 365 days after the first donation. The observed repeat donation rates were compared using the chi-square test, and year-specific multivariable logistic regression models were fitted to identify factors associated with repeat donation. Direct standardization was performed using the 2023 joint distribution of age group and collection-site type as the standard weights to calculate the standardized repeat donation rate for 2024. Results A total of 218 408 first-time whole blood donors were included, comprising of 113 656 donors in 2023 and 104 752 in 2024. The observed repeat donation rate decreased from 14.27% (16 214/113 656) in 2023 to 12.05% (12 622/104 752) in 2024, an absolute decrease of 2.22 percentage points (χ²=233.490, P<0.001). The standardized repeat donation rate for 2024 was 11.50%, which was 2.77 percentage points lower than the observed rate in 2023 and 0.55 percentage points lower than the observed rate in 2024. Changes in the selected age-group and collection-site composition did not explain the annual decline; the difference was mainly reflected in an overall reduction in repeat donation rates within the age-group-by-collection-site strata. In the year-specific regression models, donors giving 300 mL had higher adjusted odds of repeat donation than those giving 200 mL in 2023, but lower adjusted odds in 2024. Donors giving 400 mL still had higher adjusted odds than those giving 200 mL in 2024. In 2024, donors recruited at fixed donation sites or mobile blood-collection vehicles had lower adjusted odds of repeat donation than those recruited through workplace-based collections, and weekday-donors had lower adjusted odds than weekend-donors. Conclusion The repeat donation rate among first-time whole blood donors was lower in 2024 than in 2023. Changes in the age-group-by-collection-site composition examined in this study did not explain the annual decline. In addition to comparing observed repeat donation rates, annual monitoring should assess changes within comparable donor groups and collection settings to guide stratification of donor retention.

Key words: First-time donor, Whole blood donation, Repeat donation, Direct standardization, Stratified management