Risk factors for vasovagal reactions in blood donors: a systematic review and meta-analysis

Background While blood donation is generally safe, some donors experience vasovagal reactions (VVRs) that may lead to injury and reduce likelihood of future donation. Several risk factors for VVRs have been identified, but the consistency, magnitude, and validity of their associations have not been systematically evaluated. Therefore, this systematic review and meta-analysis synthesised evidence for VVR risk factors. Methods Database searches identified English-language studies published before February 2024 describing VVR risk factors in voluntary whole blood donors. Study characteristics, VVR and risk factor assessment methods, and effect sizes were extracted. Random-effects models pooled estimates across all studies and subgroups of geographical context, study quality, donation experience, and outcome severity. Inconsistently and infrequently reported risk factors were narratively synthesised. Results 71 studies reporting a total of 19 million total donations were included. Female sex, new donor status, younger age, smaller blood volume, and lower blood pressure were positively associated with higher VVR risk. Donation-related fear, anxiety, and disgust were associated with higher VVR risk in narrative syntheses. Substantial between-study heterogeneity (I2 > 90%) was observed for the majority of risk factors, while there was no clear evidence of subgroup variability and small study effects. Conclusion This systematic review and meta-analysis provides a comprehensive synthesis of risk factors for VVRs across wide-ranging blood service contexts and symptom severities, reinforcing evidence for previously identified factors. The heterogeneous associations of several risk factors motivate large-scale studies that enable comprehensive multivariable adjustment to evidence donor selection criteria and preventative intervention allocation.