# ABO Blood Types Are Not Associated with Recurrence After the Surgical Treatment of Chronic Subdural Hematoma—A Retrospective Cohort Study

## TL;DR

This study found no link between ABO blood types and recurrence of chronic subdural hematoma after surgery, emphasizing that hematoma architecture is a better predictor.

## Contribution

The study provides new evidence that ABO blood type does not predict recurrence of chronic subdural hematoma after adjusting for clinical and radiological factors.

## Key findings

- ABO blood type showed no association with recurrence in univariate or adjusted models.
- Meta-analysis confirmed no significant link between any ABO blood type and recurrence.
- Hematoma architecture remained a strong predictor of recurrence (p < 0.001).

## Abstract

Objective: Chronic subdural hematoma (cSDH) is a common neurosurgical condition with rising incidence in the aging population. Recurrence after surgical evacuation remains frequent, affecting up to one third of patients. Prior studies have proposed an association between ABO blood type and recurrence risk, though the findings are inconsistent. This study aimed to determine whether ABO blood group independently predicts cSDH recurrence after adjusting for clinical and radiological risk factors and to contextualize these findings in the context of previously published studies. Methods: We conducted a retrospective cohort study of all consecutive patients surgically treated for cSDH at University Hospital RWTH Aachen between 2015 and 2023. Clinical, laboratory, and imaging data, including hematoma volume, laterality, and architecture, were extracted from medical records. The primary outcome was recurrence requiring reintervention. Associations between ABO blood type and recurrence were assessed using chi-square tests and multivariable logistic regression. A random-effects meta-analysis was performed to integrate our findings with all identified prior studies reporting recurrence stratified by blood type. Results: Of 630 treated patients, 488 with documented ABO blood type and recurrence status were included. Recurrence occurred in 31.1% of these patients. ABO distribution matched European population frequencies. Univariate analysis showed no association between blood type and recurrence (p = 0.434). Adjusted models likewise showed no significant differences between type A and type B (OR 1.43, 95% CI 0.67–3.00), type AB (OR 2.35, 95% CI 0.74–7.24), and type O (OR 0.95, 95% CI 0.57–1.58). Hematoma architecture remained strongly associated with recurrence (p < 0.001). A meta-analysis of available studies similarly demonstrated no association between any ABO blood type and recurrence, with pooled odds ratios near unity across comparisons. Conclusions: ABO blood type was not associated with cSDH recurrence in our cohort, and pooled evidence from previously published studies confirms the absence of a meaningful effect. Hematoma architecture and volume remain the most important predictors of recurrence. Based on these results, blood type should not influence postoperative surveillance or counseling, and future work should focus on modifiable biological and imaging-based determinants of recurrence.