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Introduction: In patients with an increased bleeding tendency, extensive diagnostic blood testing is often performed. When results of tier 1 assays of primary haemostasis are normal, protocols recommend additional testing to rule out rare disorders including coagulation factor XIII (FXIII) and α2‐antiplasmin (α2AP) deficiency. Aim: To evaluate the added diagnostic value of FXIII and α2AP levels in patients with a bleeding disorder of unknown cause (BDUC). Methods: A retrospective monocentre cohort study between August 2011 and August 2023 was conducted. In all patients with bleeding tendencies and normal diagnostic tests for von Willebrand disease and platelet function, FXIII and α2AP were measured. Results: We included 158 consecutive patients; mean ISTH‐BAT scores were 8.2 (SD ± 3.7) in children, 6.2 (SD ± 2.1) in men and 10.6 (SD ± 3.3) in women. Median age was 37 (range 5–79) years, 88.6% of patients were female. Patients displayed median FXIII activity of 111% (IQR = 97–131) and median α2AP activity of 112% (IQR = 103–119). Three (1.9%) patients had FXIII levels < 50%, respectively 43%, 45% and 46%. Corresponding ISTH‐BAT scores were 7, 12 and 14. No α2AP levels < 60% was observed. No significant association was found between FXIII levels and ISTH‐BAT scores. Conclusion: In our cohort of BDUC patients, no clinical relevant FXIII deficiencies were detected; absolute values were well above the 30% cutoff considered adequate for normal haemostasis. No α2AP deficiencies were detected. These data suggest that in BDUC patients, measuring FXIII or AP activity is of limited value. [ABSTRACT FROM AUTHOR]