A 28-year-old nulliparous woman with heavy menstrual bleeding is evaluated. Coagulation studies are normal. Pelvic ultrasound shows a normal uterus and ovaries. Endometrial biopsy reveals proliferative endometrium with increased vascularity and edema but no structural lesions. Which single finding best distinguishes dysfunctional uterine bleeding (DUB) from bleeding due to coagulopathy in this patient?