Background and Clinical Significance: Isolated prolongation of activated partial thromboplastin time (aPTT) without a bleeding tendency presents a frequent diagnostic challenge and often leads to prolonged, inconclusive evaluations. Case Presentation: We report the case of a pregnant woman with long-standing isolated aPTT prolongation in whom clinical exome sequencing enabled a definitive diagnosis. Two compound heterozygous variants in F12 were identified: NM₀00505. 4: c. 1561G>A, p. (Glu521Lys), previously reported in Factor XII deficiency, and a novel in-frame insertion, NM₀00505. 4: c. 1423₁425dup, p. (Cys475dup), absent from population databases and the prior literature. Familial genetic testing confirmed a trans configuration. Factor XII activity was markedly reduced to 1%, and mixing studies showed complete correction, consistent with coagulation factor deficiency without inhibitors. Variant interpretation using ClinGen specifications within a Bayesian framework classified both variants as likely pathogenic. Despite significant laboratory abnormalities, the patient experienced no bleeding or thrombotic complications and underwent cesarean delivery without adverse events. Conclusions: This case highlights that early integration of next-generation sequencing and quantitative variant interpretation frameworks can facilitate timely diagnosis, clarify clinical significance, and support appropriate management in patients with unexplained isolated aPTT prolongation.
Park et al. (2026) studied this question.