a) The Activated Partial Thromboplastin Time (APTT) is traditionally used for identifying quantitative and qualitative abnormalities in the contact("intrinsic") and common pathways of coagulation, for monitoring anticoagulant therapy with unfractionated heparin, and for detecting inhibitors of blood coagulation, the most common of which is the lupus anticoagulant.Although short APTTs are generally considered to be laboratory artefacts of problematic blood collections, there is mounting evidence that short APTTs may reflect a hypercoagulable state, potentially associated with increased thrombotic risk.We prospectively evaluated the phenomenon of short APTTs in 110 consecutive samples compared to an equal number of age-matched normal APTT samples.We found a significant elevation in many procoagulant test parameters in the short APTT group, in particular Factor (F) Ⅴ, FⅧ, FⅪ, FⅫ, von Willebrand factor antigen and Collagen binding assay.Interestingly, there was a significant negative association for Fibrinogen and although elevated, there was no significant association for FⅨ.We also observed a significant increase in the respective level of procoagulant phospholipids present, as assessed using a novel assay (XACT).We also identified several patients with multiple low APTTs on several sequential days, suggesting that a proportion of laboratory defined short APTTs may represent in-vivo hypercoagulability.Thus, we conclude that short APTTs reflect a hyper-coagulant milieu that could feasibly contribute to thrombotic risk.