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- Janna M. Journeycake, MD*
- George R. Buchanan, MD*
- *Division of Hematology-Oncology, Department of Pediatrics, The University of Texas Southwestern Medical Center at Dallas, and the Center for Cancer and Blood Disorders at Children’s Medical Center of Dallas, Dallas, TX
- APCR: activated protein C resistance
- AT: antithrombin
- DDAVP: desmopressin acetate
- DIC: disseminated intravascular coagulation
- DVT: deep vein thrombosis
- F: factor
- INR: International Normalized Ratio
- LA: lupus anticoagulant
- LMWH: low-molecular weight heparin
- PC: protein C
- PFA: platelet function analyzer
- PS: protein S
- PT: prothrombin time
- PTT: partial thromboplastin time
- TF: tissue factor
- vWD: von Willebrand disease
- vWF: von Willebrand factor
Objectives
After completing the article, readers should be able to:
Determine when factor replacement should be given to patients who have hemophilia.
Describe the major long-term problems for hemophilia patients.
List the variables that affect von Willebrand factor measurements.
Delineate the most common risk factor for the development of deep vein thrombosis in children.
Characterize the normal range of coagulation proteins according to age.
Introduction
Hemostasis is a process by which the body repairs damage to a blood vessel wall to prevent hemorrhage. At the site of injury, a hemostatic plug is formed by the interaction of the vessel wall, platelets, and coagulation factors. As the plug is formed, the coagulation system generates thrombin from prothrombin, thus initiating the formation of a fibrin clot (Fig. 1). This system is well controlled by multiple modulators and inhibitors at each step in the pathway. The major trigger for coagulation is the exposure of tissue factor (TF) in the injured vessel wall. TF binding to Factor VII (FVII), along with calcium and platelet-derived phospholipids, activates FX. FX, in conjunction with FV, prothrombin, calcium, and phospholipids, generates thrombin. Thrombin can initiate further thrombin production by the contact factor cascade (intrinsic pathway). It also converts fibrinogen to fibrin and activates FXIII to crosslink and stabilize fibrin. There are several inhibitors of this coagulation mechanism. Antithrombin (AT), heparin cofactor II, and alpha-2-macroglobulin directly neutralize the activity of thrombin. Protein C (PC), protein S (PS), and endothelial-bound thrombomodulin indirectly inhibit thrombin generation by inactivating FV and FVIII. A disorder of hemostasis can be manifested by either bleeding or thrombosis.
The coagulation system. F=factor, TF=tissue factor.
The hemostatic system is dynamic throughout childhood. Specifically, the concentrations of the vitamin K-dependent proteins (FII, FVII, FIX, FX) and contact factors (FXII, high-molecular weight kininogen, prekallikrein) are reduced in the first 6 months …
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