Transglutaminases catalyze the posttranslational modification of proteins by transamidation of available glutamine residues. This action results primarily in the formation of epsilon-(gamma-glutamyl)lysine cross-links but includes the incorporation of polyamines into suitable protein substrates as well. The covalent isopeptide crosslink is stable and resistant to proteolysis, thereby increasing the resistance of tissue to chemical, enzymatic, and mechanical disruption. The plasma transglutaminase, factor XIIIa, is formed at sites of blood coagulation and impedes blood loss by stabilizing the fibrin clot. The squamous epithelium constituting the protective callus layer of skin is formed by the action of keratinocyte transglutaminase (TGK) and epidermal transglutaminase (TGE). The tissue transglutaminase (TGC) is a cytoplasmic enzyme present in many cells including those in the blood vessel wall. TGC function is unknown, although it could function to stabilize intra- and extra-cellular molecules in a wide variety of physiologic or pathologic processes. The amino acid sequences of factor XIII, TGC, and TGK establish them as a homologous gene family and also reveal a striking homology to the erythrocyte membrane protein, band 4.2. This review summarizes the current information on structures, functions, and evolution of the most prominent members of this gene family.

The D-dimer antigen is a unique marker of fibrin degradation that is formed by the sequential action of 3 enzymes: thrombin, factor XIIIa, and plasmin. First, thrombin cleaves fibrinogen producing fibrin monomers, which polymerize and serve as a template for factor XIIIa and plasmin formation. Second, thrombin activates plasma factor XIII bound to fibrin polymers to produce the active transglutaminase, factor XIIIa. Factor XIIIa catalyzes the formation of covalent bonds between D-domains in the polymerized fibrin. Finally, plasmin degrades the crosslinked fibrin to release fibrin degradation products and expose the D-dimer antigen. D-dimer antigen can exist on fibrin degradation products derived from soluble fibrin before its incorporation into a fibrin gel, or after the fibrin clot has IntroductionFibrinogen is a soluble plasma glycoprotein that is transformed into highly self-adhesive fibrin monomers after thrombin cleavage. 1 A detailed overview of the process of fibrin formation was recently published. 2 In brief, in the first step of D-dimer formation, thrombin cleavage exposes a previously cryptic polymerization site on fibrinogen that promotes the binding of either another fibrinogen or a monomeric fibrin molecule. 3 Fibrin monomers then bind to one another in an overlapping manner to form 2 molecule thick protofibrils ( Figure 1). 4,5 Plasma remains fluid until 25% to 30% of plasma fibrinogen is cleaved by thrombin, 6 allowing time for fibrin to polymerize while simultaneously promoting thrombin activation of plasma factor XIII. 7 Thrombin remains associated with fibrin, 8 and as additional fibrin molecules polymerize, it activates plasma factor XIII bound to fibrinogen. 9 The complex between soluble fibrin polymers, thrombin, and plasma factor XIII promotes the formation of factor XIIIa before a fibrin gel is detected. 6 In the second step of D-dimer formation, factor XIIIa covalently cross links fibrin monomers via intermolecular isopeptide bonds formed between lysine and glutamine residues within the soluble protofibrils and the insoluble fibrin gel. 10 D-dimer antigen remains undetectable until it is released from crosslinked fibrin by the action of plasmin. In the final step of D-dimer formation, plasmin formed on the fibrin surface by plasminogen activation cleaves substrate fibrin at specific sites ( Figure 1). 11 Fibrin degradation products are produced in a wide variety of molecular weights, including the terminal degradation products of crosslinked fibrin containing D-dimer and fragment E complex (Figure 1). 12,13 It is uncommon to detect circulating terminal fibrin degradation products (D-dimer-E complex) in human plasma, whereas soluble high-molecular-weight fragments that contain the "D-dimer antigen" are present in patients with DIC and other thrombotic disorders. 14 These fragments may be derived from soluble fibrin before it has been incorporated into a fibrin gel, or alternatively may be derived from high-molecular-weight complexes released from an insoluble clot (Figure 2). 15,16 "D-...

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The PFA-100 system is a platelet function analyzer designed to measure platelet-related primary hemostasis. The instrument uses two disposable cartridges: a collagen/epinephrine (CEPI) and a collagen/ADP (CADP) cartridge. Previous experience has shown that CEPI cartridges detect qualitative platelet defects, including acetylsalicylic acid (ASA)-induced abnormalities, while CADP cartridges detect only thrombocytopathies and not ASA use. In this seven-center trial, 206 healthy subjects and 176 persons with various platelet-related defects, including 127 ASA users, were studied. The platelet function status was determined by a platelet function test panel. Comparisons were made as to how well the defects were identified by the PFA-100 system and by platelet aggregometry. The reference intervals for both cartridges, testing the 206 healthy subjects, were similar to values described in smaller studies in the literature [mean closure time (CT) 132 s for CEPI and 93 s for CADP]. The use of different lot numbers of cartridges or duplicate versus singleton testing revealed no differences. Compared with the platelet function status, the PFA-100 system had a clinical sensitivity of 94.9% and a specificity of 88.8%. For aggregometry, a sensitivity of 94.3% and a specificity of 88.3% were obtained. These values are based on all 382 specimens. A separate analysis of sensitivity by type of platelet defect, ASA use versus congenital thrombocytopathies, revealed for the PFA-100 system a 94.5% sensitivity in identifying ASA users and a 95.9% sensitivity in identifying the other defects. For aggregometry, the values were 100% for ASA users and 79.6% for congenital defects. Analysis of concordance between the PFA-100 system and aggregometry revealed no difference in clinical sensitivity and specificity between the systems (p > 0.9999). The overall agreement was 87.5%, with a Kappa index of 0.751. The two tests are thus equivalent in their ability to identify normal and abnormal platelet defects. Testing 126 subjects who took 325 mg ASA revealed that the PFA-100 system (CEPI) was able to detect 71.7% of ASA-induced defects with a positive predictive value of 97.8%. The overall clinical accuracy of the system, calculated from the area under the ROC curve, was 0.977. The data suggest that the PFA-100 system is highly accurate in discriminating normal from abnormal platelet function. The ease of operation of the instrument makes it a useful tool to use in screening patients for platelet-related hemostasis defects.