Antithrombin test

The tests carried out for the evaluation of the blood anticoagulant activity of sulfated N-carboxymethyl chitosan are the same as those for the determination of heparin, i.e. the antithrombin test for the thrombin inhibition and the heparin test for the factor Xa inhibition. Both of these tests were done by spectrophotometry at 405 nm on the p-nitroaniline liberated from a chromo-genic substrate. 

Laboratory tests for hypercoagulable states should be done only when the cause of the stroke cannot be determined based on the presence of well-known risk factors. Protein C, protein S, and antithrombin III are best measured in steady state rather than in the acute stage. Antiphospholipid antibodies are of higher yield but should be reserved for patients aged less than 50 years and those who have had multiple venous or arterial thrombotic events or livedo reticularis. 

Another inhibitor is present in plasma that can inactivate thrombin, plasmin, and, to a much lesser extent, the other proteinases as well. This inhibitor, a2-macroglobulin, inhibits by a completely different mechanism from that of the SERPINS. It entraps the proteinases in a cavity that is created by the four subunits of the ai-macroglobulin molecule. The active sites of the entrapped proteinases are sterically hindered from protein substrates but are accessible to low-molecular-weight chromogenic substrates that are used in some laboratory coagulation tests for heparin and antithrombin. 

Purified thrombin is added to plasma samples and the time for clotting is measured. This test, the thrombin time (TT), primarily reflects the concentration of fibrinogen. However, it also reflects the ability of the fibrinopeptides to be cleaved and the polymerization of fibrinogen. Separation of these three contributions requires a quantitative measurement of the concentration of fibrinogen that is not related to time for clot formation. The thrombin time can be prolonged if heparin is present in the patient s plasma sample because it will promote preferential inactivation of the added thrombin by antithrombin and reduce the amount of thrombin that can act on fibrinogen. 

The components are all mentioned in the case. Because of the current attention on APC resistance, it is almost certainly that this test would be performed first. Antithrombin is indicated as being measured first because of its simplicity compared to protein C and protein S measurements. If there is no APC resistance, thrombomodulin defects might be considered. 

Factor Vlll and V levels should be decreased in DlC, but results of these tests may be quite variable because of the systemic activation of the coagulation system. The most specific findings of DlC are a low platelet count associated with an elevated D-dimer level, fibrinopeptide A, and prothrombin 1 and 2, along with depressed antithrombin and fibrinogen levels. 

Results from a substrate assay may be reported in several different ways. For example, six different ways to report the level of the thrombin inhibitor, antithrombin III, have been used (S25). One system fiimiliar to biochemists would be nanokatals (nkat) per unit of volume. A nkat is defined as the amount of enzyme which converts one nanomole of substrate per second. The second system, which would be more fiimiliar to the coagulationist, reports results in terms of percent of normal or plasma equivalent units (PEU), one PEU being the amount of a substance present in 1 ml of a standard laboratory plasma pool drawn from normal donors. This question on reporting units becomes more important when the substrate assays are substituted in the global assays, PT and PTT. The latter clot-endpoint tests measure the total effect of many fiictors and it is known that the PT and PTT are insensitive to changes in some of the fectors over a wide range, i.e., 60-120%. This point will be discussed later in Sections 6 and 7. 

Based on the work of Magnusson and co-workers with bovine prothrombin, the sequence of -Ile-Glu-Gly-Arg- has been identified as being selective for Factor Xa (M2). The commercial substrates intended to detect this enzyme therefore contain the sequence or a close derivative (A6, A7, MI). Two areas of concern in attempting to measure Factor Xa activity in biological samples are the interference from any generated thrombin and the inhibition by antithrombin III and heparin. Because of the low concentration of Factor X in plasma, only 1/10 that of prothrombin (F6, L5), the test sample can only be diluted 40- to 50-fold and one must be more concerned over fibrin formation. Van Wijk and co-workers have recommended the use of the... 

Direct thrombin inhibitors are able to inactivate thrombin in blood by directly binding to the catalytic site of thrombin rather than by acting through antithrombin molecule. Hirudin, a naturally occurring anticoagulant, was one of the first direct thrombin inhibitors to be tested for its utility in ACS. The Global Use of... 

As shown in Table 5.1, there are over 130 of products for medical applications that have been - or are being - considered as candidates for production using transgenic systems. In this section, we will discuss those biopharmaceuticals that are currently commercially available for non-hu-man use (plant-derived avidin, trypsin, and aprotinin), and one product that has been submitted for regulatory approval, namely milk-derived antithrombin. Ten therapeutic proteins derived from transgenic systems that were (or are) included in human clinical testing will be also discussed. These products are being considered for the treatment of inherited diseases (milk-de-... 

Antinuclear antibody test Antiscleroderma antibody Anti-smooth muscle antibody Antispermatozoal antibody Anti-SS-A and anti-SS-B antibody Antistreptolysin O titer Antithrombin III Antithyroglobulin antibody Antithyroid microsomal antibody Apolipoproteins Aspartate aminotransferase Atrial natriuretic factor Basophils Bilirubin Bleeding time... 

To test the model predictions, the single-pass clearance of heparin was determined as a function of time in three different sheep. For each experiment, the volume of beads, the initial enzyme loading, the animal s antithrombin level, the hematocrit, the inlet heparin concentration, and the half-life of the enzyme were used to generate model predictions for the clearance of heparin as a function of time. 

Liberators of antithrombin and/or heparin cofactor. The latter proteins appear to be secreted by the liver, and result in hypocoagulability in thrombin titration tests and increase the sensitivity of the coagulation time and other tests to heparin. Papain releases antithrombin. ... 

Mechanism and effects Regular heparin binds to and activates endogenous antithrombin 111 (ATlIl). The heparin-ATIll complex combines with and inactivates thrombin (activated factor 11) and several other factors, especially factor X. In the presence of heparin, antithrombin III inhibits the coagulation factors approximately 1000-fold faster than in its absence. Low doses of heparin also coat the endothelial walls of vessels and reduce the activation of clotting elements by these cells. Because it acts on preformed blood components, heparin is also active in vitro—almost instantaneously. The action of heparin is monitored with the activated partial thromboplastin time laboratory test (aPTT or PTT). 

To possibly obtain a mini-hirudin retaining the highly potent antithrombin activity of full length hirudin, but lacking the susceptibility to proteolysis, we selected the best-performing amino acid exchanges tested in our previous work (i.e., ValltBug,... 

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