Thromboplastin time assay

Several of the PK/PD models described in Chapter 19 have been employed to explore the relationship between circulating protein concentrations and pharmacodynamic endpoints. For example, a dog model of hemophilia was used to study the activity of recombinant FIX (79). Activity was determined in a bioassay, a modified one-stage partial thromboplastin time assay with pooled human plasma as the internal standard. As shown in Figure 32.14, the relationship between activity and recombinant FIX (BeneFIX) concentration was linear (r = 0.86), suggesting that for every 34.5 ng/mL of FIX, there would be a corresponding 1% increase in FIX activity. In 11 males with hemophilia B, it was necessary to use a sigmoid Emax... 

Activated partial thromboplastin time (aPTT) is a coagulation assay, which measures the time for plasma to clot upon activation by a particulate substance (e.g., kaolin) in the presence of negatively charged phospholipids. 

II Umbilical cord, joint, and mucosal tract bleeding Prolonged prothrombin (PT) and activated partial thromboplastin time (aPTT) Specific assay for prothrombin... 

Treatment options are largely supportive. An assessment should first be made for airway patency and adequacy of breathing. Circulation may become affected as shock develops secondary to severe gastroenteritis. The following laboratory studies are recommended for all symptomatic patients computerized blood count, electrolytes, and coagulation studies (prothrobin time, activated partial thromboplastin time). In cases of uncertain or unknown exposure, there is an enzyme-linked immunosorption assay test available for the detection and verification of the presence of ricin... 

Ingram, G. I. C., Comparative assays of factor VIII by thromboplastin generation test and partial thromboplastin time techniques. In The Hemophilias, Intern. Symp., Washington, D.C., 1963 (K. M. Brinkhous, ed.), pp. 20-28. Univ. of North Carolina Press, Chapel Hill, North Carolina, 1964. 

Accepting the above limitations of our knowledge of the true in vivo reaction sequence, a series of clot-endpoint tests has been used for many years to determine the hemostatic balance in any patient. The first, known as the thrombin time test, can be used to measure the concentration of fibrinogen (B15). The second assay, the activated partial thromboplastin time test (APTT), measures the components of the intrinsic pathway (M4). These include the serine proteases (Factors XII, XI, X, and II), the cofactors (Factors VIII and V) and again, fibrinogen. Most of the hereditary deficiencies... 

Anticoagulant Activity Assay. The anticoagulant activities of IV-acetylated heparin and all further derivatized heparins were determined based on activated partial thromboplastin time (APTT) assay methods (15). Bovine blood was collected in 3.8% sodium-citrated (9 parts blood to 1 part citrate solution) and centrifuged at 5000 x g for 15 min. The supernatant plasma was collected and pooled for subsequent APTT testing. Prior to testing plasma was kept refrigerated no longer than 6 h after... 

In healthy volunteers orally administered garlic capsules at the manufacturer s recommended dose (amount not specified) daily for two weeks, no effects on platelet function or other hematological parameters were observed, including prothrombin time, partial thromboplastin time, thrombin time, bleeding time, the collagen/epinephrine assay, or the collagen/adenosine diphosphate assay. Aspirin was used as a positive control and markedly inhibited platelet function (Beckert et al. 2007). 

Heparin resistance is defined as the requirement of more than 35,000 units in a 24-h period to maintain a therapeutic activated partial thromboplastin time. In some cases, elevated factor VIII levels can factitiously lower the aPPT without influencing heparin activity measured by anti-Xa assays, leading to a misdiagnosis of heparin resistance. A case of apparent heparin resistance due to increased factor VIII levels in an elderly male with infective endocarditis was reported [18 ]. 

The prothrombin time (PT), as proposed by Quick, is the most commonly performed coagulation function test. It is used to monitor oral anticoagulant therapy and also as a preoperative screening test to warn of possible bleeding risk in patients with a personal or family history of bleeding (Table 36-2). Measured clotting times are extremely dependent on the animal and tissue source and the quality of the thromboplastin used. Variability can be expected because of the assay s dependence on the number of tissue factor molecules and the quantity of... 

Specific factor assays are variations on the APTT or PT tests. In the APTT and PT, dilutions of the patient s plasma are made into a deficient, or depleted, substrate plasma. The assays are then performed in the usual way. The clotting times are compared with those obtained from dilutions of pooled normal plasma, commonly 1 10, 1 20, 1 50, and 1 100. A graph of the logarithm of the clotting time (y axis) versus the logarithm of the concentration as percentage of normal x axis) is used to determine the amount of the factor activity in the patient s plasma. The normal pooled plasma is conventionally assigned a value of 100% activity. Many variations exist for specific factor assays, e.g., the venom of Vipera russellii and phospholipids may be substituted for thromboplastin in a PT-like assay. An enzyme in Russell s viper venom rapidly and relatively specifically activates factor X. In conjunction with a factor X-deficient substrate plasma, this provides a specific factor X assay. 

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