Thromboplastins

Protein C activator pLOOD, COAGULANTS AND ANTICOAGULANTS] (Vol 4) Thromboplastin [9002-05-5]... 

Extrinsic Pathway. Coagulation is initiated when tissue extracts with Hpid—protein properties are released from the membranes of endothehal cells following injury or insult. These substances, collectively designated tissue thromboplastin, complex with circulating Factor VII and in the presence of calcium ions subsequentiy activate Factor X (Fig. 1). In vitro evidence suggests that Factor X can be activated less rapidly through the interaction of kaUikrein [9001-01-8] with Factor VII. 

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. 

The international normalized ratio (INR) is a method to standardize repotting of the prothrombin time, using the formula, INR = (PTpatie t/PTcontroi)ISI, where PT indicates the prothrombin times (for the patient and the laboratory control), and ISI indicates the international sensitivity index, a value that varies, depending upon the thromboplastin reagent and laboratory instrument used to initiate and detect clot formation, respectively. 

Most laboratories report results for the INR along with the patient s PT and the control value. The INR was devised as a way to standardize PT values and represents a way to correct the routine PT results from different laboratories using various sources of thromboplastin and methods of preparation for the test. The INR is determined by a mathematical equation comparing the patient s PT with the standardized PT value. Some institutions may use only PT, others PT/INR, and some may use INR. The INR is maintained between 2 and 3. 

Before administering the first dose of heparin, the nurse obtains the patients vital signs. The most commonly used test to monitor heparin is activated partial thromboplastin time (APTT). Blood is drawn for laboratory studies before giving the first dose of heparin to obtain baseline data (See the discussion on preadministration assessment for the oral anticoagulants.)... 

Hematological Effects. Leukocytosis and decreased platelet counts were reported in a group of subjects shortly after they ingested an unknown amount of endosulfan (Blanco-Coronado et al. 1992). One subject from that study, who eventually died, had prolonged partial thromboplastin time and prothrombin time with thrombocytopenia, and decreased fibrinogen two days after being admitted to the hospital. Elevated white cell count was also observed in an additional case of fatal acute poisoning with... 

Antihemophilic factor B, Christmas factor, plasma thromboplastin component (PTC)... 

A number of laboratory tests are available to measure the phases of hemostasis described above. The tests include platelet count, bleeding time, activated partial thromboplastin time (aPTT or PTT), prothrombin time (PT), thrombin time (TT), concentration of fibrinogen, fibrin clot stabifity, and measurement of fibrin degradation products. The platelet count quantitates the number of platelets, and the bleeding time is an overall test of platelet function. aPTT is a measure of the intrinsic pathway and PT of the extrinsic pathway. PT is used to measure the effectiveness of oral anticoagulants such as warfarin, and aPTT is used to monitor heparin therapy. The reader is referred to a textbook of hematology for a discussion of these tests. 

Anticoagulant or heparin use <48 h before onset with elevated partial-thromboplastin time Prothrombin time >15 s Platelet count <100,000/mm ... 

FIGURE 7.2 LMWH inhibits factor Xa and minimally affects factor Ha thus, activated partial thromboplastin time is not used to measure its anticoagulant activity. (Reprinted from the American Family Physician published by the American Academy of Family Physicians, February 15th, 1999, in an article entitled Low-molecular-weight heparin in outpatient treatment of DVT. )... 

Patient is coagulopathic or has received heparin within past 48 h and has an elevated activated partial thromboplastin time (aPTT)... 

Check complete blood count, prothrombin time, international normalization ratio, activated partial thromboplastin time, fibrinogen levels... 

Baseline complete blood count (CBC) and coagulation tests (activated partial thromboplastin time and International Normalized Ratio) should be obtained, as most patients will receive antithrombotic therapy, which increases the risk for bleeding. 

---