Parameter which should be tested before staing heparin therapy: September 2011
Correct Answer: Activated paial thromboplastin time
Description: Ans. D: Activated paial thromboplastin time Monitoring of heparin therapy is done by either whole blood clotting time (kept 2-3 times the normal) or activated paial thromboplastin time (kept upto 1.5-2 times the normal) Paial thromboplastin time/ PTT/ activated paial thromboplastin time (aPTT/ APTT It is a performance indicator measuring the efficacy of both the "intrinsic" (now referred to as the contact activation pathway) and the common coagulation pathways. Apa from detecting abnormalities in blood clotting, it is also used to monitor the treatment effects with heparin, a major anticoagulant. It is used in conjunction with the prothrombin time (PT) which measures the extrinsic pathway. Kaolin cephalin clotting time (KccT) is a historic name for the activated paial thromboplastin time Blood samples are collected in tubes with oxalate or citrate to arrest coagulation by binding calcium. The specimen is then delivered to the laboratory. In order to activate the intrinsic pathway, phospholipid, an activator (such as silica, celite, kaolin, ellagic acid), and calcium (to reverse the anticoagulant effect of the oxalate) are mixed into the plasma sample . The time is measured until a thrombus (clot) forms. The test is termed "paial" due to the absence of tissue factor from the reaction mixture The typical reference range is between 25 seconds and 39 s (depending on laboratory). Shoening of the PTT has little clinical relevance. Normal PTT times require the presence of the following coagulation factors: I, II, V, VIII, IX, X, XI, and XII. Notably, deficiencies in factors VII or XIII will not be detected with the PTT test. Prolonged APTT may indicate: - Use of heparin (or contamination of the sample) - Antiphospholipid antibody (especially lupus anticoagulant, which paradoxically increases propensity to thrombosis) - Coagulation factor deficiency (e.g. hemophilia) To distinguish the above causes, mixing tests are performed, in which the patient's plasma is mixed (initially at a 50:50 dilution) with normal plasma. If the abnormality does not disappear, the sample is said to contain an "inhibitor" (either heparin, antiphospholipid antibodies or coagulation factor specific inhibitors), while if it does correct a factor deficiency is more likely. Deficiencies of factors VIII, IX, XI and XII and rarely von Willebrand factor (if causing a low factor VIII level) may lead to a prolonged aPTT correcting on mixing studies.
Category:
Pharmacology
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