Activated Partial Thromboplastin Time (APTT)
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Plasma is incubated with an activator (which initiates intrinsic pathway of coagulation by contact activation). Phospholipid (also called as partial thromboplastin) and calcium are then added and clotting time is measured.
(1) Kaolin 5 gm/liter: This is a contact activator.
(2) Phospholipid: Various APTT reagents are available commercially, which contain phospholipids.
(3) Calcium chloride 0.025 mol/liter.
(1) Mix equal volumes of phospholipid reagent and calcium chloride solution in a glass test tube and keep in a waterbath at 37°C.
(2) Deliver 0.1 ml of plasma in another test tube and add 0.1 ml of kaolin solution. Incubate at 37°C in the waterbath for 10 minutes.
(3) After exactly 10 minutes, add 0.2 ml of phospholipidcalcium chloride mixture, start the stopwatch, and note the clotting time.
(1) Hemophilia A or B.
(2) Deficiencies of other coagulation factors in intrinsic and common pathways.
(3) Presence of coagulation inhibitors
(4) Heparin therapy
(5) Disseminated intravascular coagulation
(6) Liver disease
(1) Screening for hereditary disorders of coagulation: Since deficiencies of F VIII (hemophilia A) and F IX (hemophilia B) are relatively common, APTT is the most important screening test for inherited coagulation disorders. APTT detects deficiencies of all coagulation factors except F VII and F XIII. PT is also performed along with APTT. Prolongation of both PT and APTT is indicative of deficiency of coagulation factors in common pathway. Normal PT with prolongation of APTT is indicative of intrinsic pathway deficiency (particularly of F VIII or IX).
(2) To monitor heparin therapy: Heparin potentiates the action of natural anticoagulant antithrombin III which is an inhibitor of thrombin and activated factors IX, X, and XI. Full dose heparin therapy needs monitoring by APTT to maintain the dose in the therapeutic range (1.5 to 2.5 times the upper reference limit of APTT).
(3) Screening for circulating inhibitors of coagulation: APTT is prolonged in the presence of specific inhibitors (which are directed against specific coagulation factors) and non-specific inhibitors (which interfere with certain coagulation reactions).