PT and APTT intro
Prothrombin time
Principle
• The prothrombin time measures the clotting time of recalcified
plasma in the presence of an optimal concentration of tissue extract
(thromboplastin) and indicates the overall efficiency of the extrinsic
clotting system
• Although originally thought to measure prothrombin, the test also
depends on factors V, VII and X and on the fibrinogen concentration
of the plasma.
• In the PT test, thromboplastin (TF/calcium mixture) is added to
citrated PPP, and the time for fibrin formation is measured.
Activated partial thromboplastin time
• The test measures the clotting time of plasma after the activation of
contact factors and the addition of phospholipid and CaCl2 but without
added tissue thromboplastin and so indicates the overall efficiency of the
intrinsic pathway.
• The plasma is first preincubated for a set period with a contact activator
such as kaolin, silica or ellagic acid.
• During this phase of the test, FXIIa is produced, which cleaves FXI to FXIa,
but coagulation does not proceed beyond this point in the absence of
calcium.
• After recalcification, FXIa activates FIX and coagulation follows.
• The test depends not only on the contact factors and on factors VIII and IX,
but also on the reactions with factors X, V, prothrombin and fibrinogen
• The APTT uses two reagents: an activated partial thromboplastin and
Ca++.
• The partial thromboplastin reagent simulates activated platelet
surfaces by providing phospholipids on which enzymatic reactions in
the coagulation cascade can occur.
• In addition the reagent contains an activator (kaolin, celite,
micronized silica, or ellagic acid) that provides the negatively charged
surface for the activation of FXII and prekallikrein.
• The citrated PPP and APTT reagent are incubated at 37°C for
approximately 3 minutes.
• Activation of the contact factors occurs during the incubation.
• Next, calcium chloride is added, and the time required for a fibrin clot
to form is measured.
Thrombin time
Principle
• Thrombin is added to plasma and the clotting time is measured.
• The TT is affected by the concentration and function of fibrinogen and
by the presence of inhibitory substances.
• The clotting time and the appearance of the clot are both
informative.