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How does the activated partial thromboplastin time test assess the intrinsic clotting pathway?
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bleeding and blood clotting
Red blood cells (erythrocytes) trapped in a mesh of fibrin threads. Fibrin, a tough, insoluble protein formed after injury to the blood vessels, is an essential component of blood clots.
The activated partial thromboplastin time (aPTT) test assesses the intrinsic pathway of coagulation by measuring the time required for a fibrin clot to form under controlled laboratory conditions.1
The test procedure functions as follows:
- Preparation: Plasma is collected and treated with a citrate buffer, which removes functional calcium ions, preventing spontaneous clotting.1
- Activation: A negatively charged substance, such as kaolin, is added to the plasma to activate factor XII into factor XIIa. This initiates the intrinsic cascade, but the process is halted at the activation of factor IX because calcium ions are absent.1
- Measurement: Calcium ions and a phospholipid preparation (acting as an artificial membrane) are added to the mixture. The laboratory then records the time it takes for a visible clot to form.1
By comparing the clotting time of the test plasma to that of normal plasma, clinicians can identify potential issues. A prolonged aPTT indicates delayed clotting, which may result from a deficiency in one or more clotting factors or the presence of a chemical inhibitor.1