Partial Thromboplastin Time

Partial Thromboplastin Time

The coagulation cascade, showing the intrinsic pathway which is measured by the Partial Thromboplastin Time test.

Partial Thromboplastin Time (PTT) is a laboratory test used to evaluate the efficiency of the intrinsic pathway of the coagulation cascade. It measures the time it takes for blood to clot in response to an activator substance. PTT is an important diagnostic tool in assessing bleeding disorders, monitoring anticoagulant therapy, and evaluating liver function.

Procedure

The PTT test involves collecting a blood sample from the patient and adding an activator substance, such as kaolin or celite, to initiate the clotting process. The time it takes for the blood to clot is then measured using a coagulation analyzer. The result is reported as the number of seconds it takes for clot formation.

Interpretation

The normal range for PTT varies depending on the laboratory and the specific reagents used. Generally, a PTT value within the normal range indicates that the intrinsic pathway of the coagulation cascade is functioning properly. However, an abnormally prolonged PTT may indicate a bleeding disorder, such as hemophilia or von Willebrand disease, or the presence of an inhibitor to clotting factors.

Clinical Applications

PTT is commonly used in the diagnosis and monitoring of various conditions:

1. Bleeding Disorders: PTT is a valuable tool in diagnosing and differentiating between different types of bleeding disorders. Hemophilia A and B, which are characterized by deficiencies in clotting factors VIII and IX respectively, typically result in prolonged PTT. Von Willebrand disease, a common inherited bleeding disorder, can also be detected through an abnormal PTT.

2. Anticoagulant Therapy: PTT is used to monitor patients receiving anticoagulant medications, such as heparin. Heparin works by inhibiting the activity of clotting factors, thus prolonging the PTT. Regular monitoring of PTT levels ensures that the patient is receiving the appropriate dosage of anticoagulant therapy.

3. Liver Function: PTT can be used as an indicator of liver function. The liver produces many of the clotting factors involved in the intrinsic pathway. Therefore, liver diseases, such as cirrhosis or hepatitis, can lead to abnormalities in PTT.

Limitations

While PTT is a useful test, it has certain limitations:

1. Sensitivity to Reagents: PTT results can vary depending on the specific reagents used in the test. Different laboratories may use different activators, which can lead to variations in results. Standardization of reagents is important to ensure consistent and accurate interpretation of PTT results.

2. Lack of Specificity: PTT measures the overall efficiency of the intrinsic pathway, but it does not provide information about specific clotting factors. Additional tests, such as factor assays, may be required to identify the specific factor deficiency or abnormality.

Conclusion

Partial Thromboplastin Time is a valuable laboratory test used to assess the intrinsic pathway of the coagulation cascade. It plays a crucial role in diagnosing bleeding disorders, monitoring anticoagulant therapy, and evaluating liver function. However, it is important to consider the limitations of the test and interpret the results in conjunction with other clinical findings.

See Also

• Coagulation Cascade
• Hemophilia
• Von Willebrand Disease
• Anticoagulant Therapy
• Liver Function Tests