Coagulation:- part 4 – Partial Thromboplastin Time (PTT)
Partial Thromboplastin Time (PTT)
- The blood sample is taken in the anticoagulant.
- The PTT blood sample is 1.8 ml of blood and anticoagulant (maybe ESR solution) 0.2 ml.
- Blood 1.8 ml + 0.2 ml ESR solution.
Indications for Partial Thromboplastin Time (PTT)
- Used to monitor the heparin therapy and control its dose.
- It is part of the coagulation panel workup.
- It evaluates:
- Extrinsic Pathway.
- Common Pathway.
- Advised to evaluate abnormal bleeding.
- It is recommended before the surgery.
- It evaluates factors I, II, V, VIII, IX, X, XI, and XII (1, 2, 5, 8, 9, 10, 11, 12).
Pathophysiology of Partial Thromboplastin Time (PTT)
- To understand the coagulation mechanism, go through the following diagrams, which give the various coagulation factors and their role.
- Hemostasis and the coagulation process represent the hemostatic balance which is between the clotting factors and factors encouraging clot dissolution.
- The first reaction of the body is the constriction of the blood vessels. In small blood vessels, this constriction may be enough to control the bleeding.
- In large blood vessels, a clot is needed to stop bleeding. This process will be as follows:
- The First stage is the changes in the platelets.
- The next phase is called the intrinsic system.
- Factor XII and other proteins form the complex on the subendothelial collagen in the damaged blood vessels.
- Activated factor XIa is formed, and in turn, it activates factor IXa.
- Now complex forms consist of factors VIII, IX, and X.
- Activated factor Xa enters the common pathway.
- At the same time, the extrinsic system is activated.
- There is a complex formation between the tissue thromboplastin (Factor III) and factor VII. Activated factor VIIa forms.
- VIIa can directly activate factor X.
- VIIa can also activate factors IX and X.
- When activators are added to the PTT reagent to shorten the clotting time. This process is called activated partial thromboplastin time APTT.
- The blood for APTT will be drawn 30 to 60 minutes before the next dose of heparin.
- If APTT is <50 seconds, then the dose of heparin is increased.
- When APTT is >100 seconds, it indicates an overdose of heparin.
Heparin mechanism of action:
- Heparin inactivates prothrombin (Factor II) and will prevent the formation of thromboplastin.
- It will prolong the intrinsic clotting mechanism, which may be around 4 to 6 hours between two doses of heparin therapy.
- The dose of heparin can be monitored by PTT.
PTT is a one-stage test.
- PTT serves the same function as APTT, but APTT is more sensitive.
- PTT detects the intrinsic pathway deficiency of the thromboplastin system and the common pathway.
- PTT also finds a defect in the extrinsic pathway.
- PTT screens intrinsic pathways and tests for the adequacy of factors XII, XI, IX, and VIII.
The normal value of Partial Thromboplastin Time (PTT)
- This is compared with the normal control, which may vary from lab to lab.
- Mostly with control, maybe 25 to 35 seconds.
- Another source = 30 to 45 seconds
- Patient taking anticoagulant therapy level:
- PTT level is 1.5 to 2.5 times the normal value.
Difference between PTT, APTT, and PT:
- APTT is actually PTT.
- An incomplete thromboplastin reagent and calcium are added to the patient’s plasma. The time necessary to form the fibrin clot is measured.
- The PTT reagent is only phospholipid platelets substitute without other components of thromboplastin.
- The PTT is useful in detecting intrinsic factors abnormality but is insensitive to heparin therapy.
- The APTT is sensitive to heparin therapy.
- The APTT is very sensitive to coagulation factors deficiency in the intrinsic system before the prothrombin to thrombin stage.
- APTT is also abnormal in prothrombin or fibrinogen deficiencies, but only when the defect is very severe.
- APTT is not sensitive to prothrombin abnormality as PT because the extrinsic thromboplastin used in PT is more powerful than the intrinsic system prothrombin activator complex generated by the APTT so that PT will detect even the small defect in the prothrombin.
- Platelet abnormalities don’t affect the APTT.
Effects of various anticoagulants on Partial thromboplastin test (PTT) :
|Various anticoagulants||Effects of various anticoagulants on PTT|
Abnormal PTT high level is because of:
- Liver diseases like Cirrhosis.
- Vitamin K deficiency.
- Disseminated intravascular coagulopathy (DIC).
- Heparin therapy.
- Coumarin therapy.
- Factor XII deficiency.
- Congenital factors are deficiencies as seen in:
- Von Willebrand’s disease.
- Hemophilia A and B.
Decreased PTT level:
- Early stages of DIC.
- Extensive cancers like ovarian, pancreatic, and colon.
Differential diagnoses of bleeding disorders:
|APTT||PT||Platelets count||Causes of bleeding disorders|
- PTT = >100 seconds
- APTT = >70 seconds