Activated Partial Thromboplastin Time (APTT), Partial Thromboplastin Time (PTT), Prothrombin Time (PT) and INR

Laboratory tests play a critical role in the diagnosis and management of bleeding disorders. They provide essential insights into the body’s clotting mechanism, helping healthcare professionals make informed decisions about patient care. Key tests such as Prothrombin Time (PT), Activated Partial Thromboplastin Time (APTT), and International Normalized Ratio (INR) are particularly significant. These tests evaluate the functionality of various clotting factors and monitor the effectiveness of treatments involving blood-thinning medications. By accurately diagnosing bleeding disorders and monitoring treatment progress, these laboratory tests contribute significantly to improving patient outcomes and advancing our understanding of hematology.

Why Are These Tests Performed?

1. These tests are conducted to diagnose bleeding disorders.
2. Activated Partial Thromboplastin Time (APTT) is conducted to distinguish the functionality of clotting factors I, II, V, VII, IX, X, XI, and XII.
3. APTT is utilized to monitor the treatment of patients taking heparin or other blood-thinning medications.

Collection of Sample

Venous blood samples are collected from the antecubital fossa in test tubes containing trisodium citrate (3.2%), with a blood to anticoagulant ratio of 1:9.

Precautions

1. Sample handling is crucial; incorrect ratios of blood to anticoagulant can yield false or elevated values.
2. Plasma remains stable for one hour when kept at 4º C.
3. Plasma can be preserved for 28 days if frozen.

Principles

Activated Partial Thromboplastin Time (APTT)

Activated Partial Thromboplastin Time (APTT) serves as a fundamental laboratory examination important for assessing the efficacy of the intrinsic coagulation pathway within the circulatory system. This meticulous test meticulously gauges the duration required for blood plasma to undergo clot formation when subjected to precise activators. Within the APTT protocol, plasma undergoes meticulous incubation with an activator, commonly silica or kaolin, triggering the initiation of the intrinsic coagulation cascade. Following this activation, the introduction of phospholipid (partial thromboplastin) and calcium ions into the plasma milieu further accelerates the coagulation process. The resultant clotting time is meticulously recorded, offering invaluable insights into the functionality of various clotting factors, notably factors VIII, IX, XI, and XII. APTT results, typically expressed in seconds, serve as linchpins in the diagnosis and monitoring of hemostatic disorders, in addition to facilitating the assessment of anticoagulant therapy efficacy and the evaluation of thrombotic event risks.

Plasma is incubated with an activator, initiating the intrinsic pathway of coagulation through contact activation. Phospholipid (also known as partial thromboplastin) and calcium are then introduced, and clotting time is measured.

Partial Thromboplastin Time (PTT)

This is a one-stage test distinguishing the functionality of clotting factors I, II, V, VIII, X, XI, and XII. Both APTT and PTT hold clinical significance, yet APTT is more reliable due to its heightened sensitivity.

Prothrombin Time (PT)

Prothrombin time (PT) stands as a pivotal laboratory assessment, delicately gauging the efficiency of coagulation within the bloodstream. This meticulous test measures the precise duration required for blood to transition from a liquid state to a solid clot. Its evaluation delves into the intricate workings of both the extrinsic and common coagulation pathways, meticulously scrutinizing the integrity and functionality of an array of clotting factors, notably factors I, II, V, VII, and X. During PT examination, blood plasma undergoes a meticulous fusion with tissue thromboplastin and calcium ions, igniting the complex coagulation cascade. The journey towards fibrin clot formation is charted with precision, with PT outcomes typically articulated in seconds. Prolonged PT values serve as harbingers of potential impairment in coagulation function, thereby elevating the risk of hemorrhagic complications. Beyond its diagnostic significance, PT's utility extends to the therapeutic realm, facilitating the monitoring of anticoagulant efficacy, offering insightful glimpses into hepatic performance, facilitating the pivotal diagnosis of bleeding disorders, and strategically evaluating the risks of excessive bleeding during surgical interventions.

Tissue thromboplastin and calcium are added to plasma, and clotting time is determined. This test assesses the overall efficiency of extrinsic and common pathways.

International Sensitivity Index (ISI) and International Normalized Ratio (INR)

The ISI of a specific tissue thromboplastin is established by its manufacturer, comparing it with a reference thromboplastin of known ISI. To standardize and ensure comparable results, reporting PT in the form of an INR is recommended, calculated by the formula:

INR = PT of Patient / PT of Control^ISI

Purpose of INR

• Atrial fibrillation
• Thrombophilia
• Cardiomyopathy
• Prosthesis (Replacement of heart valve)
• Venous thromboembolism
• Antiphospholipid syndrome

Normal Values

Normal values vary across laboratories due to differing techniques. Typically, normal values are:

• APTT: 30-40 seconds
• PTT: 60-70 seconds
• PT: 11-16 seconds
• INR: 1-1.5

Critical Values

• APTT: >70 seconds (Considered a panic value; spontaneous bleeding may occur if APTT exceeds 100 seconds.)
• INR: >5.0 (Expected INR value for Deep Vein Thrombosis (DVT) patients on warfarin treatment is between 2.0 to 3.0.)

Reasons for High Results

1. Disseminated intravascular coagulopathy (DIC)
2. Factor XII deficiency
3. Cirrhosis
4. Hemophilia A and B
5. Von Willebrand’s disease
6. Hypofibrinogenemia
7. Vitamin K deficiency
8. Malabsorption
9. Leukemia
10. Fibrin breakdown products
11. All congenital deficiencies of Intrinsic system coagulation factors
12. Drugs

The Significance of APTT, PTT, PT, and INR Test for the Layman

Patients taking blood-thinning medication or undergoing heparin treatment are advised to undergo these laboratory investigations.