Activated Partial Thromboplastin Time (APTT)
Sample Type: Serum (Note: While "serum" is sometimes referenced, APTT is most accurately performed using plasma, specifically citrated plasma to prevent clotting during collection.)

Overview
Activated Partial Thromboplastin Time (APTT or aPTT) is a laboratory test used to assess the intrinsic and common pathways of coagulation. It measures the time it takes for a blood sample to clot after specific reagents are added. APTT is commonly used to screen for bleeding disorders and to monitor patients receiving unfractionated heparin therapy.

Purpose of the Test
Evaluate coagulation function

Detect bleeding disorders such as hemophilia A or B

Monitor anticoagulant therapy, especially with unfractionated heparin

Preoperative screening for coagulation status

Assess liver function (since the liver produces clotting factors)

Mechanism
The APTT test evaluates the intrinsic pathway of the coagulation cascade by adding an activator (e.g., kaolin), phospholipid, and calcium to the patient's plasma. This initiates the clotting process, and the time taken to form a fibrin clot is measured in seconds.

Normal Range
Reference Range: 25–35 seconds (may vary slightly by laboratory)

Prolonged APTT indicates delayed clot formation, suggesting a deficiency or inhibition of certain clotting factors.

Causes of Prolonged APTT
Congenital clotting factor deficiencies:

Hemophilia A (Factor VIII deficiency)

Hemophilia B (Factor IX deficiency)

von Willebrand disease

Acquired deficiencies or inhibitors:

Liver disease

Vitamin K deficiency

Disseminated Intravascular Coagulation (DIC)

Lupus anticoagulant

Presence of heparin

Anticoagulant therapy:

Heparin (unfractionated)

Direct thrombin inhibitors

Clinical Interpretation
APTT Result Possible Indications
Normal Normal intrinsic and common pathway function
Prolonged Clotting factor deficiency (VIII, IX, XI, XII), presence of inhibitors, heparin therapy
Shortened Hypercoagulable state (less common and not typically clinically significant)

Important Considerations
APTT is not affected by deficiencies in factors VII or XIII (extrinsic pathway).

Sample handling is crucial—improper collection or contamination can lead to inaccurate results.

Should be interpreted alongside PT (Prothrombin Time) and other coagulation studies for a complete picture.

Limitations
APTT may be falsely prolonged by:

Improper sample collection (e.g., underfilling citrate tubes)

High hematocrit

Delayed processing or incorrect storage

APTT is less useful for monitoring low molecular weight heparin (LMWH) or warfarin therapy.

Conclusion
The Activated Partial Thromboplastin Time (APTT) is a key diagnostic test for assessing the functionality of the intrinsic and common coagulation pathways. It provides critical insight into bleeding risks, coagulation disorders, and the effectiveness of certain anticoagulant treatments. Correct interpretation, in conjunction with clinical findings and other lab results, ensures accurate diagnosis and effective patient management.