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SOP

Activated Partial Thromboplastin Time (APTT)

SOP for evaluating the intrinsic and common coagulation pathways and monitoring unfractionated heparin therapy.

Last verified 1 month ago 2 min read Hematology Coagulation Laboratory Techniques #Coagulation #APTT #Clotting Time #activated partial thromboplastin time #heparin #intrinsic pathway
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Document Control
Version
1.0
Effective
Aug 29, 2026
Reviewed
Aug 29, 2026
Department
Coagulation
Principle

The APTT evaluates the intrinsic (Factors XII, XI, IX, VIII) and common (X, V, II, Fibrinogen) coagulation pathways. A contact activator (e.g., silica) provides a surface to activate Factor XII, and phospholipid substitutes for platelet factor 3. After an incubation period, calcium is added to reverse citrate and initiate the cascade. The time to fibrin clot formation is measured.

Standard Operating Procedure: Activated Partial Thromboplastin Time (APTT)

1. Purpose and Scope

To measure the time required for plasma to clot after the addition of a contact activator, phospholipid, and calcium, evaluating the intrinsic and common coagulation pathways.

2. Specimen Requirements

  • Type: Platelet-poor plasma.
  • Anticoagulant: 3.2% (0.109 M) Sodium Citrate (9:1 ratio).
  • Processing: Centrifuge at 1500-2000 × g for 15 mins within 2 hours.
  • Stability: 4 hours at room temperature.
  • Rejection: Clotted, under-filled, or hemolyzed samples.

3. Safety and Precautions

  • Standard PPE. Handle plasma as potentially infectious.

4. Equipment and Reagents

  • Automated coagulation analyzer.
  • APTT reagent (containing a contact activator like silica, kaolin, or ellagic acid, and phospholipid).
  • 0.025 M Calcium chloride solution.
  • Citrate plasma controls (normal and abnormal).

5. Step-by-Step Procedure

  1. Bring reagents and controls to room temperature or 37°C.
  2. Run two levels of QC. Verify results are within range.
  3. Load patient plasma into the analyzer.
  4. The analyzer dispenses plasma and APTT reagent, incubating the mixture (typically for 3-5 minutes) to allow contact activation.
  5. Pre-warmed calcium chloride is added to initiate clotting.
  6. The instrument detects clot formation and records the time in seconds.

6. Quality Control

  • Run QC daily and with each new reagent lot.
  • Monitor reagent open-vial stability as per manufacturer instructions.

7. Reference Ranges

  • The canonical adult reference interval for APTT is shown in the Reference Ranges panel below.

Decision Limits

  • Therapeutic heparin range: Typically 1.5-2.5 times the normal control value (or per lab-specific anti-Xa correlated range).

8. Reporting Results

  • Report APTT in seconds.
  • Flag critically prolonged values for immediate clinical notification.

Reference Ranges

Browse all ranges

25–35 seconds

Frequently Asked Questions

The incubation period allows the contact activator to fully activate Factor XII and other intrinsic pathway factors before calcium is added to start the final clotting cascade.
Common causes include under-filled citrate tubes, heparin contamination from a line draw, or a severely lipemic or icteric sample interfering with optical clot detection.