Activated Partial Thromboplastin Time
aPTT· also: PTT, APTT
Clinical Overview
aPTT measures the time for blood to clot via the intrinsic coagulation pathway, which includes factors XII, XI, IX, VIII, X, V, II, and I. It is used to detect hemophilia A and B, monitor unfractionated heparin therapy, and screen for lupus anticoagulant.
Why This Test Matters
aPTT is the primary monitoring test for unfractionated heparin (UFH) therapy. A prolonged aPTT with normal PT isolates the defect to the intrinsic pathway — specifically factors VIII (hemophilia A) or IX (hemophilia B). Lupus anticoagulant causes a paradoxical prolonged aPTT yet increases clotting risk rather than bleeding risk.
Reference RangesWHO/IFCC standards
| Reference Range | Unit | Notes |
|---|---|---|
| 25 – 38 | seconds | Normal (not on heparin) |
| 60 – 100 | seconds | Therapeutic range on UFH |
Critical (Panic) Values
Critical High: > 120 seconds. Values outside these limits require immediate clinical attention.
What Causes Abnormal Results?
High aPTT Causes
- Unfractionated heparin therapy (therapeutic or supra-therapeutic)
- Hemophilia A (factor VIII deficiency)
- Hemophilia B (factor IX deficiency)
- Von Willebrand disease (severe type)
- Lupus anticoagulant (antiphospholipid syndrome)
- Liver disease
- DIC (disseminated intravascular coagulation)
- Factor XII deficiency (prolonged aPTT but no bleeding)
Low aPTT Causes
- Not clinically significant on its own
- Acute phase response (factor VIII is an acute phase reactant)
Signs & Symptoms to Watch For
How to Prepare for This Test
Blood must be drawn in a citrate tube and processed promptly. The citrate-to-blood ratio must be exact — underfilling the tube prolongs aPTT. Inform the lab if you are on any anticoagulants.
Factors That Can Affect Results
- Heparin contamination from arterial line flushes (falsely prolongs aPTT)
- Underfilled collection tube (falsely prolongs aPTT)
- Lupus anticoagulant (causes prolongation independent of factor deficiency)
- Sample clotting before testing (falsely shortens aPTT)
Related Topics
Frequently Asked Questions
How is aPTT used to monitor heparin?
Unfractionated heparin (UFH) prolongs aPTT in a dose-dependent manner. The therapeutic aPTT range is typically 60–100 seconds (1.5–2.5× the upper limit of normal). Hospitals use weight-based heparin nomograms to adjust dosing. LMWH (enoxaparin) and direct oral anticoagulants (DOACs) do NOT require aPTT monitoring.
What is lupus anticoagulant and why does it prolong aPTT?
Lupus anticoagulant is an autoantibody seen in antiphospholipid syndrome that binds to phospholipids in the coagulation assay, prolonging aPTT in the test tube. Paradoxically, in the body it promotes clotting rather than bleeding, causing venous and arterial thrombosis and recurrent miscarriage. It is diagnosed by a mixing study (aPTT does not correct when normal plasma is added).