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Activated Partial Thromboplastin Time (APTT)
Synonyms
- APTT
- aPTT
- Partial Thromboplastin Time with Activator
- Activated PTT
- PTT (when activator used; modern term is APTT)
Units of Measurement
s (seconds), sec
Description
Activated Partial Thromboplastin Time (APTT) is a coagulation screening test that measures the time (in seconds) it takes for plasma to clot via the intrinsic and common coagulation pathways.
It evaluates the activity of the following factors:
- Intrinsic pathway: VIII, IX, XI, XII
- Common pathway: II, V, X, fibrinogen
- Also prolonged by heparin, lupus anticoagulant, factor inhibitors
The test is essential for diagnosing bleeding disorders, monitoring unfractionated heparin therapy, and detecting coagulation factor deficiencies.
Physiological Pathway Tested
APTT evaluates:
- Factor XII → XI → IX → VIII (Intrinsic Pathway)
- Factor X → V → II → I (Common Pathway)
It requires:
- Platelet-poor plasma
- Calcium
- Phospholipid
- A surface activator (kaolin, silica, ellagic acid)
Clinical Significance
Prolonged APTT
Occurs in:
1. Coagulation Factor Deficiencies
- Hemophilia A (FVIII deficiency)
- Hemophilia B (FIX deficiency)
- FXI deficiency
- FXII deficiency (no bleeding tendency)
- Fibrinogen abnormalities
- Combined factor deficiencies (liver disease, DIC)
2. Anticoagulant Therapy
- Unfractionated heparin (APTT is the monitoring assay)
- Direct thrombin inhibitors (argatroban, dabigatran)
3. Lupus Anticoagulant / Antiphospholipid Syndrome
- Prolonged APTT but increased risk of clotting, not bleeding
4. Other Conditions
- Von Willebrand disease (due to low FVIII)
- Severe sepsis
- Massive transfusion
- Vitamin K deficiency (occasionally)
- Liver disease
Shortened APTT
Occasionally seen in:
- Acute inflammation (high fibrinogen)
- Pregnancy
- Post-surgical states
- Elevated Factor VIII (acute phase)
- Disseminated intravascular clotting (early phases)
Clinical significance of shortened APTT is limited.
Reference Intervals
Typical reference range:
- 25 – 35 seconds
However, modern lab analyzers define method-specific ranges:
- 23 – 35 sec
- 25 – 38 sec
- 28 – 40 sec
Each laboratory must validate its own range based on:
- Reagent (ellagic acid, silica, kaolin)
- Coagulometer system
- Calibration and quality control data (CLSI H47-A2)
APTT in Heparin Monitoring
APTT is used to guide unfractionated heparin (UFH) dosing.
Target therapeutic range:
- 1.5 – 2.5 × normal APTT
OR - 60 – 100 seconds (varies by reagent)
Low-molecular-weight heparin (LMWH) does NOT affect APTT reliably
→ Monitor with anti-Xa assay instead.
Mixing Studies (APTT 1:1 Correction Test)
Differentiates factor deficiency from inhibitors:
Correction after mixing → Factor deficiency
- Hemophilia
- Liver disease
- Vitamin K deficiency
No correction → Inhibitor present
- Lupus anticoagulant
- Factor VIII inhibitor
- Heparin contamination
Sample Handling Requirements
Critical for accurate results:
- Use 3.2% sodium citrate
- Maintain 9:1 blood-to-anticoagulant ratio
- Avoid hemolysis
- Centrifuge to achieve platelet-poor plasma (<10,000/µL)
- Test within 4 hours
- Freeze plasma if delayed
Underfilled tubes → falsely prolonged APTT.
| Unit | Meaning |
| s | seconds |
| sec | seconds |
No conversion required.
Clinical Pearls
- APTT prolonged only, PT normal → think intrinsic pathway defect (Hemophilia A/B).
- APTT prolonged + PT prolonged → common pathway problem (DIC, liver failure, severe deficiency).
- Lupus anticoagulant prolongs APTT but increases clotting risk.
- Heparin contamination is a common cause of unexpected prolongation.
- Factor XII deficiency causes prolonged APTT but no bleeding.
Interesting Fact
APTT was developed in 1953 to improve the older PTT test by adding a surface activator - making results faster and more reproducible. It revolutionized clinical coagulation testing.
References
- Tietz Clinical Chemistry and Molecular Diagnostics, 8th Edition - Coagulation Testing.
- CLSI H47-A2 - Approved Guideline for APTT Validation.
- Mayo Clinic Laboratories - APTT Test Catalog.
- ARUP Consult - Coagulation Pathway Disorders.
- ISTH (International Society on Thrombosis and Haemostasis) Guidelines.
- NIH / MedlinePlus - Coagulation Tests Overview.