Activated Partial Thromboplastin Time (aPTT)
aPTT measures the intrinsic + common clotting pathways. The classic test for monitoring heparin therapy and screening for haemophilia and similar factor deficiencies.
What it measures
aPTT is a coagulation screening test that times how long plasma takes to clot when exposed to a phospholipid surface activator. It tests the intrinsic pathway (factors XII, XI, IX, VIII) plus the common pathway (factors X, V, II, fibrinogen). Two clinical uses dominate: (1) monitoring unfractionated heparin therapy — therapeutic range usually 1.5–2.5× control, depending on the indication; (2) screening for bleeding disorders — prolonged aPTT with normal PT/INR points to intrinsic-pathway problems (haemophilia A or B, von Willebrand disease, lupus anticoagulant). With INR (extrinsic pathway), aPTT (intrinsic) and platelets + fibrinogen, you have the four-marker basis of any bleeding work-up.
What a high value can mean
- Unfractionated heparin therapy — expected and therapeutic.
- Haemophilia A (factor VIII deficiency) or B (factor IX deficiency) — congenital; usually known by adulthood but mild cases present late.
- Von Willebrand disease — commonest inherited bleeding disorder; mucocutaneous bleeding pattern.
- Lupus anticoagulant — paradoxically prolongs aPTT in vitro but causes thrombosis in vivo.
- Severe liver disease — failing factor synthesis affects both PT and aPTT.
- Vitamin K deficiency — late finding; PT/INR usually prolongs first.
- Disseminated intravascular coagulation (DIC) — consumed factors.
What a low value can mean
- Acute-phase reaction — factor VIII rises in inflammation, can shorten aPTT.
- Hypercoagulable state in pregnancy — late pregnancy, immediately postpartum.
- A short aPTT alone is rarely actionable; clinical correlation needed.
When to discuss with a doctor
aPTT is rarely a stand-alone test for patients to interpret. In the bleeding-disorder context, a prolonged aPTT with normal PT and normal platelets warrants haematology evaluation for von Willebrand disease, haemophilia or lupus anticoagulant. In a heparinised patient the value belongs to the medical team adjusting the infusion rate. Mediora.AI shows aPTT alongside INR, platelets and fibrinogen for the full coagulation picture.