Activated partial thromboplastin time (APTT)
Pathway · detail · Intrinsic limb Heparin
Citrated plasma is recalcified with phospholipid and a contact activator (kaolin, silica or ellagic acid) but no tissue factor, so clotting must run through the contact and intrinsic limbs into the common pathway. It therefore interrogates factors XII, XI, IX and VIII plus X, V, prothrombin and fibrinogen.
Traced from the start
- Contact activation system (factor XII, prekallikrein, high-molecular-weight kininogen)
- Activated partial thromboplastin time (APTT)
Detail
- Normal range
- About 25-38 seconds, reagent dependent
- Measures
- XII, prekallikrein, HMWK, XI, IX, VIII, X, V, prothrombin and fibrinogen — everything except factor VII and factor XIII
- Prolonged by
- Haemophilia A and B, von Willebrand disease, factor XI or XII deficiency, unfractionated heparin, dabigatran, lupus anticoagulant, DIC and severe liver disease
- Mixing study
- Correction on 1:1 mixing with normal plasma indicates factor deficiency; failure to correct indicates an inhibitor — a lupus anticoagulant (immediate) or a factor VIII antibody (time and temperature dependent)
- Monitors
- Intravenous unfractionated heparin, target ratio about 1.5-2.5; LMWH requires an anti-Xa level, not the APTT
When it goes wrong
Prolonged APTT that fails to correct on mixing, in a patient with thrombosis rather than bleeding and recurrent miscarriage
Antiphospholipid syndrome — the lupus anticoagulant binds the phospholipid in the assay and prolongs it in vitro while causing thrombosis in vivo
Practise this structure
No question in the bank is tagged to Activated partial thromboplastin time (APTT) yet. Questions appear here automatically once one is — the question bank is free to browse in the meantime.