Activated partial thromboplastin time (APTT)

Principle

• Plasma is incubated with a contact activator (kaolin, silica, celite or ellagic acid) and phospholipid (partial thromboplastin, which lacks tissue factor); calcium chloride is then added and time to clot is measured.
• Tests the intrinsic and common pathways: prekallikrein, HMWK, factors XII, XI, IX, VIII, X, V, II and fibrinogen.

Specimen

• 3.2% sodium citrate, 9:1 blood to anticoagulant, correctly filled; platelet-poor plasma.
• Test within 4 h of collection; samples for unfractionated heparin monitoring should be centrifuged within 1 h because platelets release PF4, which neutralises heparin.

Procedure

• Mix equal volumes of plasma and APTT reagent; incubate at 37 °C for the time stated by the reagent (commonly 3–5 min).
• Add pre-warmed 0.025 M calcium chloride, start timing and record the clotting time.

Results

• Typical about 25–35 s (range depends on reagent and analyser); local ranges apply.

Interpretation

• Prolonged: unfractionated heparin, haemophilia A (VIII) or B (IX), factor XI deficiency, factor XII, prekallikrein or HMWK deficiency (no bleeding tendency), von Willebrand disease (low VIII), lupus anticoagulant, direct thrombin inhibitors, liver disease, DIC, vitamin K deficiency.
• Shortened: high factor VIII (acute phase response, pregnancy) or an activated sample from difficult collection.
• Isolated prolonged APTT is investigated with a mixing study and factor assays.

Quality control and pitfalls

• Reagents differ in sensitivity to heparin, factor deficiencies and lupus anticoagulant; each lab sets its own heparin therapeutic range (often calibrated to anti-Xa 0.3–0.7 U/mL).
• Heparin contamination from lines, under-filling, high Hct, clots and haemolysis cause errors.
• Run normal and abnormal controls each shift.

Clinical use

• Monitoring unfractionated heparin, screening for haemophilia and other intrinsic factor deficiencies, pre-operative screening, detection of lupus anticoagulant.