Activated Partial Thromboplastin Time (APTT)

Coagulation Cascade

• Intrinsic Pathway: – Factors VIII, IX, XI, and XII.– Activated on surface of exposed

endothelium.– Complexes form on platelet

phospholipids.

• Common Pathway:– Factors X, V, prothrombin and

fibrinogen

Activated Partial Thromboplastin Time

• APTT test measures – the intrinsic pathway factors – & common pathway factors

• Reagent contains phospholipids and a surface activator• Calcium Chloride reagent added to start the reaction.• APTT reagent mimics the surface of a platelet.

Expected APTT Values

• Normal Range: 25 to 43 seconds

• Slightly Elevated: 45 to 65 seconds

• Extremely Elevated > 70 seconds

When is it ordered?

• When a patient presents with unexplained bleeding or bruising,

• It may be ordered as part of a pre-surgical evaluation for bleeding tendencies,

• When a patient is on intravenous (IV) or injection heparin therapy, the APTT is ordered at regular intervals to monitor the degree of anticoagulation.

What does the test result mean?

• APTT prolongations are caused by either factor deficiencies (especially of factors VIII, IX, XI, and/or XII),

• Inhibitors (most commonly, lupus anticoagulants)

• Therapeutic anticoagulants such as heparin

Sample

• One blue top (citrate) tube; 3.2% citrate tubes

• Ratio of anticoagulant to blood should be 1:9

Procedure

• Prewarm sufficient volumes of CaCl2 (0.025 M) to 37oC

• Label 2 test & 2 control tubes• Add 0.1 ml of sample or control to the

appropriate tube• Add 0.1 ml of APTT reagent to each tube• Incubate at 37oC for 5 minutes

• Add 0.1 ml of prewarmed CaCl2 & start stop watch immediately

• Record the time at which clot is detected