Activated partial thromboplastin time (aPTT)

What it measures

Measures clotting time through the intrinsic and common pathways (factors XII, XI, IX, VIII, X, V, II, fibrinogen). Used to screen for bleeding disorders and to monitor unfractionated heparin.

Specimen and preparation

• Specimen: Citrated plasma (3.2% sodium citrate, 9:1)
• Tube: Light_blue top
• Preparation: Correct fill is essential. For heparin monitoring, centrifuge within 1 h of collection and test within 4 h; otherwise test within 4 h.

Reference range

Reference ranges differ between laboratories, methods and populations. Always use the range printed on your own lab report.

Causes of high values

• Heparin therapy
• Hemophilia A or B, factor XI deficiency
• von Willebrand disease (some cases)
• Lupus anticoagulant
• Liver disease, DIC
• Factor XII deficiency (no bleeding)

Causes of low values

• Acute phase with high factor VIII
• Difficult draw with sample activation

Method

Clot-based assay with contact activator, phospholipid and calcium on coagulometer

Good to know

A prolonged aPTT should be followed by a mixing study. Reference intervals are reagent-specific; each lab sets its own.

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