partial thromboplastin time (PTT)

Indications

• assessment of coagulation disorders
• monitoring therapy with unfractionated heparin

Contraindications

• anti-factor Xa heparin assay indicated
  • patients with lupus anticoagulant
  • patients also taking warfarin
  • patients refractory to heparin (> 40,000 units/day)
  • baseline elevated aPTT

Reference interval

• normal range: < 35 seconds (method-dependent)
  • 25-35 seconds
• therapeutic range for heparin therapy: 60-90 seconds

Principle

Assessment of coagulation factors

• factor II
• factor V
• factor VIII
• factor IX
• factor X
• factor XI
• factor XII

The assay is performed by activating contact factors with kaolin, ellagic acid or celite, adding a standardized phospholipid preparation as a platelet substitute & measuring the time to clot formation after adding excess Ca+2 to citrated plasma.

Increases

• causes of increased PTT*
  • decreases in coagulation factors < 30-40% of normal*
    • factor II deficiency
    • factor V deficiency
    • factor VIII deficiency
    • factor IX deficiency
    • factor X deficiency
    • factor XI deficiency
    • factor XII deficiency
  • decrease in plasma prekallikrein
  • decrease in plasma high molecular weight kininogen
  • plasma fibrinogen levels < 50 mg/dL or dysfibrinogenemia
  • circulating anticoagulant: lupus anticoagulant
  • drugs
    • heparin
    • chlorpromazine
    • valproic acid
  • failure for correct for hematocrit
  • insufficient filling of vacutainer

* ONLY factor VIII deficiency or factor IX deficiency associated with frequent or severe bleeding Plasma mixing studies are performed to assess whether increased PTT is due to deficiency vs inhibitor

• causes of increased PTT with normal PT
  • factor V deficiency
  • factor IX deficiency
  • factor XI deficiency
  • factor XII deficiency
  • von Willebrand's disease (severe) & factor VIII low
  • heparin exposure

• causes of increased PT & PTT:
  • factor II deficiency
  • factor V deficiency
  • factor X deficiency
  • vitamin K deficiency (late)
  • warfarin

Decreases

• compensated low-grade coagulopathy (activated coagulation system)
• traumatic venipuncture contaminating specimen with tissue thromboplastin

Differential diagnosis

• prolonged prothrombin time, normal aPTT
  • factor VII deficiency
  • disseminated intravascular coagulation (DIC)
  • liver disease
  • vitamin K deficiency
  • warfarin
• normal prothrombin time, prolonged aPTT
  • factor VIII deficiency
    • acquired factor VIII inhibitor
      • severely prolonged aPTT, NOT corrected with addition of normal plasma
  • factor IX deficiency
  • factor XI deficiency
  • factor XII deficiency
  • von Willebrand's disease (if factor VIII low)
    • lesser increases in aPTT
  • heparin
• prolonged prothrombin time, prolonged aPTT
  • factor V deficiency
  • factor X deficiency
  • factor II deficiency
  • fibrinogen deficiency
  • severe liver disease
  • disseminated intravascular coagulation (DIC)
  • vitamin K deficiency
  • warfarin toxicity
  • heparin overdose
• normal prothrombin time, normal aPTT
  • platelet dysfunction (acquired or congenital)
  • von Willebrand's disease (factor VIII not too low)
  • scurvy
  • Ehlers-Danlos syndrome
  • hereditary hemorrhagic telangiectasia
  • factor XIII deficiency

More general terms

• clinical coagulation test

Additional terms

• coagulation cascade
• plasma mixing studies
• prothrombin time (PT)

Component of

• PT/aPTT/fibrinogen in plasma
• DIC panel
• coagulation screening panel

Patient information

activated partial thromboplastin time (aPTT) patient information