Activated partial thromboplastin time (APTT)





aka partial thromboplastin time (PTT), kaolin cephalin clotting time (KCCT), partial thromboplastin time with kaolin (PTTK)


Activated partial thromboplastin time (APTT) measures the intrinsic coagulation pathway (as per classical model).

APTT measures the clotting time of plasma
... after an activator (e.g. kaolin) and calcium are added to plasma sample

NB:

  • The test is called "partial" because of absence of tissue factor


APTT is sensitive to:

  • Circulating anticoagulants
    • e.g. lupus anticoagulant in antiphospholipid syndrome
  • Heparin

NB: Antiphospholipid syndrome is an HYPERcoagulable state.

Use

APTT is used in:

  • Heparin monitoring
  • Screening for...
    • Haemophilia A (factor VIII deficiency)
    • Haemophilia B (factor IX deficiency)
    • Coagulation inhibitors

Abnormal results

Normal range = 26.0 to 33.5 seconds
[OHCLI: p206]

NB: I believe APTT is not yet standardised (similar to PT)


Normal results requires the presence of:

  • Fibrinogen (factor I)
  • Prothrombin (factor II)
  • Factor VIII, IX, XI, XII
  • Factor V and X (PT/INR will also be raised)

NB:

  • APTT will NOT detect deficiencies in factors VII or XIII

Raised APTT

  • Haemophilia
  • Heparin
    • Rx
    • Contamination
  • Circulating anticoagulant
  • DIC
  • Liver disease
  • Massive blood transfusion
  • Oral anticoagulatns (modest increase in APTT)
  • Rarely, von Willebrand disease
    • ?? Factor VIII decreased only in type 2N, and type 3

Presence of anticoagulant

e.g.

  • antiphospholipid antibody (e.g. lupus anticoagulant)
  • antibodies against coagulation factors

If an inhibitor is suspected, normal control plasma can be added and APTT repeated.

APTT will be corrected if the previously raised APTT was due to factor deficiency.

APTT will not be fully corrected if it was due to an inhibitor.