Thromboplastin--a theoretical substance in blood/tissues that, in the presence of Ca, converts prothrombin to thrombin. In fact, refers to groups of coagulation factors which in combination with Ca can produce Va & convert prothrombin; so "extrinsic thromboplastin" = factors of the extrinsic pathway; "intrinsic thromboplastin" = factors of the intrinsic pathway + PF-3; the former is tested in the PT assay; the latter is tested with the PTT.

Method measures "intrinsic thromboplastin": combine

  1. Pt's plasma (from citrated blood)
  2. + CaCl2
  3. + PF-3 (a partial thromboplastin)
  4. + activating agent (e.g. micronized silica)
  5. & record time till 1st appearance of fibrin threads

Increases with Heparin, deficiencies of factors VIII & IX (but also X, XI, XII) & 10% of SLE pts (circulating anticoagulants)