ISSN 2398-2942      

Hematology: activated partial thromboplastin time (APTT)


Synonym(s): aPTT, PTT, Partial thromboplastin time, (Kaolin-cephalin clotting time)


  • Evaluating intrinsic (XII, XI, IX, VIII) plus common (X,V, II, I) pathways help determine whether bleeding is due to one or more defects in coagulation (secondary hemostasis), from other causes (eg trauma, ulceration, neoplasia) or possibly primary disorders (like von Willebrand's disease). 


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Manual, optical, electrical or electromagnetic  

  • A. Citrated plasma is incubated with surface activator (eg Kaolin) and procoagulant phospholipids (eg cephaloplastins, partial thromboplastin) at 37°C (e.g water bath, heating block or cuvette/cup). 
  • B. Calcium ions (factor IV) are added (counter-acting the citrate chelation) and the solutions-plasma mixed. 
  • C. Time taken for fibrin to then form is measured (eg visible fibrin/gel-clot, light transmission (spectro-) or scatter (nephro-), electrode voltage difference, oscillating magnetized ball). 
  • D. Testing (especially manual) is often performed in duplicate with an average value reported.  


  • Varied bench top automated analyzers available for in house (mainly PT, APTT and/or ACT) 
  • Most clinical pathology laboratories offer a basic hemostatic/clotting profile  Hematology: complete blood count (CBC), coagulation times (PT and APTT) and possibly fibrin degradation products (FDP) Fibrin degradation products or D-dimers Hematology: D-dimers.   
  • Individual factor assays typically require sending to specific referral labs. 



  • More sensitive than activated clotting time (ACT) Hematology: activated clotting time
  • Still relatively insensitive since activity of clotting factor must be <30% to result in abnormal elevation. However, this may correlate well with clinically evident bleeding. 


  • Does not require presence of blood’s cellular components (eg platelets). 
  • Varies with method and potential in vitro factors as listed.  

Technique (intrinsic) limitations

  • Some manual methods or automated instrument platforms are designed for human testing, so less suitable for canine specimens (faster clot formation relatively). The instrument and method should be validated for each specific species and canine specimens. 
  • Reference intervals are often poorly defined and data on healthy sampling conditions may not translate to patients with systemic diseases or if on anti-coagulant therapy.  

Result Data

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Further Reading


Refereed papers

  • Recent references from VetMedResource and PubMed.
  • Couto C J (1999) Clinical approach to the bleeding dog or cat. Vet Med 450-459.
  • Sheafer S E & Couto C J (1999) Clinical approach to a dog with anticoagulant rodenticide poisoning. Vet Med 466-471.
  • O'Brien S R, Teresa, Sellers T, Meyer D (1995) Artifactual Prolongation of the Activated Partial Thromboplastin Time Associated With Hemoconcentration in Dogs. JVIM 3, 169-170 Wiley

Other sources of information


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