Felis ISSN 2398-2950

Hematology: activated partial thromboplastin time (APTT)

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

Contributor(s): Kathleen P Freeman, Karen L Gerber, Roger Powell


  • 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). 


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Manual, optical, electrical or electromagnetic  

  • Citrated plasma is incubated with surface activator (eg Kaolin) and procoagulant phospholipids (eg cephaloplastins, partial thromboplastin) at 37°C (eg water bath, heating block or cuvette/cup). 
  • Calcium ions (factor IV) are added (counter-acting the citrate chelation) and the solutions-plasma mixed. 
  • 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). 
  • 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 small animal specimens (faster clot formation relatively). The instrument and method should be validated for each specific species and 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 VetMed Resource and PubMed.
  • Couto C J (1999) Clinical approach to bleeding dog or cat. Vet Med May, 450-459.
  • Lisciandro C L, Hohenhaus A, Brooks M (1998) Coagulation Abnormalities in 22 Cats with Naturally Occurring Liver Disease. JVIM 12, 71; 71-75 Wiley 
  • Green R A, White E  (1977) Feline  factor  XI1  (Hageman) deficiency. Am  J Vet Res 38, 893-895. 

Other sources of information

  • Kaneko J J, Harvey J W & Brass M L (eds) (1997) Clinical Biochemistry of Domestic Animals. 5th edn. Boston: Academic Press.
  • Jain N C (1993) Essentials of Veterinary Hematology. Philadelphia: Lea & Febiger
  • http://eclinpath.com/hemostasis/tests/.