ISSN 2398-2942      

Hematology: prothrombin time


Synonym(s): PT, one-stage prothrombin time, OSPT


  • Evaluating extrinsic hemostasis (VII) and 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 possible primary disorders (like von Willebrand's disease). 


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Manual, optical, electrical or electromagnetic  

  • Citrated plasma is incubated at 37°C (eg water bath, heating block or cuvette/cup) separately to a reagent containing thromboplastin (tissue factor / platelet factor 3 and phospholipids with calcium ions (factor IV).  
  • 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. 
  • Individual factor assays typically require sending to specific referral laboratories. 



  • Relatively insensitive since activity of clotting factor must be <30% to result in abnormal elevation. However, this may still 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.
  • Furlanello T, Caldin M, Stocco A, Tudone E, Tranquillo V, Lubas G, Solano-Gallego L (2006) Stability of stored canine plasma for hemostasis testing. Vet Clin Pathol 35 (2), 204-207 PubMed
  • Couto C J (1999) Clinical approach to the bleeding dog or cat. Vet Med May, 450-459.
  • Sheafer S E & Couto C J (1999) Clinical approach to a dog with anticoagulant rodenticide poisoning. Vet Med May, 466-471.

Other sources of information

  • Kaneko J J, Harvey J W & Brass M L (eds) (1997) Clinical Biochemistry of Domestic Animals. 5th edn. Academic Press, USA.
  • Jain N C (1993) Essentials of Veterinary Hematology. Lea and Febiger, USA.


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