Felis ISSN 2398-2950

Hematology: activated clotting time

Synonym(s): whole blood clotting time WBCT, clotting time, activated clotting time, ACT

Contributor(s): Kathleen P Freeman, Andy Torrance


  • Evaluates entire secondary blood clotting system (common and intrinsic pathway and platelet).
  • Test is independent of platelets (unless the platelet count is <10 x 10*9/l).


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Capillary tube

  • Blood collected in fine capillary tube; time noted.
  • Break off end of tube every 30 seconds until appearance of fibrin is noted.
  • Measure time interval.

Activated clotting time

  • Time taken for whole blood to clot inside glass tube containing contact activator (diatomaceous/earth).
  • Less variable and more sensitive than capillary tube method.
  • Performed at room temperature or 37°C (tube must be prewarmed).
  • Special Vacutainer tube required (also allows clot retraction time to be measured).
  • ACT: time taken to notice obvious clotting following addition of blood.


  • In veterinary practice.



  • Relatively insensitive but acceptable to help differentiate/evaluate presentations with bleeding.
  • Evaluates same pathways as activated partial thromboplastin time (APTT).

Predictive value

  • Prolonged ACT: presence of severe coagulopathy or platelet <10 x 10*9/l.

Technique (intrinsic) limitations

  • Deficiency of factor VII has no effect (extrinsic system of less importance).
  • Does not evaluate extrinsic pathway.
  • Be sure to concurrently evaluate platelets (automated and/or manual count) and correlate with examination of blood smear. Marked thrombocytopenia (<10 x 10*9/ml) may result in prolongation of ACT or WBCT.

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.

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 Haematology. Philadelphia: Lea & Febiger.